version of 2007-04-12

The TableHooters modification FAQ

[guide for safer circuit-bending]
This FAQ is about the modification of cheap music keyboards and electronic sound toys to convert them into unusual music synthesizers.

Maintainer of this FAQ is CYBERYOGI =CO=Windler (e-mail: windle_c@informatik.fh-hamburg.de). This FAQ comes with absolutely no warranties - particularly I can not guarantee that the technical explanations and safety tips in this FAQ are fully correct, complete and up to date, therefore whatever you may do by instructions and tips found in this FAQ, you will do it solely at your own risk. I also can and will not teach you in this FAQ the general basics of electronics or synthesizers; on the internet are plenty of other sites to explain these.

The newest version of this FAQ can always be found on the WarrantyVoid site:

Why "tablehooters"?

In Germany cheap sounding electronic beginner's keyboard instruments are called a "Tischhupe". "Tisch" means table, "Hupe" is a car- or alarm horn, though the possibly best English translation for this nickname is a "tablehooter".

What the FAQ is circuit-bending?

Circuit- bending means to hack/ modify/ abuse the hardware of electronic sound toys or such instruments in completely different ways than their manufacturer has intended - namely as experimental musical (or not so musical) instruments.

The simplest (but not always healthiest) form of circuit- bending is the naughty act of simply opening the case of a low voltage (battery operated) sound device and stick ones hands straightway into the circuitry to temporary short electric connections by fingers to play sounds on it. Though circuit- bending can be basically regarded as a cyberage's anarchic successor of phono record scratching, which also only got possible by systematically ignoring all grannies warning: "Don't touch the precious gramophone discs with your smeary, sweaty fingers!". ;-) The same way circuit- bending lives from systematically ignoring any "warranty void" warning stickers on its explorative mission to boldly hear what no man has heard before...

In more elaborated forms of circuit- bending additional potentiometers, buttons and other controls get soldered to the circuit of a sound device to modify it into a synthesizer- like musical instrument. I always thought I would be the only electronics hobbyist with the strange hobby of modifying sound toys, until I discovered the homepage of the artist Reed Ghazala (see here), who since long times does quite the same and created for this the term "circuit- bending". Already in my childhood I dismantled my Casio MG-880 melody/ game calculator, made its remains' melody pitch (clock speed) adjustable and played it at full volume through the record player input of my grandma's radio. I also placed my Casio VL-Tone 1 onto the radio and switched the latter to AM or SW, which makes tons of funny distorted synth sounds receivable in the radio (depending on the radio's tuned frequency). I also often wiggled the VL-Tone batteries at their contacts, which made often quite bizarre sounds, melodies and symbols appearing on the LCD.

Nowadays I modify instruments usually more planful than I did in my childhood, and unlike Ghazala* (who encourages people to ignore theory and simply play around with shorting connections on the PCB by trial and error without caring much what they technically do) I am e.g. particularly systematically searching for "eastereggs", i.e. extra chip functions those were not wired to buttons because they were intended for a more expensive version or test purpose etc. E.g. many old keyboard instruments (with one button per rhythm) contain way more rhythms than buttons are present on the control panel. But I also mess around with the clock oscillator (including exploring effects of crashes by overclocking), add potentiometers to the individual sound channels of an accompaniment circuit, distort things and much more.

*) Reed Ghazala calls his techniques "anti- theory", but sometimes a bit of scientific/ technical theory can help a lot, so far it doesn't transform people into herd- animalish cookbook engineers incapable to think by themselves. (I went successlessly at the Technical University of Hamburg- Harburg through 6 semesters of "technical computer science" (which is based on electrical engineering) before I started my software- techniques studies, though I learned plenty of "theory" (and white science's prejudices) there.) Look at Friedensreich Hundertwasser; he also needed some architecture theoretical basics to become capable to create buildings those don't collapse; in spite of this he didn't repeat the same boring and shape- aggressive rectangular concrete block rubbish that other architects do. Important is just to stay capable to think differently in spite of all that "theory" (to which my enlightenment enabled me). I may think like an engineer scientist, but I feel like an alchemist.

My approach is much more systematic than what Ghazala tells the world, but not to understand me wrong - circuit bending needs no studies of electrical engineering and simple forms can be done by everyone - even a kid can find interesting sounding results by trial and error. But in spite of this, blindly shorting IC connections can be quite dangerous for electronics and result in expensive and annoying damages. I think that an instrument that you build by this method should last as long as any professionally manufactured one and not burn out after a few days or months for obvious or even totally unobvious reasons, and it also should not become a health risk. I therefore wrote down this FAQ for safer circuit- bending and tablehooter modifications...

a few words about soldering irons:

To solder electronics, a small soldering iron with not more than 25W or a temperature regulated soldering station is necessary. Soldering fumes typically contain lead and many other brain damaging substances those must not be inhaled; good ventilation is therefore very important. (I built a smoke sucking appliance which fan sucks much of the smoke away from my face.)

It is often much easier to abuse a soldering iron to melt holes into a plastic case than to completely drill/ carve them out by hand. Especially with rectangular and irregularly shaped holes this is much easier than messing around with a drill or Dremel milling tool or jig saw; such tools also tend to produce much dust that easily crumbles e.g. between the contacts of switches or potentiometers and makes them fail to work, and they also make it strictly necessary to remove all PCBs from that case area, which can be awkward and risky when there are many fragile ribbon cables on them (e.g. with early analogue Casio instruments). For switches I rarely use round holes, because this type of switches wastes space with a bulky body and a long lever that tends to crack off easily, and they tend to be expensive too (circuit bent instruments often need dozens of them). I therefore prefer tiny slide switches or DIP switch blocks those need rectangular holes. After doing the main work with a soldering iron, I use cheap household- and nail scissors to carve out the rest and smoothen the rims.

But to melt holes into plastic it is strictly necessary to use a temperature controlled soldering device, because a too hot soldering iron will cause way more toxic smoke when attempting to melt or weld plastic, and you also easily melt away too much plastic with a too hot iron. Plastic cases are mostly of polystyrene or PVC, which emits toxic substances (e. g. styrene or chlorine) when overheated; the iron therefore must be adjusted not much hotter than barely necessary to melt the plastic (typically below 200°C). Very small soldering irons work best for making fine holes, but they also cool down quickly while melting plastic, which can make the work awkward when making larger openings into a case. (Turning it hotter would cause more toxic fumes.) The best solution is often to melt a small hole with the soldering iron and then carve out the rest using a household scissors. If present, use a different soldering iron tip (or different iron) for plastic and for electronics soldering because melted plastic remains on the tip can emit toxic fumes when heated up to soldering temperature.

other important tools:

You will certainly need a multimeter, test cables with alligator clips, screw drivers and various other stuff those are a matter of course with electronics work and those I won't mention here.

But one of the most important tools next after the soldering iron is a small low temperature hotglue gun. You can insulate contacts and quickly mount cables and many other components with it. Normal high temperature hotglue glues even stronger and can be also used (I use both), but it also can warp thin plastic parts by excessive heat and emits more toxic phthalate fumes. Unlike normal household tube glue, hotglue solidifies within less than 2 minutes and also emits no brain destroying solvent odours.

Also a black permanent felt pen (like used for writing on CDRs) is very important to mark discovered connections directly on the PCB. To remove the permanent ink, use isopropanol and Q-Tips. Non- permanent (water soluble) felt pens are too awkward to use because they tend to smear badly when touched with sweaty fingers.

A cheap household scissors helps a lot to carve or enlarge drilled holes in plastic cases, and also can be used for removing cable insulation.

Another almost essential tool for my work is a digital camera with display. Before you modify anything, make a photo of the previous state of the wiring, thus when you accidentally crack off a cable, you can much easier track back on the camera display where it came from. Although you could theoretically also draw on paper where the cables go, the camera is a huge help and can prevent a lot of trouble. Especially tube electronics and cheap Chinese sound toys can contain a horrible component mess that you really don't want to document on paper (see Jörgensen Clavioline, Golden Camel-11AB or MeiKe MK-320B for example). Another crucial application for the digicam is to photograph the back side of a PCB to compare both sides and examine the wiring without continuously flipping the PCB back and forward (which easily can crack off  rigid wires or ribbon cables). This way you can e.g. watch the trace side on the display while the PCB is screwed back into place (which e.g. is necessary with control panel PCBs those have many separate slide switch wipers on the plastic panel of the case). The hi-res PCB photos also help very much to compare the hardware of multiple keyboards without dismantling them all.

(My "Jenoptik Jendigital JD 4.1 x z3" camera unfortunately focusses very badly in dimly illuminated rooms, thus I have to shine with a small LED torchlight on the object to be focussed while pushing the trigger halfway down and then remove the torch light to avoid overexposed white areas. With dim light it also makes either crumbly pixels or blurs the picture by shaky hand when shot with reduced sensitivity. (I read that modern Sony Cybershot cameras compensate shaky photos automatically.) The JD 4.1 x z3 also displays with sunlight dark grass green way too bright an pale, and dark violet as almost light blue, which is annoying when taking keyboard photos for this site. The camera also uses the NiMH battery capacity badly because it turns itself off by any voltage drop despite the battery contains still half the energy. With disposable alkaline batteries it even don't work at all and (wrongly) considers them empty after only about 13 shots or 2 minutes of use.)

cleaning electronic devices, plastic & rubber care:

Q-Tips are useful to clean or apply substances on small spots within electronic devices. For larger areas use paper towels, paper handkerchiefs or kitchen paper. Also toilet paper may be used, but since it crumbles apart easily in humid state, it tends to make more mess. To clean plastic or metal cases, simply wipe them with the paper moistened by dipping it into a bowl of water with dish washing soap. Do not use too much water to avoid it from leaking into the electronics. Such paper can be also used to dry it again where necessary.

Isopropanol is one of the best substances to clean switch contacts and to remove paint stains (e.g. kid's crayon marks on old sound toys), lubricant residues and other dirt. But be careful with cleaning shiny plastic or painted case parts, because they may tarnish or get damaged by isopropanol. Also regard that solvent vapours are flammable, thus wait until it has vapoured out before you operate the device again. But generally isopropanol is less toxic than most organic solvents and also than flammable spirt (camping oven fuel), which can contain poisonous methanol and even when it is made from ethanol (the same poison like in booze), it contains the denaturizer diethyl phthalate, which is toxic, smells awful and can damage rubber parts. Although isopropanol may possibly harm rubber during long term exposure, it is at least safe enough to remove spilled oil residues from rubber parts in tape drives to make the rubber tacky again. (I even apply isopropanol to prepare latex rubber when I glue it with non- toxic rubber cement.) The only exception seems to be polyurethane (PU) rubber, which can dissolve by it (I have such a PU- coated dive suit).

When you find very dirty modern electronics PCBs (e.g. in trash) those need to be completely cleaned, do not fear to wash the removed PCB in ordinary water with (dishwashing) detergent. Important is only that no electric live parts (e.g. integrated batteries) stay connected during this, that the PCB has no water soluble parts (like paper diaphragms of speakers) and that the water can not flow into semi- encapsulated components (like motors, closed potentiometers, rotary capacitors or shielded coils) where it can not get out well again. Rinse the PCB thoroughly with fresh water and towel it dry with kitchen paper and/ or use a hairdryer. The only important thing is to dry it thoroughly and not to operate electronics in moist state (even when the voltage/ current is too low to risk electric shocks or burning out components) because this may cause electrolytic corrosion. For safety I also recommend not to soak mains transformers in water unless they are hermetically sealed into plastic resin.

To remove price tag glue stains, residues of dissolved foam rubber and similar sticky substances from plastic cases, apply a drop of ordinary food oil on them, rub it with kitchen paper and then remove oil and dirt with water and dishwashing detergent. But watch out not to spill the oil on rubber or foam rubber parts, because they can easily dissolve from it.

Generally rubber parts (besides silicone rubber) must be kept away from fats, oils (beside silicone oil) and plasticized soft PVC (e.g. fresh PVC cables, PVC dust cover envelopes, cheap imitation leather, PVC inflatables etc.) because these dissolve the rubber into a smeary pulp. Even the (smellable) vapours of oil, organic solvents or plasticizers (phthalate) can damage it over time. Also UV radiation (direct sun light) and especially the ozone produced by it make e.g. latex, butyl rubber and PU turn grey and brittle. Especially foam rubbers react extremely allergic against airborne pollutants, those make it often crumble apart after only few years (see UniSynth XG-1 guitar). To avoid this, best store the rubber in airtight polyethylene or polypropylene containers (e.g. PE plastic bags or Tupper- style fridge boxes) when not in use.

To lubricate mechanical parts, I strictly recommend to use only viscous silicone oil, because any other fats and oils can dissolve rubber parts (beside silicone) and can even make certain hard plastic sorts brittle and crack apart over time. Even the (smellable) vapour of normal oils damages rubber over time; that's the main reason why belts and rubber wheels in tape drives turn hard or sticky after a decade. Wrong oils can also solidify into wax- like residues. But do not spray the silicone oil into the mechanism, because inhaling the vapour causes lung damage, and sprays tends to spread way to far and thus reach components those must stay unoiled to work properly (e.g. rubber wheels in tape drives, or electric contacts). Unfortunately most silicone lubricants are only offered in spray cans, but viscous silicone oil is also sold in sexshops as a latex care product in normal plastic bottles. The only negative side effect of spilled silicone oil is that its high electrical insulation property can increase static friction electricity, which theoretically might be a potential risk for microchips. But yet I never had trouble with this inside electronic devices. Also dedicated special silicone grease is sold in tubes, but it is quite expensive  (e.g. 6€ for only 15ml Äeonix Spezial- Silikonfett at Conrad).

It is a common false myth that rubber would disintegrate by itself over time. When stored correctly (away from air pollutants, UV radiation, direct contact with copper or silver alloys, mechanical overload, humidity, fats, plasticizers and strong heat - e.g. in airtight PE bags) it can basically last forever. With tape decks and record players it is always wrong oil or ozone from brush motors that makes rubber wheels turn hard or belts turn into chewing gum. I found 30 years old cassette recorders with all rubber parts still perfectly intact, while belts in a wrongly oiled (but than temporary well working) VCR turned gummy after only few months. Thus never oil a cassette recorder etc. with any other lubricants than silicone. If you have no silicone oil, it is still much safer to leave it completely unoiled (only thoroughly remove dirt and hardened old oil residues) than to use wrong oil. And always watch out not to smear lubricant (e.g. from oily hands) on rubber friction parts (like belts or wheels) because even silicone oil would make them slip and thus fail to work. To unoil rubber parts, wipe them off thoroughly with a dry paper handkerchief. Especially when the lubricant residue is not silicone, you may also clean it first completely with some isopropanol to prevent further oil damage, but do not expose the rubber to long to it because isopropanol may harm rubber also.

Rubber switch contacts in electronic devices are usually made from silicone rubber, which does not decay this way. But also contacts and buttons of butyl rubber exist those may need more protection, and unfortunately there are even ones of odorous plasticized PVC (smells like flammable spirt, e.g. Cyber Drum Center drumpads), those toxic phthalate vapour might endanger other rubber objects exposed to it. Thus be careful not to store soft PVC parts (e.g. cables) or anything that is sticky or contains fat or oil (e.g. leather) upon rubber buttons, and store things separately those badly stink of phthalate. Even certain kinds of hard case plastic can get badly damaged from direct contact with such cables (see Antonelli). Excess phthalate oil residues in soft PVC can be also reduced by thoroughly washing that PVC object (or detached component) multiple times in fairly hot soap water. Although this will not remove it entirely (without plasticizer the PVC would turn brittle), it will at least reduce the emitted vapour to a more tolerable level.

Sometimes reckless parcel senders ruin original packaging boxes with brown plastic packaging tape. This nasty tape consists of a thin transparent plastic foil with brown glue underneath and deserves to be morally banned through public petitions like landmines, because it ruins everything with brown glue stains despite it has absolutely no benefits compared to the same kind of tape with transparent glue. To remove this tape and its residues from cardboard boxes without damage is almost impossible, but at least there are some tricks to reduce the damage. The tape loosens easier by applying short fast jerks rather than slow peeling motion. When you peel or pull at it, pull the tape backward in an u-turn (180°C angle), which causes the least damage to the cardboard surface. Also cold makes the glue adhere less, thus putting the box in a fridge or in winter outside helps to remove it, and you also may put ice in a small container and "iron" the tape with it to cool it down. (Don't use coolant spray - the gas causes brain damage and pollutes the environment.) Glue residues can be removed by rapidly smacking the residues with the glue side of a piece of adhesive film, which will make the residues rejoin with the glue surface and loosen from the cardboard. (Also a piece of the just removed brown tape can be used although there is a small risk of causing additional stains.) But crucial for this is not to smack too hard and especially do not let both surfaces come in contact for longer than a fraction of a second. It works best to wrap the adhesive film around a finger with the glue outside for this, and then tap with the finger on the brown residues rapidly like a sewing machine. A small amount of dust or powder on the adhesive film reduces the risk of cardboard damage by reducing the glue force.

Plastic and paper parts also should not be unnecessarily exposed to UV radiation (direct sunlight), because it can make reddish dyes bleach out, stain the material yellow and even make white polyethylene or polypropylene plastics turn brittle (likely by photochemical reactions with titan dioxide dyes).

eliminate chemical odours:

Especially cheap Chinese electronic devices often stink unbearable of acrid chemicals. E.g. some of my Yongmei keyboards stank brand new so horrible of toxic chemicals (styrol, phthalate, organic solvents, formaldehyde?) that I got headache and a sore throat whenever I played them for longer than 5 minutes. The simplest countermeasure is to let them vapour out in a preferingly unused and well ventilated room; unfortunately this can take many weeks or even months. It will vent out faster when you open the case for better ventilation, but this needs space and also makes the electronics catch dust when stored in this state for longer.

Sometimes buttons and pseudo- rubber contacts are made from cheap plasticized soft PVC (instead of silicone), that can stink very badly of phthalate (a toxic oily plasticizer that smells like flammable spirt). To reduce that odour, take these parts out and wash them many times in boiling hot soap water (which will leave oily phthalate residues in the bowl). But do not attempt to extract all of the phthalate (e.g. chemically using oil or alcohol) because this would make the plastic brittle. Also foil button pads are often made from stinky phthalated PVC foil. To reduce their odour, you can carefully clean them with a rag and warm detergent water. Never use solvents like alcohol here, because this can would make the foil crumble apart. (On a flea market I bought a toy laptop with such a dissolved keypad.)

The odours inside the case can be also reduced by smell absorbents, which has the benefit that the toxic vapours will be locked away inside the absorbent instead of infesting the room air elsewhere. Solid absorbers are e.g. active carbon smell filters for household kitchen hoods. The normal type looks like black foam rubber and can be placed inside the case, but regard that this foam rubber may conduct electricity, thus to prevent short circuits it is necessary to put them inside an envelope of air permeable cloth or such paper (e.g. a peeled off layer from 4 layer paper handkerchiefs) and mount them in a way that the envelope can not be penetrated by electrical components (like spiky wires from a PCB back). Other kitchen hood smell filters look like small plastic boxes with a grid those contain carbon granulate. Also these may be usable, but to prevent shorts, watch out that the granulate can not crumble through the grid holes (use an envelope when necessary). Another effective smell absorbent is Febreze textile deodorizer (based on the harmless sugar- like substance cyclodextrine). Unfortunately it comes in a water solution that sprays things very wet during application, thus it may cause corrosion and damage paper parts (e.g. speaker diaphragms) when sprayed directly on electronics. But it certainly can be used this way so far the device is not connected to electricity (take batteries out) and the moisture inside the case is immediately dried with a hair dry (not too hot to prevent case deformation). But a better idea is to spray the Febreze on a piece of kitchen paper let it dry it and mount only the dried paper inside the case like a solid smell absorber. The treated paper will not conduct electricity and can be easily replaced when it looses its effect, thus this is likely the safest method of choice.  (I haven't used Febreze directly on electronics yet, but it worked well (and didn't damage the diaphragm) with the amplifier cabinet of my Tuttivox organ.)

gluing case parts:

Small cracked off hard plastic (usually polystyrene) case parts can be glued best with superglue (cyanacrylate). Wear gloves during use and hold small parts with tweezers or an alligator clip. Unlike the manufacturers claim, superglue does not always glue within seconds but sometimes can takes up to about an hour to dry. To speed up drying, use a hairdryer (avoid excessive heat to prevent case deformation); also exhaling on the glue joint can help, since the humidity speeds it up too. Larger or mechanically stressed internal case parts (e.g. screw mounting posts) can be also glued from inside with normal (high temperature) hotglue.
To repair a cracked off plastic key, take out the key assembly (usually multiple keys hanging on a comb- like common plastic strip) and hotglue at the key's bottom side a thin piece of polyethylene or polypropylene sheet plastic over the crack. Well works sheet plastic cut out from transparent plastic packaging. Use the hotglue gun nozzle to flatten the glue joint to prevent the key from getting stuck.

fixing crooked keys & plastic:

Especially with Yongmei keyboards often the plastic keys or their fixtures are so severely crooked that they get stuck, protrude upwards or miss their key contact underneath. But also with other  keyboards made of thermoplastics this can happen when they were exposed to heat (e.g. left in a hot car). To re- align deformed keys, do not attempt to bend keys back in cold state by raw force since they may crack off. Instead take out the plastic key assembly (usually multiple keys hanging on a comb- like common plastic strip) and heat it with hot water (e.g. a hot shower). This should make the keys flexible enough to bend them back into normal position. The same can also be done with other case parts. Alternatively a hot hairdryer can be used to heat the keys (also in installed state), but be careful not to heat them too much, since this may warp keys or other case parts even more in unwanted ways.

fixing cracked PCBs:

Defective electronics often fails by fine cracks in the PCB traces, those can be difficult to see and cause random failures.  Such damages happen especially by too hard button presses (e.g. in sound toys ill- treated by kids) in the area around switches and at other mechanically stressed parts. To fix a cracked trace, first remove the insulating paint at its end with fine sand paper or a screw driver. Then bridge the now bare copper ends with a drop of solder. When the gap is too wide or a piece of trace is missing, bridge the missing part with a thin copper wire (e.g. coil wire or a single thin wire pulled out of a wick cable) before you apply the solder. It is a good idea to mark all found cracks with wipe- proof felt pen, because the melting resin core of the solder tends to spill over the cracks and makes them very hard too see as soon you start soldering.

Sometimes the PCB is not only cracked but smashed to pieces those need to be mechanically glued together for stabilization. The likely strongest glue for this purpose is 2 part epoxy, but because its vapours are highly poisonous and cancer causing, I recommend not to use it. Superglue (cyanacrylate) works also well and makes less extreme vapours (but they are still acrid and contain cyanide that should not be inhaled). Use a hairdryer to make the glue dry faster. When it is sufficient to glue a single sided PCB only from the component side and the PCB doesn't run hot during operation, also normal (high temperature) hotglue can be used as the least harmful alternative. Sometimes it can be also sufficient to reinforce given traces by soldering thick copper wires to them instead of using glues.

Unfortunately there is no easy fix for cracked multi layer PCBs (like in modern PC mainboards) because they contain multiple stacked traces inside. The only fix would be to find out the starting and ending point of each torn trace (e.g. by shining with a bright lamp through the PCB or by measuring at the crack surface with an ohmmeter against all other solder joints) and then re- connect them by soldering thin cables or coil wire to matching solder joints from outside. But at least with non- transparent PCBs such a task may rather take months than hours or may not work at all, thus don't expect quick success here. Fortunately such PCBs are rarely used in old keyboards and cheap sound toys. Casio's VL-Tone keyboards and calculators indeed contain simple multi layer PCBs, but at least they are transparent. Newer keyboards often contain an additional layer of conductive carbon traces printed over the normal traces for rubber button contact surfaces and wire bridge replacement. These carbon traces can not be soldered, thus you can only reconnect broken carbon traces by soldering wires to the copper traces those were interconnected by the carbon. Also a special conductive paint may be usable to repair such carbon traces (including those on foil cables), but I have no experience with this.

fixing faulty LCDs:

Small LCD displays sometimes turn dim or segments fail by bad contacts. Most small LCDs are connected to the PCB by silicone rubber strips those contain flexible carbon contacts and squeeze against bare metal traces on the PCB and transparent contacts on the glass of the LCD. To fix them, take out the LCD, clean all the contact surfaces on silicone, glass and PCB with a Q-Tip with isopropanol. Then re-assemble the display. (Write down or make a digicam photo during dismantling when you are not sure about the correct part placement.) Other LCDs (especially by Casio) are connected with a flimsy plastic foil cable, which carbon traces are glued to the contacts on PCB and LCD glass. These glue joints tear off very easy. I know no means to re- glue them, but a strip of adhesive window insulation foam rubber can be used to squeeze the foil cable back into place, which is sufficient to make the display work again when aligned properly.

When an old LCD does not show anything despite the hardware looks ok and you are not sure if the previous owner had dismantled it, then always check first if the polarization filter foil is missing. While with newer displays the filter is glued to the glass, with old ones it was a loose part that was usually only held in front of it by the case frame and could easily get lost during repair attempts; without polarization filter foil you will not see a picture (and the filter turned wrongways inverts the picture). A new filter may be available in camera or telescope stores; also cardboard 3D glasses with grey foil glasses (from a cinema) contain these foils or you may carefully peel it off from the display of a cheap or broken LCD clock or similar.

using power supplies - yes or no?

A common myth says that with circuit- bent instruments generally AC- adapters must not be used but only non- rechargeable batteries to prevent electric shocks or fire by short circuits. IMO this is very exaggerated regarding that disposable batteries pollute the environment and constitute a severe waste of raw materials. In reality it depends very much on the type of power supply you use. When the power supply is short- circuit proof (like dedicated toy grade transformers or laboratory power supplies) there is no higher risk to cause electric shock or fire than with batteries. With a finished and technically correctly modified instrument you also will not need a short- circuit proof PSU; only during experimentation or when you are inexperienced in electronics it is very recommended to have a short- circuit protected power supply.

Especially the internal power supply of an opened instrument should not be used because typically there are bare mains voltage components touchable when open, those can cause a lethal electric shock or fire. When its use can not be avoided (e.g. by the lack of a battery compartment or unknown voltages) and you exactly know what you do, cover its life parts with insulating plastic and watch out that there are replaceable fuses in the DC output lines to prevent damage when you accidentally short its DC voltage.

Also be very careful with cheap plug- style no-name AC- adapters; sometimes they contain a horrible wire mess or loose solder blobs those can easily cause a short between mains and DC voltage. In one such power supply I e.g. found the metal core of the transformer resting directly on the spiky end of the mains plug pins - only separated by a few millimetres of crumbly styrofoam (no joke!). These AC- adapters also rarely contain replacable fuses but typically only soldered low ohmed resistors those need to be desoldered when toasted by overload. When there is no fuse at all, the transformer may even overheat by a short and thus catch fire or melt and output mains voltage on the output side, which can be very dangerous. Also the electronics of small switched power supplies tends to burn out when shorted and may in the worst case output mains voltage. Thus always open a cheap AC- adapter and check what's inside before you use it for experimentation or circuit bent instruments with touch sensor contacts.

For all my experimentation I upgraded a cheap and unstabilized 1A AC- adapter (without switched transformer) with an externally accessible fuse holder and a slow blowing 1.2A fuse; the fuse yet blew a few times by overloads and also the cross shaped multi- plug broke and was replaced multiple times, but the thing still works without problem.

Another argument not to use power supplies is that non- grounded ones tend to output mains hum and HF dirt from the power line, which can distort the sound in buzzy ways and pollutes you nervous system with electric smog while playing on instruments with analogue touch sensor contacts. Grounding the GND line of the instrument may help to prevent this, but especially do not use switching power supplies for this, because their output voltage is particularly strongly infested with RF smog.

An alternative to disposable batteries are rechargeable (secondary) ones, but regard that their output currents can be much higher than with primary ones, which may also cause fire or make the batteries explode when shorted. Especially shorted or overcharged lithium batteries are reported to occasionally explode like fireworks rockets and spill toxic chemicals, and also the cadmium from leaking NiCad cells is badly poisonous, thus I recommend only to use cheap NiMH rechargeables and wire a fuse or 1Ohm resistor in series when you expect to cause unrecognized shorts during experimentation.

general IC treatment hints:

ICs must NEVER be operated with reversed supply voltage ("+Vs") polarity because this generally destroys them. Also a too high +Vs is very dangerous and with normal ICs any input voltages must NOT be lower than GND nor higher than the +Vs because already 0.5V too low or too high can destroy the chip. By this reason ICs also must not be operated with interrupted or disconnected GND connection, because though the result can be the same like feeding a too low voltage into its inputs. Into my modified instruments with AC- adapter jack I therefore generally install a polarity protection diode and where necessary a voltage regulator to prevent accidental damage by a wrongly set external power supply, so far these are not already present.

Adding a voltage regulator is only necessary when the digital hardware has no own one (i.e. when e.g. the CPU is directly connected to the supply voltage, which is typical for cheap battery operated sound toys), because many digital ICs can burn out by already 1V overvoltage. In many devices a regulator is already present for the digital part, but the power amplifier IC (and often other analogue parts) has none, because it runs on higher voltage and typically also can survive some additional volts without immediate damage. With such devices it is not strictly necessary to add an external voltage regulator for the entire electronics, however adding one protects the analogue section against chronic overvoltage damage and can help to reduce mains hum, and it also may still help to protect the digital ICs better, because the factory- installed regulator for the digital section tends to be very small (e.g. a 100mA regulator IC or transistor with zener diode) and thus can overheat and eventually fail when accidentally fed with too high input voltage (which then can destroy the entire digital electronics by the resulting overvoltage passed through the destroyed regulator). The output line from the voltage regulator and battery pack must not be connected directly, because otherwise the battery would empty itself through the regulator. To prevent this, a diode must be placed into both output lines. Due to normal silicon diodes cause each a voltage drop of about 0.7V, I recommend to use Schottky diodes at least for the output line of battery packs to reduce the voltage drop. (This diode also prevents damage by wrongly inserted batteries.) Some AC- adapter jacks also contain a switch contact that mechanically disconnects the battery pack when the AC- adapter plug is inserted; here it is sufficient to use only one diode in the regulator output line. This works well, but so far the contact fails (e.g. when a too thin plug is inserted), it can make inserted batteries leak or even explode when no 2nd diode is present and thus the AC- adapter current flows into the batteries.

With unknown devices power supply jacks do NOT test by trial and error which polarity works, because (depending on chip type, voltage and current) a wrong polarity can burn out ICs within less than a second. Instead open the device to identify the correct polarity (see e.g. electrolytic capacitors polarity) or compare with an ohmmeter the jack polarity with the one of the device's battery compartment. With power supply plug inserted, normally one contact of the jack stays always connected with the battery compartment, while the other is interrupted to prevent battery damage. The always connected jack contact has the same polarity like the one in the battery compartment it is connected with. Also never try to connect an AC power supply (transformer without rectifier) to unknown devices, since this will also destroy unprotected DC electronics by wrong polarity. Very hazardous is that Yamaha and Casio keyboards use the same type of power supply jack, but with different polarity. And because older Casio instruments tend to have no a protection diode, any attempts to operate them with standard polarity (center pin = +, outside = GND) can by lethal for them. When missing, I therefore add this diode to ANY devices with ICs and a standard AC adapter jack, and additionally I modify my Casio instruments to standard polarity to prevent confusion.

(Power supply jacks have usually a 3rd contact that disconnects one pole of the battery pack by a leaf switch when the AC adapter plug is inserted into it. When modifying to standard polarity, it is important that also this contact will change polarity and thus needs to be soldered into the line to the other battery pole. Sometimes this leaf switch is bridged with (typically) 2 diodes in series; these are not polarity protection diodes but are intended to keep the RAM contents of the device backed up by a fraction of the battery voltage when the power supply is plugged into the jack but not into the wall socket. These diodes must be also reversed and soldered into the other line when the polarity is modified. If the device contains a shielding (e.g. aluminiumized cardboard) that is connected with the battery compartment cable to GND, regard that after polarity modification the leaf switch contact must not disconnect it together with the battery pack, thus the shielding has to be connected through a separate cable directly to the GND of the electronics to prevent EM interferences or mains hum in the speaker when using the AC adapter.)

During measurement or test connections directly at the pins of SMD ICs it happens easily that the device suddenly stops working without any visible reason. This happens because by touching them with test leads etc. small metal particles tend to get stuck very easily between the narrow IC pins and make a short circuit. Thus if a device suddenly fails after such a test, don't panic, but disconnect the device from power (remove batteries etc.) and carefully scratch with a very fine screwdriver or needle at the gaps between all of the tested IC pins to remove the shorting particles; usually this will fix the problem. (Never use excessive force here - be very careful not to accidentally cut PCB traces or damage IC pins with the cleaning tool!)

It is generally difficult and time consuming to find a particular pin at large ICs, thus it is very recommended always to use a permanent (or at least wipe proof) felt pen with narrow tip (e.g. a CD labelling pen) to mark interesting connections on the PCB to retrieve them easier. Especially SMD ICs have not only many pins but are mechanically delicate to handle, thus it is also much safer to connect things to given PCB solder joints or traces than to the flimsy IC pins itself, therefore first measure with a continuity tester which trace at the IC goes where, and then use the felt pen to mark the pin numbers and/ or functions of interesting IC pins at connected solder joints for easier access. The felt pen helps also to trace connections between both sides of double sided PCBs, and is therefore one of the most important gadgets to analyse PCB circuits.  (Unwanted or faulty felt pen marks can be later removed with isopropanol and a Q-Tip.)

about COB ("black blob") ICs:

Did you ever wonder why ICs in modern toys and consumer devices are often welded in a black blob of plastic directly to the PCB? It is a false myth that the blob of such COB (chip on board) ICs serves the purpose to prevent commercial espionage by hiding the circuit inside. In reality it is simply cheaper to mount the silicon die of a chip directly to a small PCB (so far it needs no heat sink) than to package it inside a rectangular plastic or ceramic case and then solder its legs to the PCB. The plastic blob protects the die against corrosion and mechanical damage, and the blob is not black to enshroud the interior against spectators but to shield it against light, which would otherwise generate electricity in the silicone (like with solar cells) that prevents proper working.

Particularly with some old COB ICs the blob was made instead from hard plastic resin from soft silicone rubber. Be very careful not to scratch or tear such coatings, because it may easily destroy the chip or its fragile bond wires. Also be generally careful not to burn the blob with a soldering iron. In some old handheld electronic games I even found bare microchips without blob, those were only protected by a hollow hard plastic cap that was bolted to the PCB; by the lack of corrosion protection the life span of such ICs is very questionable. I own an LCD game which has a hole in that cap (production fault) and continuously crashes.

do not ill-treat ICs - chip cancer!:

It is important to regard that an IC that once has been overloaded by short circuits, wrong voltages or static electricity can still starve many days or months later even when it seems to have survived without damage. This is caused by small internal short circuits on the silicon die, those can come into being by the "lightning strikes" of static electricity or local overheating from too high currents. These so-called "hot spots" make the chip draw more current and make its silicon locally overheat again. The local overheat softens the different layers of the chip and thus makes them slowly melt together (electro- migration) , which enlarges the heat producing short circuit area. At the beginning such a chip can still behave normal and only run hotter, but when such an area of "chip cancer" has grown large enough, it will make the chip fail. Very typical for chip cancer is that such a chip still works perfectly for some seconds up to many minutes, but always makes mess after the device has fully heated up. While at the beginning the problem only occurs after a long time under hot conditions, it will later often happen after few seconds when the chip is finally fried "well done".

Chip cancer can not be undone again; only a sufficiently big external heat sink (optionally with fan) and possibly a reduced supply voltage may make such a chip behave normal and possibly stop the progress of the cancer so far the hot spot is cooled down well enough and is not too severe yet. Thus for prevention of chip cancer it is crucial to regard safety measures against static electricity, not to apply wrong voltages (especially no reversed polarity) and better not to short output pins for long time, even when a chip seems to survive the latter without noticeable damage.

Generally when an intact IC runs hot enough to burn your fingers, then this will reduce its life time severely; install a heatsink or other cooling device (like with PC CPUs) when ever possible. With rare or irreplaceable ICs undervolting can help to reduce heat stress and increase their life span. Undervolting can sometimes also help to make damaged or faulty ICs work correctly (see e.g. Penrod AJ-430). But always regard that with most ICs the signal input voltages must not exceed its supply voltages (in positive and negative direction), thus when possible turn down the voltage of the entire hardware section and not only of a single IC, because the latter may stress an intact IC even worse than keeping it on its original voltage. (At least measure whether its signal line currents increase beyond healthy limits when you turn down the supply voltage of only a single IC. If yes, insert each a diode or resistor into the affected signal lines.) In many cases one or a chain of multiple silicon diodes in series can be used to turn down the supply voltage. Undervolting analogue parts can also increase distortion, which can be used as a sound effect. Undervolting digital parts too much can cause system crashes (also see shitshot).

shorting IC outputs:

Outputs of normal ICs (data lines etc.) should NOT be shorted directly to GND nor +Vs nor to other outputs (especially not for longer than few seconds), because despite some ICs are short circuit proof, with many other it will slowly fry the output transistors. (Such internal transistors are only 1/1000 mm small or so, though their overheat typically won't feelable heat up the IC package.) The damage often will not occur immediately but the IC can die MANY months later of slowly growing chip cancer resulting from the previous local overheat. Cutting a PCB trace (and putting a switch into it) is generally much safer than shorting output lines.

If you want to pull an output line down to GND or up to +Vs or anything else, instead cut the trace from it and solder a resistor (typically between 1 kOhm and 22 kOhm) into the line. (Use a little smaller value than the largest that still works. 5 kOhm works well with most digital low power circuits.) Behind the resistor you now can add your switch, pot or whatever to bend the voltage. If a pin is as well input as output line, use 2 resistors and connect your short circuit contact between them to protect the ICs at both ends. (For tests you may short IC outputs directly, but better avoid to short them for longer than 2s.)

IC outputs those are connected by a given external resistor with +Vs are "open collector"; such outputs can usually safely be pulled to GND without additional resistors. When there instead is a given resistor to GND, than it can likely safely be pulled to +Vs. If an IC has multiple supply voltage (e. g. 5 and 12V), only use the lower one to short lines with, so far the circuit doesn't anyway pull it to the higher one during normal operation. Even when an IC has additional negative supply voltage inputs, this does not imply that all its connections are proof to these voltages; therefore never pull other pins of such an IC to negative voltages unless they get also regularly pulled to that voltage in other situations.

Through an average IC pin (e. g. data line) there typically should not flow any higher current than 1 ma for longer, therefore I recommend to measure the AC and DC current while shorting. (Attention: Multimeters may measure very inaccurate at high signal frequencies.)

If you want to make a signal line short circuit DC- controllable, a 4066 IC can be used. A 4066 even permits to (sort of) modulate things which analogue voltages which may be interesting for circuit- bending.

adding touch sensor contacts to ICs:

ICs very easily get destroyed by static electricity, because there input voltages must neither get lower than GND nor higher +Vs while static charges are typically about thousands of volts. If you intend to add sensor contacts to ordinary IC pins, it is therefore crucial to add 2 diodes "wrongways" from GND to the sensor contact and from the contact to +Vs to protect the IC from any voltages those exceed the supply voltage range in positive or negative direction. Also a zener diode of the highest allowed voltage value against GND can be added between sensor and GND for protection. (The zener diode shorts voltages higher than its value to GND and though protects the IC.) The zener diode may be instead also soldered between the +Vs and GND line, which prevents the supply voltage line of the chip to be pulled out of range by static electricity spikes running through  the 2 "wrongways" diodes. But so far there is an electrolytic capacitor between +Vs and GND of the IC, it will normally also protect the IC quite well without this zener diode, because an average static electricity spike contains despite the high voltage not enough energy to charge up the capacitor out of range. (But check if this cap stays also connected with the IC when the power switch is in "off" position; otherwise the protection will be less effective when off and thus the zener diode is recommended.)

A resistor (between 1 kOhm and 10 kOhm) should be soldered in series to each sensor contact to protect the IC against direct (accidental?) shorts when metal parts touch them, and also to limit the maximum body current. To limit the body current, also always add a resistor of at least 1 kOhm to sensor contacts from the "+" voltage line when an input line shall be drawable to "+" or GND, because the player in reality will likely touch "+" and GND simultaneously (even when GND is only a non- sensor metal part) and though make a short circuit.

But generally I can only WARN AGAINST adding sensor contacts to random parts of unknown/ unanalysed IC based circuits because:

Digital circuits spit out a lot of pulsed high frequency crap that can seriously maladjust your body cybernetics. It is scientifically proven how harmful EM radiation can be. It is e.g. well measurable that pulsed microwave radiation of mobile phones alters brain waves within many meters of distance for some hours (reducing mental concentration), that this radiation causes multiple living cells to melt together, that it opens the blood- brain barrier (thus lets dirt from blood circuit into brain) and can crack DNA into pieces by electro- mechanical resonance effects, which kills (e.g. brain) cells and can lead to cancer. Even ordinary 50Hz/ 60Hz mains magnetic fields from a normal household transformer cause chickens in hen eggs bred upon the transformer to die or makes of them crippled freaks. These are fully reproducible laboratory results those are just played down and denied by the electronics industry by banal profit reasons. I therefore also fight against the spread of those brainfryers named mobile phones.

I am researcher of neuronomy and own various mind machines; one of them works by sending weak electric NF frequency pulses via hand electrodes through the body, and there is even a chart in the manual that explains exactly which frequency increases or decreases which neurotransmitter etc. Thus I would be extremely careful with recklessly sending an unknown mess of electric pulses through your body. Instead of body contacts I therefore recommend to rather add potentiometers to the PCB (although it may be mechanically complicated to build things like a real pitchbend wheel with a centring spring). Even DC currents sent through the body produce all kinds of toxic/ corrosive molecules (radicals) and therefore better should be either avoided or at least the current must be kept as low as possible.

(For me (CYBERYOGI =CO= Windler) the electric smog and current of digital sensor touch contacts would be particularly fatal due to they can easily maladjust my skin's nervous system, which proper operation is crucial for my survival because as a yogi I need it for the morphic resonance to keep my data transfer to the network of cosmic consciousness working (in a modem- like way) of which I am terminal. For explanation see Logologie-FAQ.)

I once made a very bad experience with a so-called "Glidepoint" mouse replacement product (nowadays integrated in most laptops) which also uses a pulsed HF capacitance effect to scan the position of the user's finger moving over a small matrix- addressed PCB trace grid. After I had bought my PC which came with this device, every time I started working with the Glidepoint I felt something like very weak electric shocks at my finger's tip. When I worked longer then ca. 30 min, my right arm (which operated the Glidepoint) got a tremor, i.e. it began to shake and involuntarily hop all over the mouse pad while I felt extremely stressed and I lost the control over my autonomous nervous system like in a sort of fever. After I replaced this cruel torment device with a normal mouse I never had these problems again.

But don't worry too much about it; during tests I also often touch for some minutes a PCB to find places to wire my pots and I know too well how many different bizarre sounds can be produced by fumbling around especially on contacts of partly analogue keyboard instruments. Many of these sounds are difficultly reproducible with other means due to the hum caused by adding additional cables and the effect of touching multiple solder joints simultaneously. Possibly the sort of circuit- bending by grasping on the bare PCB can even be well understood as a successor of what the first DJs did when they began to scratch with phono records (despite remembering that granny that always had told: "Don't touch the precious gramophone discs with your smeary, sweaty fingers... ". ;-] In some of my later instruments I indeed have integrated sensor contacts for low DC currents or NF, but I never connect HF signals from a clock oscillator or digital HF pulses directly.

a few words about potentiometers:

When potentiometers are operated with DC voltage (like often necessary for circuit- bending) they tend to make a lot of crackling noise when they haven't been used for some weeks. To fix this, just turn them a few times completely left and right again. (Don't fear about this; even the Minimoog - one of the most famous, beloved and respected analogue synthesizers - had this problem. Regard it not as a defect but rather as a request by your instrument to play it more often. ;-) )

Often a fixed value resistor has to be replaced with a potentiometer, while the old preset value of the resistor shall stay still available. So far only 2 pins of the potentiometer are needed, instead of adding a switch (which tends to fit badly into tiny toys), you can simply take the potentiometer apart and cut the carbon trace at one (e.g. the left) end; this way the pot will behave like non- existent when fully turned to that end. You can now solder the old preset resistor to that "loose" end of the potentiometer and the other end to the wiper pin, and the intact pot end directly or through a small serial resistor to the other end of the preset resistor. This way at the one end the pot will select the old preset resistor value, while when turned, it will freely adjust the value without having the preset resistor in series to it. A potentiometer with cut carbon trace is also useful when a sound shall be altered (e.g. distorted) by the pot, but at one end it shall stay completely unaltered, which often would need a so high ohmed pot that the value would become awkward to adjust.

In certain circuits a logarithmic potentiometer can be better suited to tweak a value (e.g. CPU clock speed) than a linear one, even when for this the turning direction needs to be reversed.

Inside small keyboards and sound toys is often too little space to mount normal potentiometers. But sealed trimmer pots with attached plastic capstan take way less space and are not less robust than normal small pots and even tend to be much cheaper. (Pots tend to be the most expensive parts in many circuit- bend instruments.) The only disadvantage of trimmers is that they have no nut fixture and thus need to be hotglued into place (which works quite well), and that they tend to be not available as logarithmic types. You can also mount knobs with too large diameter (e.g. toothpaste tube lids) using hotglue. When the diameter is way too big, insert a piece of cable insulation as an adapter to prevent the knob from rotating excentrically. When this adapter can still move inside the knob, you can even later readjust the zero position of the knob relative to the pot.

things you shouldn't try to touch: (ouch!)

Before you look for interesting contacts by touching a running PCB with your hands, you must regard the following rules:

things you must not short:

adding LEDs:

If you add LEDs to normal digital IC pins, always use a 1 kOhm resistor in series to it to avoid overloading the output pin. If you find the LEDs too dark in this state, use a "super- bright" (=more efficient) LED or buffer the signal with an additional transistor or a 74LS. . driver IC. This way buffered lines can be loaded with a smaller resistor. LEDs must be equipped with a resistor in series because the current through it otherwise gets way to high as soon its supply voltage ascends only a tiny bit higher than the voltage it needs; even when the LED gets warmer, its "needed" voltage gets lower, therefore a resistor is crucial here to avoid burning out the LED or its driver transistor.

how to distort sounds/ control volume of single- transistor amps:

Sound toys often have a single- transistor "class A" power amplifier. The entire current of such devices typically flows through the loudspeaker, and often even the sound IC gets its supply voltage from there through the base contact of the transistor. By inserting a pot into the base line, the sound not only gets quieter, but often also gets distorted at low volume because the DC voltage component at the base drops too. As a volume control, wire the base with the pot's wiper pin, and the sound output from the IC with the clockwise pin. Wire an electrolytic cap of about 10 µF against with the anti- clockwise pin to prevent distortion. Pulling the base voltage through another pot to GND (recommended with NPN transistor) or +Vs (recommended with PNP transistor) will change the amount of distortion, but important is here especially with a current increasing pots (i.e. against +Vs with NPN or against GND with PNP transistor) to put always a resistor (ca. 200 Ohm.. 5 kOhm?) into the line from the pot to the base because a too high base current (positive with NPN or negative with PNP transistor) would overheat the transistor and speaker. Putting a capacitor into the sound output line of the IC and bridging this cap with a 2nd pot makes it possible to control the distortion amount in a different and very safe way. So far an IC stops working after cutting its sound output line to the base, then this IC needs a resistor (ca. 200 Ohm.. 10 kOhm) to +Vs or GND (depending on the kind of transistor (NPN or PNP)) to get its supply voltage. Also the cap at the anti- clockwise pin of the volume pot can be connected to GND through another pot to control the distortion amount differently. Usually all distortion methods sound a little different, and also the capacitor and potentiometer values influence the sound. The correct polarity for the electrolytic capacitors can be measured - the IC won't reverse it during operation. (The principles of such simple transistor amplifiers can be read in many electronics books.)

In devices without power switch it is important to check that the modified circuit must not draw too much current in standby mode ( <1 mA is a good value, measure with all pot settings) to prevent excessive battery consumption. If it draws more current than 1 mA, add a power switch.

To add a sound output jack, connect a 100 nF capacitor (or similar) to the connection between transistor and loudspeaker and the cap's other end to the center pin of a cinch jack. Ground the jack's outer ring. To add a speaker mute switch it is necessary to switch the now open transistor output instead against a 2W resistor of the same or a slightly higher resistance as a dummy load, because the circuit normally gets its current through the speaker which therefore needs to be replaced by the resistor when muted.

Hint: The distortion is a typical key element in the sound appearance of most multi-voiced squarewave musics; without distortion it sounds quite boring.

In rare cases (see e.g. Bontempi B50 keyboard) there are also digital single- transistor power amplifiers those get a high frequency bit stream of on/ off pulses by the IC those gets formed into audio signals by an electrolytic cap behind the transistor. Such badly distorting beasts can make a lot of tinitus, awful earbleeding sounds and may destroy your hifi tweeter when modified incorrectly. To replace the internal amplifier it is therefore necessary to connect a resistor with a capacitor to GND to the IC output line to turn the bit stream into something analogue before processing it further.

how to change pitch/ mess up the program:

Sound toy ICs often control their clock speed by an external resistor; modifying its value changes the sound pitch and a too high frequency often crashes the program and though makes wild sound mess. The resistor can be found by touching it by hand; touching the right one will change the sound pitch/ speed (like turning a phono record faster/ slower) . When replacing it by a pot, a safety resistor should be placed into the line to the IC to prevent overload by a too low resistance. The value of the resistor should be chosen in a way that the current to the IC does not exceed 1 mA. Sometimes putting GND or +Vs at the other end of the pot enlarges the accessible frequency range. When the pot is turned to far, typically in both directions no sounds get produced anymore due to stopping the clock completely or crashing the program. To make use of the entire motion range of the pot therefore additional resistors or trimmer pots can be added to limit the effective resistance range to sensible values and though make the instrument more playable. Also coarse and fine value pots can be wired in series to improve frequency control.

Attention: While in some toys at the clock resistor there is simply a DC control voltage, in other toys there can be up to many MHz HF on this line those can be unhealthy when touched for longer and those may mess up radio and TV reception in the neighbourhood when connected to unshielded cables. With these toys I can't recommend at all to use touch sensor contacts to implement a pitchbend function. When such an IC's frequency resistor sits between 2 IC contacts (and not one against GND or +Vs), 2 separately shielded cables (each shielding to GND) should be used to connect the pot, because otherwise the pitch can tend to howl without touching anything by the capacitance between both leads.

When an IC has a resistor controlled clock frequency (that varies when touching the resistor), you can find out without oscilloscope if it is DC or HF controlled by grounding the resistor pins through a small capacitor (a few pF or nF). When the device still plays sounds of correct pitch during this, then it is DC controlled. (Possibly you need to switch off and on the power after connecting the capacitor, because the pulse of its previous charge voltage may crash the CPU during the 1st contact.) Only DC controlled clock inputs can be safely connected with sensor touch contacts for pitchbend. It is recommended to verify by oscilloscope that there are really no HF signals on these lines before adding the sensors; also connecting a tiny capacitor (some pF?) against ground permanently will further reduce possible HF remains, but a too large one will make the pitchbend respond too slow and might hinder the CPU from proper resetting.

CPUs with DC controlled clock oscillator often have the clock resistor connected between the clock input pin and a dedicated output pin instead of the CPU supply voltage. This output pin usually outputs a specially stabilized positive reference voltage that prevents the pitch from howling when the battery supply voltage drops by e.g. playing loud sounds through the speaker. Thus when you replace in such hardware the clock resistor with potentiometers, connect their  positive line to the stabilized output instead of +Vs to avoid howling. Watch out not to confuse the clock input with the reference voltage output when you wire potentiometers, since shorting this output (e.g. through the wiper) against GND or +Vs may destroy the CPU by overload even when the pitch control first seems to work. Connecting the reference voltage output to a touch sensor contact is not recommended (it would reduce the accessible pitch range and need an additional diode pair for static electricity protection) since howling doesn't disturb here. Better connect the the positive sensor contact through a resistor (about 1kOhm) with the positive CPU supply voltage.

In devices with a crystal oscillator the crystal can sometimes be replaced by a coil and a small capacitor (usually some pF to nF) to make the frequency adjustable. To adjust it, an adjustable ferrite core coil or rotary capacitor can be used (but the adjustable frequency range is typically way smaller than the one achievable with devices those use a resistor to set the clock speed). To tune a crystal clocked instrument a little down, it is often sufficient to wire only a trimmer capacitor parallel to the crystal instead of replacing it. Because all these parts are exposed to HF frequency, they should neither be touched for longer, nor be connected with unshielded cables to minimize HF radiation. It is also possible to replace the crystal with a voltage controllable HF oscillator circuit to make it safely controllable through potentiometers or touch sensor contacts or external inputs within a much wider frequency range, but adding such an oscillator can be complex and fairly expensive task.

Theoretically overclocking an IC too much can damage it by overheat, but practically in toys and low- end music keyboards this problem is unlikely to appear due to the anyway very low power consumption of these ICs. (So long an IC doesn't feel unpleasantly hot, there should be no risk of damage. If it indeed gets hot, mount a cooler or don't overclock it that far.) But other people claim that they indeed toasted chips by overclocking, thus be careful especially not to operate the device with its crystal or clock coil/ capacitor completely disconnected, because this can drive it into a far too high frequency. (I once toasted a fluorescent tube transformer in a small LCD monitor this way.) Resistor controlled clock oscillators usually rather stop their clock with resistor disconnected, than pushing the frequency dangerously high.

But very few IC types may also overheat themselves or damage other things (e.g. by sending DC through their speaker) when the clock frequency gets completely stopped. If you want to underclock such ICs very much, just either avoid to make it possible to turn the clock that low (e. g. by a resistor in one of the pot lines) or when the IC itself gets hot place a resistor into its supply voltage line. Also LCD displays tend to get damaged by a completely stopped clock, and also single- transistor power amplifiers might overheat themselves or the speaker by a continuous DC current drawn through it when the clock is stopped. Especially tiny plastic toy speakers with flimsy plastic diaphragm melt quickly when overloaded. Against these problems many solutions are possible; e. g. the amp could be modified to get the AC sound signal from the IC through a capacitor to stop DC components, or simply a pocket torch light bulb can be placed in the speaker line; the bulb will light up and though increase its resistance when too much current flows for a certain time, and this will reduce the current flow. A bulb with a higher power consumption has less resistance and though would let more energy pass to the speaker and shut off the current slowlier. But the bulb might also cause the sound to howl (which can be desired or not) . Instead of the bulb also a resistor bridged with an electrolytic cap can be used, but this reduces also the maximum volume.

A well working clock frequency potentiometer can be used as a pitchbend effect and it can also make a lot of similar sound effects like known from record scratching, with the difference that a knob can be turned much faster than a turntable may accelerate. In many toys the addition of a resistor or light bulb into the IC's supply voltage provides a howling pitch envelope which may be useful as an effect.

hard reset & shitshot:

To quickly reset a crashed toy IC, simply add an "opener" button switch (one that lets current flow as long it is not pressed) into the positive supply voltage of it. Such a button is also good to stop/ cut long sound samples from it when the toy itself has no button for this function. In some devices this kind of button can also be used as a "shitshot" function when pressed very short to shit data mess into the RAM and though crash the program in more or less predictable ways. If the shitshot effect is not desired, an electrolytic cap behind the switch (at the IC's +Vs input) can sometimes prevent this behaviour. In some devices such a button causes a disturbing pop in the loudspeaker when pressed; by bypassing the internal amp/ speaker supply voltage (i.e. connecting only the digital ICs supply with the switch but not the amp) the pop noise can often be avoided.

A more precisely controllable kind of shitshot can be achieved by inserting an additional potentiometer of a few kOhm into the supply voltage line of the IC, because this way the supply current can be turned down slowly until the program freaks out. By bridging that pot with a switch, the instrument can be switched back and forward between normal and shitshot operation, which in combination with the reset button permits to control and explore the crash sound behaviour better. (Whether this feature has benefits depends much on the hacked hardware.)

how keyboard matrices works and how to find eastereggs:

Keyboards with many buttons/ keys would need a lot of pins at the IC when each switch would be wired to an own IC pin and pull it to GND or +Vs. Due to a high pin count makes ICs and PCBs expensive, typically keys in digital circuits are connected as groups in a way that multiple keys can share the same IC input lines by the trick that the IC selects the actually active group by setting only its output line high while the rest remains low (or vice versa) in a way that at any time only one group gets current to influence the input lines when its keys are pressed. Though the logical keyboard wiring can be thought of a matrix with rows (the key groups) connected to the IC's output lines while the columns are connected to the input lines; at each of the crossing points can be a key switch. Due to the IC knows which output line (key group) is active, it can determine which keys are pressed by the voltages at its input lines. By periodically activating all rows one after the other all actual key presses can be sensed this way.

In old home keyboard instruments and other devices there are often combinations of input and output lines at those no key/ button exists despite that the ICs were designed to support functions activateable by a switch soldered there. The reason for this is often that the company intentionally left these buttons away to reserve them for more expensive models. Another reason can be that such functions turned out to be not working correctly due to an IC design flaw, or because they only make sense in combination with other components those are not present in this device (like e. g. a stereo effect section in a mono music keyboard, or connections for a longer piano keyboard than the one of the actual model). Such hidden IC features are called "eastereggs". A keyboard matrix easteregg function that the manufacturer never has activated in any commercially released instruments (and thus yet was never heard before by other people than its developers) is called a "golden egg". To find eastereggs, simply imitate what the keyboard does by shorting rows with columns and write down what effect each combination has. (Also here I recommend a resistor in your test cable to prevent to accidentally short +Vs with GND or similar.) Afterwards you can wire buttons to desired keyboard eastereggs you found.

In polyphonic keyboard instruments with more than 2 voices each key switch is normally wired with a diode in series, because when more than 2 keys/ buttons are pressed simultaneously, otherwise they could short key groups with each other which would make it impossible for the IC to determine to which groups the actually pressed keys belong. To avoid disturbing the polyphony it is therefore recommended to also use diodes in series to your own test cables and buttons when the instrument contains such diodes (there are typically many in a row). Besides the simple row- output/ column- input principle there are also keyboard matrices which rows and columns are treated by the IC by turns as inputs and outputs. This (seldomly used) keyboard scanning scheme permits to connect more keys to the same IC pin count because now 2 switches can be sensed between each 2 lines by using diodes in different directions. For exploring such a keyboard matrix this simply means to turn your test cable with the diode around after testing all combinations and then test again, because here switches can be placed in both diode directions.

Especially in cheap toys also key matrices those connect multiple lines per key exist (e.g. Xin Anda - 8-Melody Letter Study Piano). But typically a keyboard matrix uses separate input and output lines at the IC. The input lines can be found by connecting IC pins through a resistor with either GND or +Vs; with input lines one of these voltages will make the instrument behave like if many keys or buttons would be pressed simultaneously. The output lines can be often identified by touching IC pins with a test lead connected through a high ohmed resistor (e.g. 22 kOhm) with the input of the instrument's own sound amplifier (i.e. a connection that produces mains hum in the speaker when touched with bare hands); with matrix output lines this will typically produce a characteristic buzzing or tooting continuous tone, which sounds normally similar among all matrix outputs of the instrument, but depending on the used hardware individual matrix output signals can also sound differently. It is very recommended to use a permanent felt pen to mark the input and output line sections on the PCB itself to ease further analysis. (Do not write down only the IC pin numbers on paper; it would be terribly confusing to retrieve them.) Then test all input/ output combinations with a test lead + diode (and safety resistor) and write down a table with all found functions.

In some devices the keyboard matrix also controls LEDs, those are connected with each 2 output lines and light up when both lines output different voltage (one high, the other low) with the polarity of the LED, and because during the poll of the keyboard matrix this situation always happens for a fraction of the time, such LEDs tend to always glow dim even when intended to be off, so long the device polls the keyboard matrix. To light the LEDs, the device changes the matrix poll timing in a way that the time phase during that the selected LED lights up is prolonged in relation to the others (thus technically the LEDs are always flickering, which is more or less visible depending on the poll frequency). In some devices LEDs are also placed between input and output lines instead, and the device switches the input lines as outputs in an odd way between the keyboard poll cycles to light up the LEDs. In such keyboard matrices it is difficult to distinguish input and output lines with the sound amplifier method (and even by oscilloscope), because by the LEDs there is also a buzzing or tooting signal on the input lines. Also closed locking switches in the keyboard matrix can make inputs appear like outputs, which can be confusing. Particularly in old Casio keyboards such switches are sometimes even wired through a diode between 2 "output" lines, those apparently alternatingly become inputs to poll the state of the switch. In some devices (e.g. certain old Casio keyboards) diodes are soldered directly between particular input and output lines (behaving like an always closed switch) to set the CPU always into a certain fixed mode, or to permanently activate the function of an omitted switch (to explore what they do, temporary disconnect the diode).

When a keyboard matrix contains an entire group of  many (e.g. 8) omitted control panel buttons as eastereggs, it is often easier than adding that many new OBS buttons to rewire a given button group that already shares the same row lines like that group by adding a mode switch that exchanges the given button group's column line with the column of the new easteregg group. (See e.g. Bontempi BS 2010. Rows and columns stand here for matrix inputs and outputs or vice versa, depending on the actual circuit.) Some instruments also expect additional multi- contact slide switches instead of simple buttons to trigger an easteregg function, which can be mechanically difficult to implement as an upgrade. But especially with later Casio keyboards the last selected matrix place (i.e. closed contact) of such sliders is memorized by the chip when all of its contacts open again, thus the contacts don't need to stay permanently closed and thus simply can be treated like a button group. Also here the trick with the additional mode switch can be used to assign the slider functions either to a given button group or even to a given slider that shares the same rows. To modify a given slider, disconnect its original column line and connect the original and the new column(s) through each [a diode in series to a button switch] with the column line of the slider. (See e.g. Casio MT-540.) This way the slider position is only sensed when you press the button of the desired (original or new) function, and thus you can independently set the value of multiple functions with the same slider (much like the number entry dial on many digital synthesizers).

If you want to make the effect of a matrix key press (= connecting 2 data lines) DC- controllable, a 4066 IC can be used. When polarities and inputs/ outputs are exactly known, also a correctly wired AND or OR gate (74LS...) can be used instead. (For details see Yamaha SHS-10.)

Very modern polyphonic keyboard instruments sometimes have no keyboard matrix at all but instead one IC pin per key; to look for keyboard eastereggs means here simply to look for unused IC pins.

fixing keyboard matrix flaws:

When IC input lines of keyboard matrices are too high ohmed, it can happen that they pick up EM signal mess from other data lines or hold their previous voltage level too long by capacitance effects and thus fail to work properly. This e.g. causes polyphony bugs with certain music keyboards those play additional erroneous "ghost notes" when multiple keys are pressed simultaneously, because the IC confuses the mess with additional key presses. Such problems particularly occur with empty batteries or when the hardware is overclocked by a modification, but also certain poorly designed toy keyboards have this flaw. To solve this problem, simply solder a row of pull-up resistors (correct value may vary, but 22 kOhm usually works) to all matrix input lines and connect their other ends with that voltage that the input lines shall have when no key is pressed (i.e. either GND or +Vs of the IC).

Other poorly designed toy keyboards produce ghost notes when more than 2 keys are held down because although their CPU can play more than 2 note polyphony, the necessary key matrix diodes were omitted to save costs (1 diode per key, although diodes would cost only few cents). To fix this flaw, each key must be upgraded with 1 diode in series, which can be a lot of flimsy solder work.

I also found a toy keyboard which refused to sense certain 2 key combinations because a key matrix output drove an LED without resistor in series, which simply overloaded the output causing a too high voltage drop; to fix this, the missing LED resistor needs to be added. With other toy keyboards certain 2 key combinations failed because the input(?) lines were too low ohmed and thus caused a too high voltage drop on the output lines when 2 keys on the same output(?) line were pressed. Soldering a diode into each input(?) line fixed this.

velocity & pressure sensitive keys:

Keyboards with velocity sensitive keys normally contain 2 contacts per key and simply measure the time between the key press signal from the earlier (upper) and the later closing (lower) key contact to determine the volume of the played note. Because this hardware measures only a duration between 2 events instead of the actual key position, it is not capable to sense the key pressure of a held key, which would need genuine analogue (e.g. resistance) measurement instead of simple digital timing values. That's why pressure sensitive keyboards are still very uncommon and usually much more expensive than velocity sensitive ones despite pressure sensitivity can improve the intuitive expressivity of held notes dramatically. (But I am sure that with actual DSP hardware it should indeed be possible to design a cheap pressure sensitive MIDI keyboard as a mass product. Unfortunately yet nobody dared to do so.)

Annoying is also that classic electronic keyboards with mini or midsize keys (like Yamaha PortaSound) are generally not velocity sensitive. Don't ask me why, but I guess the manufacturers itself considered them rather as beginners toys than serious instruments, despite smaller keys permit a lot of special play techniques those are impossible with the establishment piano's fullsize ones. Thus if you want pressure sensitive small keys, there are only very few modern keyboards available (e.g. the MicroKorg synth) those tend to be expensive.

signal feedback loops:

Toys often have LEDs or light bulbs those flash in the rhythm of played sounds. By creating a feedback line from such lamp outputs through a diode to key inputs it is possible to automatically re- trigger a sound in a loop again and again. By inserting potentiometers and capacitors etc. into this feedback line, the timing (operating point) of this feedback can be made adjustable to produce drum loops of varying lengths and similar effects. It may also be possible to feedback the sound output itself into key inputs, but I didn't do this yet. It is important not to exceed the input voltage range; use resistors (voltage dividers) to reduce too high amplitudes and use diodes to filter out wrong polarity voltage spikes. To simulate a particular key press in a key matrix, also here 4066 or AND/ OR logics ICs can be added. Also the supply voltage line of an IC can be made be modulateable through resistors after a shitshot potentiometer has been inserted there.

build your own FM synthesizer:

Most old FM home keyboards with a separate soundchip can be ridiculously easy converted into a real FM synthesizer. Simply solder a DIP switch block into the (usually 8) data lines between CPU and FM soundchip. Solder at the soundchip side a pull-up resistor (each about 22 kOhm) to each data line and connect their ends to a single alternating switch ("0/1 switch") that pulls the resistors against GND or against +Vs. To use the instrument as normal, switch all DIP switches "on". (The pull-up resistors are necessary because the open data input lines would otherwise pickup EM noise and thus cause random results. The setting of the 0/1 switch is an additional mean to modify the sound by setting the bits selected by the DIP switch to either 0 or 1.)

To create a new sound, simply select a preset sound, set the 0/1 switch, switch some DIP switches off, select a different preset sound (possibly multiple in sequence) and switch them on again. (This procedure writes a mutilated version of the 2nd selected preset sound into the internal FM synthesis registers of the sound chip, while some bits from the 1st preset sounds stay in the registers.) Regarding the effort, the variety of new sounds you can get is really outstanding. Depending on the instrument, the behaviour can be even predictable enough to write down your own sound library by making notes of the switch settings and preset combinations you use. You can also get all kinds of bizarre experimental crash sounds out of it by playing on the instrument or enabling rhythm etc. with some DIP switches turned off, which will produce rather random results. This easy modification can be likely added to any old Yamaha FM home keyboard and it provides you a universe of new exciting sounds to explore for only about 3€. (I have done this e.g. with my Fujitone 6A. For details see here.)

(A similar DIP switch block assembly can be also inserted e.g. into electronic speech toys (e.g. from Texas Instruments) between CPU and speech synthesizer chip or between ROM and CPU to make all kind of bizarre crash noises. If you want to quickly access the switches to perform music on them in realtime, also a row of normal switches can be used, but the benefit of DIP switches is that they easily fit even into tiny electronic devices.)

speech synthesizer toys:

A very widespread class of circuit- bending candidates are old electronic speech toys those contain an LPC speech synthesizer chip with separate ROM. These chips were originally invented by Texas Instruments in late 1970th as a mean to reduce the memory demand of speech samples by magnitudes to make them fit into a tiny ROM. But in fact they are basically nothing else than a monophonic virtual analogue music synthesizer with a real, very advanced programmable resonance filter and complex envelope control. Despite sound output is only 8 bit and restricted to a rather low sample rate (about 10 kHz), they can technically create lots of other sounds far beyond speech. Toys of the Texas Instruments "Speak'n'..." hardware class (e.g. Speak'n'Spell or Touch'n'Tell) contain the first generation of this chip. These old toys had separate CPU, sound chip and ROM, and it is very easy to mess up the program to produce lots of partly very bizarre noises by connecting data lines with resistors, putting switches into PCB traces etc. Later toys had higher sound quality but were often integrated into a single chip, which limits systematic influences on the program, thus only simple shitshot controls can be added here (those mess up clock speed or reduce supply voltage), which produce mostly random results.

When there is an expansion cartridge port in a speech toy, then a DIP switch block can be soldered into the data lines between device and cartridge for better controllable results. Even if you have no cartridge, you can still make a dummy cartridge that connects data lines through resistors (and possibly other components in series) with each other by switches. This kind of modification works e.g. with most old talking toy laptops. (Regard that a deep crash could theoretically damage the LCD display by DC operation, but at least the toy laptops I yet examined have a separately clocked display controller IC that keeps updating the screen even during a deep crash of the rest.) Speech toys for small children are most interesting because they often already include unique synthesized sound effects like animal sounds etc., those have a typical gritty timbre (similar like coarse granular synthesis). Unfortunately newer speech toys (later than 1995) tend to be less suitable for such modifications, because at least ones with a small vocabulary nowadays mostly lack a speech synthesizer but just play back samples from an internal ROM. The more robotic and gritty the voice of such a toy sounds, the less likely it plays just samples. Also certain typical synthetic sound effects like "boing" or "ääheonng" or animal voices those sound like sung by a human voice hint that a toy contains a speech synthesizer chip.

It would be interesting to connect a PC with the cartridge port of such a toy to examine the sound capabilities in detail and possibly program own sounds for the speech chip or even emulate it as software on PC. The MAME arcade game emulator and Visual PinMAME pinball emulator for PC already contain simulations of similar speech chips (released as GNU open source software written in C++), thus programmers can find here likely many useful routines for such a project. I think that making a ROM cartridge that transforms a Speak'n'Spell or toy laptop into a real programmable synthesizer would be certainly interesting, because normal circuit bending with these devices tends to produce mainly random or semi- random results instead of behaving like a conventional musical instrument.

Speech toys with OBS alphabet letter buttons those each say the English name of a letter can be also nicely abused for tekkno to form sentences (like "I M A DJ"), because most letters also sound like English words. When the buttons can immediately retrigger their sound (without waiting until the chip has stopped talking), they can be played like drumpads for sound effects.

hardware class identification:

Multiple electronic instruments are in the same hardware class when they contain identical digital hardware (including internal ROMs) that controls their sound and behaviour. Multiple instruments are in the same hardware family when although their hardware or internal ROM software partly differs, they still sound and behave very similar and employ the same sound generation technology.

E.g. Casio and Yamaha have released over multiple years many of their music keyboards with a different case but electronically identical hardware inside. While some variants look almost identical, others are very different and even may lack some features or have a shorter keyboard than others. The rarity, used price and collectors value of different variants can vary a lot despite the sound is basically the same. For finding or avoiding to buy versions with the same sound, as well as for easteregg searching it is therefore important to identify to which hardware class an instrument belongs.

If present, the best identification mark of the hardware class are definitely demo melodies (with their exact arrangement), because these were usually changed rapidly with every new generation and thus are unique to them. Besides this, the rhythm and preset sound names are an important hint, but it is to regard that with OBS presets some can be missing in cheaper versions. Even worse than with Casio and Yamaha is the situation with brandless no-name manufacturers those sold their hardware to many plastic case manufacturers, because this way technically identical instruments can look totally different and even the same sound and rhythm presets can have varying names in different case variants, and a similar case can even contain different hardware. E.g. the "MC-3" (originally designed by Medeli?) exists in so many case variants that nobody really knows how many exist, and with toy keyboards (like My Music Center) the situation is even worse, because there are even many chip variants around of those some have technical flaws those in others are corrected. The demo melodies are here the most promising method to distinguish them. Unfortunately through eBay no demo tunes can be tried out, and typically not even all preset sound names are mentioned there, thus asking the vendor is the only chance to identify them.

Once you have bought an instrument, you can normally identify the real hardware class by the special ICs, those printed label normally includes a fixed type number (the "name" of the IC) that does not change among case variants. Other (usually smaller) characters can be serial numbers and thus vary among specimen. But regard that  sometimes digital ICs have behind their obvious (major) type number in the same row some (typically 3) additional digits separated by blanks or a minus sign; these appended ciphers or characters are not meaningless serial numbers but often (especially with Casio keyboards) are part of the type number to indicate the version of the software program inside the internal ROM that controls the IC's entire behaviour. Thus ICs with the same major type number but different software number are definitely not the same, but they share many technical similarities like the general pinout, electrical properties, clock rate or sound chip polyphony, which can help a lot to compare and analyze their function. But with RAMs appended ciphers normally simply indicate the speed of the RAM, thus they can vary among instrument specimen of the same hardware class (so long the RAM is fast enough for its purpose). Often the type or function of components is also printed on the PCB next to them. Unfortunately Casio gives ICs here only cryptic abbreviations those don't really help to identify their function (e.g. any complex digital ICs begin always with "µP", no matter if they are a single chip CPU, a separate sound chip or only a key matrix decoder), but at least they help to find out which numbers on the IC are the important ones to distinguish them from serial numbers.

By the way, did you ever wonder why no- name companies still sell new sound toys or cheap electronic instruments those sound like when their hardware would be at least 10 or 20 years old? Although sometimes NOS ware or electronic components of that time are indeed forgotten in stock or cargo containers for so long, this is normally not the main reason. True is that at the one hand microchips with less functions take less space, thus can be produced on a smaller silicon die which makes them cheaper. But at the other hand this cost saving measure is also limited by the necessary pin count, because the bond wire connections also need a certain space on the silicon, which surpasses the space for the electronics on very simple chips and thus it makes economically no sense to reduce the function count even further. A modern Pentium CPU e.g. contains millions of transistors while old chips had on the same silicon area only a few thousand; thus to produce chips with 15 year old circuit design in modern, costly chip factories is very uneconomical because with modern integration density the bond wires would need much more space than the electronics. My theory is therefore that the main reason for nowadays production of technically outdated microchips is that the machines of the same way outdated 1980th chip factories were not scrapped, but instead sold to 3rd world countries where they are still regularly used to cheaply produce new microchips with only the same low integration density that was possible at the time these machines were built. Otherwise it makes no sense why e.g. melody greeting cards still employ simple monophonic squarewave sound, although with modern technology on the same chip area long sound samples or at least complex polyphonic digital synthesizer hardware could be integrated for no additional cost.

case date stamps:

To find out when an instrument was made, it can help to look for copyright dates in the manual, on the box printing or on ICs containing ROM software. But many keyboards also have their manufacturing date marked as numbers or paper stickers on the PCB or embossed into the inside of the plastic case. 

In plastic cases beside normal date stamps there are often number tables with dots in rows and columns. Here you see the most common type of this table that was used in later Casio keyboards (here my Casio SA-35). But also odd concentrical patterns exist, those resemble the hands of one or multiple clock faces and can be badly confusing.

With Casio keyboards each table row stands for a year and each of the 12 columns for a month, but every table contains a lot of dots instead of only one. I first thought that these indicate the manufacturing and assembly dates of multiple components inside that particular keyboard specimen, but this makes not much sense because they are often spread among multiple years. Thus my theory how the dots come into being is the following: The manufacturer engraves a new dot into the tooling mould on the first day of every new month in that the mould is used to cast plastic cases. This way the last dot in a table indicates the month of the year in that this particular keyboard case was produced, while the others mark all previous months in those keyboard cases were made by the same mould. Thus the first dot indicates when the mould was used first time, which is often identical with the time when the keyboard model went into production.

However because tooling moulds are extremely expensive devices, they tend to be re-used later (and sometimes modified in between) for manufacturing the cases of different keyboard models in the same mould, which makes the situation a bit ambiguous. Thus when there are multiple groups of dots separated by long pauses (e.g. a year) in the table, and the keyboard model had predecessors with the same outer case shape, then there is a high change that the earlier groups stem from those predecessors due to the mould was re-used. Also fine visible outline rims of non- existing openings (e.g. for additional controls, jacks or battery compartments) on the inner or outer case surface hint that it was casted in a re-used tooling mould from a predecessor that made use of them. But also the opposite is possible, namely that the expensive mould was designed from begin on for re-usability and therefore contains modular parts for changeable case holes, those leave the additional fine outlines when not in use. Such exchangeable mould parts can basically even include individual date stamping units, those theoretically may leave multiple differing date stamps on the same plastic case part when the mould consists of older and newer modules. This method can look confusing and is rarely used, but technically simply the newest date mark is valid to date the particular plastic part.

keyboard name prefixes & manufacturers:

Keyboard model numbers often consist of a few letters followed by a number. With some brands they have a systematic meaning, with others they seem to be rather randomly chosen. Usually (but not always) higher numbers within a series stand for more expensive models. This are the prefixes of some common brands. (There may be others especially with professional stuff that I don't care much about.)


Antonelli made mainly fullsize home organs and released only few different home keyboard models, but these always were somewhat different than other brands and often had unusual accompaniment features. Most of their analogue keyboards are rare; according to their only 3 or 4 digit short serial numbers of most models only a few hundred or thousand specimen were made. Other models have here 2 short numbers separated by a "/", those may indicate production week (or month) and number of the specimen in that week (or month). The sheet metal model plates at their case bottom look exactly like with Siel keyboards, and also ICs and PCB stamps in some Antonelli instruments are labelled "Siel", thus Siel was likely a different brand label of the same company to market their more professional instruments. E.g. the Antonelli 2381 was also released as Siel MK 370, and the Antonelli 2614 as Wersi X1000, thus Siel and Wersi organ repair companies may also help to retrieve spare parts for the rare Antonelli stuff. So far I know, Antonelli and Bontempi later joined with Farfisa.

caution: A common disease of Antonelli keyboards is that the plasticizer of PVC mains cables melts itself into their grey case plastic, which looks like scratches by a hot soldering iron and locally turns the plastic surface and paint into tar- like goo. Thus watch out to avoid direct contact between plastic case and any sticky or smeary feeling PVC cables. (Better generally avoid contact with soft PVC; also certain keyboard dust covers and -bags were made of the plasticized material.) Especially the fixed mains cables of first generation Antonelli keyboards (with bulky case) seem to contain particularly aggressive plasticizers. The over 20 year old cable of my Syntorgan 2445 still feels softer than many brand new cables (and slightly smeary), while its silver grey case was disfigured by a zillion of black burn marks, those linear pattern indicate that they obviously were caused by the cable. The smeary goo residues on the case can be partly removed with vegetable oil, but be careful to wash off also the oil residues with water and dish washing detergent, since long oil contact may damage this vulnerable plastic also.

Antonelli used for most instrument models only plain numbers instead of letter combinations.


With Bontempi (see links) the prefix scheme of old keyboards is very messy and especially the same keyboards were released with very different prefixes. Specific for older Bontempi instruments is also that on the model plate the ciphers behind the dot indicate the case style (i.e. e.g. a. Bontempi HF222.21 and HF222.22 differ only in the case colour).
BK = "Basic Keyboard" old mini keys
HF = "High Fuga" old fullsize keys
HP, KF, KP = home organ
M, MS = old fullsize keys, squarewave without rhythm
HIT = fullsize toy- like keyboard
HT = ?
BN, PK = midsize chord organ
MR = midsize with LCD (only MR52 Special known)
B, X, HB, ES, MRS = almost everything without logical scheme
BS = "Bontempi Synth" midsize toy synth
KE = midsize toy keyboard/ synth
ET = old System5 mini keys (only ET-202 known)
KM = "KidsMusic" old System5 midsize keys (only KM40 known)
KS = old System5 midsize or fullsize keys
BT = old System5 midsize or fullsize keys
RX = fullsize e-piano
AZ = old fullsize MIDI keyboard
KT = System5+ midsize keys (only KT-32 known)
GT = cheap System5+ keyboard (no MIDI)
PM = "ProfiMusic" System5+ fullsize keys with General MIDI
NK = fullsize keys with General MIDI
SK = modern toy keyboard


Casio (see links) has a little messy scheme; especially some CT-# keyboards were later released as CTK-#, some SA-# as M-# and various toy keyboards had own names. With old keyboards a by 1 higher number often stands simply for a different case colour variant (e.g. brown instead of white).

old keyboards:
CT- = "Casiotone" fullsize keys (the first keyboards were named "Casiotone #" instead of CT-#")
MT- = midsize keys
PT- = "Petite Keyboard"(?) mini keys
VL- = "VL-Tone" (named after "Very Large Scale Integration" ICs) early mini keyboards
EP- = toy keyboard
SK- = sampling keyboard
CK-, KX- = keyboard with built-in radio and/ or cassette recorder
DM- = midsize dual manual (only DM-100 known)
AZ = guitar shaped keyboard
CZ-, VZ- = phase distortion synthesizers (improved FM)
HT-, HZ- = "spectrum dynamics synthesis" semi- analogue synthesizers
FZ- = professional sampler (contain also phase distortion synthesis)
RZ- = drum computer
CPS = e-piano (velocity sensitive fullsize keys)
CSM- = MIDI tone generator module
DG-, PG- = synth guitar
DH- = "Digital Horn" MIDI saxophone

newer (sample based) "ToneBank" keyboards:
CTK- = "Casiotone keyboard"(?) expensive fullsize keyboard
CA- = cheap fullsize keyboard (no MIDI, only mono?)
MA- = midsize keys
SA- = small keyboard (up to 37 mini or midsize keys)
M- = "Casio Club" like SA- series, but M-10 is much older without samples
KA-, PA- = toy keyboard
RAP- = "Rapman" DJ toy instrument
TA- = toy keyboard with cassette player (only TA-10 known, TA-1 was a data tape storage cartridge)
KT- = keyboard with built-in radio, cassette recorder and/ or CD player
LK- = fullsize key lighting keyboard
ML- = key lighting mini keyboard (or 1980th melody calculator)
VA- = "Voice Arranger" midsize effect keyboard (only VA-10 known)
GZ- = MIDI master keyboard
PMP- = velocity sensitive fullsize keys
PS-, PX- = e-piano (velocity sensitive fullsize keys)
AL-, PL- = key lighting e-piano (velocity sensitive fullsize keys)
AP- = heavy wooden e-piano (velocity sensitive fullsize keys)
WK- = "workstation keyboard"(?) velocity sensitive fullsize MIDI keyboard
MZ- = professional fullsize MIDI workstation keyboard
LD = e-drumkit

With fullsize keyboards there may be also some other prefixes but I don't care much about them. Also some other exotic names may exist. A general rule of thumb is that Casio instruments with names ending on "-1" (like VL-1, SK-1 etc.) are usually good ones (except perhaps PT-1) and particularly those ending on "Z-1" are great. Apparently all old Casio instruments (before CTK- series) with an "8" in their type number had a ROM-Pack slot and key lighting. The only known exception is the keyboard CT-8000, which was part of the ultra- rare modular stage organ Symphonytron 8000.

Jin Xin Toys

This Chinese company made the most bizarre whacky toy tablehooters straight after Yongmei. Typical for them is especially that they blatantly imitated case design or sound samples from other known keyboard brands, and that many of their products had hardware bugs and absurd funky model names full of Engrish misspellings (often containing the word "luxury", see JX-20165 for more info). The case plastic seems to be similarly flimsy like with early Yongmei instruments, and also the case paint scratches easily, thus be careful with sending them through mail. At least in Germany most of their instruments are rare, and also the rest is at least uncommon. Jin Xin Toys apparently later renamed itself J.X.T and released under this label the strange toy groovebox J.X.T 20808.

Instrument model numbers of this company often have the prefixes JX-, JT-, J.X.T. Some of them were also released by Interkobo.


Kawai released only few small keyboards and mainly professional and semi- professional stuff; there may be many other prefixes I haven't listed here.
FS, X = velocity sensitive fullsize MIDI keyboard
WK = fullsize MIDI keyboard
MS = midsize keyboard
PH = "pop synth" MIDI keyboard (only PH50 known)
*m = MIDI tone generator module (name ending on "m", number is often the same like the keyboard version)
SX- = analogue synthesizer
K = digital synthesizer
Z = professional fullsize MIDI workstation keyboard
MDK = "MIDI Datacat" midsize MIDI master keyboard 
R- = drum computer
GB- = "session trainer" (portable accompaniment workstation)
E-, RS- = home organ

Potex Toys

This Chinese manufacturer (see links) builds the most noble and stylish looking toy instruments I ever saw. Especially they make very impressive toy DJ and tekkno consoles (see e.g. Mix Evolution); despite also their toys have some flaws, Potex seems to be yet the only toy company with the right feeling for the sound and estheticism of the tekkno environment. Some of their instruments were also released with the brand names Beat Square, Kid's Com/ Happy People, Kawasaki. (Someone e-mailed that versions of their Super Jam keyboard were also labelled DSI/ Yamaha Motorcycles and possibly Oregon Scientific.) Potex instruments seem to have rather names than a specific model number scheme.


With Yamaha (see links) there are way less prefixes, although some PS-# PortaSound keyboards were later released as PSS-#. Often identical Keyboards were later re- released with a different (usually lower) number (and different control panel colours).

old keyboards:
PS, PS- = midsize or fullsize keyboard
A, B, C, D-, E, CN-, CSY-, FC, FE, FS, HC = home organ
CP = e-piano (velocity sensitive fullsize keys)
CS, CS- = analogue (later virtual analogue) synth (but CS40 is a new wood guitar and even mopeds began with CS)
GS- = preset FM synthesizer
SK = "string ensemble keyboard" (fullsize)
YC- = stage organ

Later keyboards have the following scheme:
PSR- = fullsize keys
PSS- = "PortaSound" midsize or mini keys
VSS- = "voice sampler" sampling keyboard
HS- = "HandySound" old mini keyboard (or home organ)
SHS- = guitar shape keyboard
MK- = midsize synth keyboard (only MK-100 known)
PC-, PCS- = "PlayCard" midsize key lighting keyboard
TYU- = mini key lighting keyboard
EZ- = fullsize key lighting keyboard
KB- = fullsize "ensemble" keyboard
MP- = midsize with score printer (only MP-1 known)
DX = professional FM synthesizer
SY = improved FM synth workstation
YS = cheap FM synthesizer
QR-, QY- = portable mini workstation
MC- = home organ
HC-, ME-, MW, = stage organ/ transportable home organ
DD- = "digital drumkit" e-drumkit
PF, YPR- = e-piano (velocity sensitive fullsize keys)


Yongmei (see links) is a Chinese "no- name" company that in former times made the likely worst keyboards of the world; despite modern case design they only contained a monophonic transistor beep tone generator (see Golden Camel 7A for info). But nowadays they also sell better keyboards, including even some velocity sensitive MIDI instruments (seen on their site and eBay), but at least the cheaper fullsize instruments those I bought are still ridiculously bad and e.g. have fraudulent duplicate sound/ rhythm names and stank extremely of acrid chemicals (see e.g. Yongmei DL-2300, YM-3300, YM-6700). Because Yongmei buys their components from different manufacturers (their own factory seems to be Meisheng) and apparently sell their stuff under many trade names, it is hard to track which instruments were made by them. Some companies also sell as well Yongmei as other keyboards under the same trade name.

The following prefixes likely hint to manufacturers related to Yongmei:
YM, YMS-, MS, SWT- = used by Yongmei itself
MS-, MLS-, MK-  = used by Meisheng (aka Miles, MeiKe, Meiker)
JY- = used by Jia-Yin
SK- = used by Sankai
AB = used by Keynote
CEK-, CHP- = used by Cronenwerth
DL- = used by Yongmei itself?
GC- = Golden Camel (released by Meisheng?, my 11AB has a Miles sticker)
HD- = used by Shining Star
JC- = used by Elegance
K = used by IMD Musik
JT- = used by Jin Xin Toys ??
KBX- = used by Paxton
KX- = used by Orla (only their cheaper modern keyboards look like Yongmei)
LP = used by Clifton
MK- (others) = also used by C.Aemon, Mirage, Skytec, Orla midsize
PS- = used by Panashiba
S- = used by IMD Musik
SK (others) = used by Keytone
SK- (others) = used by Shenkang, ShenKong (or ShenHong?), SongMax, Yongmei
SWT- (others) = used by Siweite, Sieweit (only highend MIDI Yongmei keyboards)
W = used by Keytone (only highend MIDI keyboards?)
XH- = used by Kamichi, Miles, Ostoy
XTS- = used by Angelet
XW- = used by Xiong Wei
XY- = used by Xin Yun
ARK-, JK-, KN-, MSD- = ?
only number = used by Carsan

Very important with these keyboards is that the number seems to indicate only the case shape, while a following letter stands for the hardware revision number. Thus keyboards with identically looking case can contain totally different hardware; the higher the letter, the later the hardware version. Thus a hypothetical YM-1234 (I don't know if it exists) may contain a monophonic transistor beep tone generator, while YM-1234A has a monophonic sound chip with separate rhythm IC and a YM-1234B contains a 2 note polyphonic My Music Center variant or the like. Another strong hint to Yongmei or Sankai related hardware is when a keyboard with plastic case has the odd count of 54 fullsize keys; other companies only released keyboards with either 49 or 61 fullsize keys, but I newer saw 54 keys on instruments by any other known brand. Also keyboards with green percussion icons on the rightmost or leftmost keys are typical for Yongmei, because they often have built a CPU into a case with more keys than that CPU can support (thinking bigger is better...) and wired the remaining keys parallel with drumpad buttons to camouflage the mismatching combination.

warning: The flimsy plastic of Yongmei keyboards (especially the older ones) is horribly brittle. I don't know if they made it from dirty recycled plastic or if the hot moulded polystyrene is shock- cooled which builds up strong tension, but it shatters like a shellac record by any hard bump. Thus when sent through mail, the parcel must be urgently padded especially at the ends of the keyboard case with multiple centimeters(!) of styrofoam, firmly crushed paper, fanfolded cardboard or similar; otherwise the case will unavoidably shatter into a thousand of pieces as soon they toss it around in the mail. (Older Yongmei keyboards original packaging contains no end padding at all and thus is absolutely unsuited(!) for mail shipping. Wrapping it in a thin layer of bubble wrap etc. doesn't help anything.) Some Yongmeis (e.g. MS-210B) even contain useless iron weights inside their case bottom, which worsens the situation even more. Also always take out the batteries (especially heavy D size ones) before shipping, else they will turn into a ballbreaker and smash out the entire battery compartment.

Yongmei (or Meisheng or whatever their genuine name is) apparently first built only their infamous monophonic beeping transistor tooters (see Golden Camel 7A) containing an analogue tone generator of discrete components, incredible cable mess and sheet metal contacts. These keyboards can be recognized by their lots of buttons labelled with alphabet letters and especially a "play/ store" slide switch; often there are also other slide switches labelled with alphabet letters, and also 3.5 mm microphone/ AC- adapter jacks with a strange (usually yellow) plastic rim are a typical feature. Nowadays Yongmei makes keyboards with fairly normal looking PCBs, (kind of) rubber contacts and digital single chip CPU. But in between there was a short intermediate period where they made keyboards with already a digital sound generator chip but still the case style and cable mess of their former transistor tooters (e.g. Golden Camel-11AB). These "transition Yongmeis" were produced only for a very short time and thus are the rarest of all Yongmei keyboards; they can be distinguished because they still have multiple slide switches but typically already real preset sound and rhythm names instead of alphabet letters on their button fields, and they often have even great unusual lo-fi percussion and sounds. But also the larger transistor tooters will likely become rare soon, since by fragility and ear tormenting loud tooting they will certainly not survive parental rage attacks and end smashed in trashcans as quickly as they were assembled together (a fate that also decimated the legions of wacky late 1980th boomboxes).

Funny is that despite all this, there is even a voting on the Yongmei site to rate the quality of their keyboard models, thus when you feel not satisfied by them, don't hesitate to set your checkmark accordingly.

other no-name keyboards:

No-name keyboards with MC and EK numbers seem to be based on Medeli hardware designs. Many other trade names of MC hardware can be found on the Letron MC-3 page, those often also have different prefixes, thus if present, the model number on the PCB is better suited to identify the hardware class. With MC keyboards a letter suffix behind the number (like in "MC-3A" vs. "MC-3") sometimes stands for a different case variant of the same hardware. The EK keyboard hardware (may be Hing Hon) looks slightly different than native MC stuff, but both generally have much more common with each other than e.g. with Casio or Yamaha, thus either they were the same company or at least exchanged or copied their technology. EK- series  instruments often have mechanically better designed electronics than MC series (e.g. with plugged instead of soldered internal ribbon cables), but otherwise tend to contain more software bugs (i.e. strange sound glitches, lousy programmed demo songs) and some have cold distorted sound (by poor amplifier design?). Strange is that most MC and EK keyboards of the 1990th were already during release technically outdated by at least 10 years, i.e. they were in sound and operation very comparable with e.g. Yamaha keyboards from early to mids of 1980th, which made e.g. the MC-3 with its only 12 extremely artificial squarewave sounds appear very ridiculous compared to the sample based 100 sound bank instruments of its competitors. Nowadays such simple electronic sounds have found many fans and don't appear stupid anymore, and unlike some absurdly faulty Yongmei stuff, none of the MC and EK keyboards ever worked badly enough to be considered unplayable. In the opposite many of them even still had plenty of OBS controls for great professional live play tricks at a time when on Casio and Yamaha's (typically more expensive) home keyboards you could only type in numbers. And the MC/ EK series fingered accompaniments even still accepted any disharmonic key combinations when those wannabe more modern Casio/ Yamaha sound bank things (see Yamaha PSS-390) were designed to ignore anything else but a few standard establishment chords.

Regard that also famous brand manufactures (Roland, Kawai etc.) may use some of the same prefixes like no-name stuff (especially MC is extremely common), but when the brand name is something obscure, the chances are high to find either variants of Medeli or Yongmei hardware inside, or one of the hundreds of My Music Center variants, those original creator is almost impossible to find out.

squarewave secrets:

Squarewave is a very archaic electronic sound style that is well known from historical videogames and early home computers, but it was also used in cheap beginners keyboards and electronic sound toys. Also many analogue musical instruments employ internally squarewave tones, those are post- processed by analogue filters to modify them into more sophisticated timbres.

Squarewave tones are based on the square waveform, which consists of theoretically rectangular pulses made from only 2 signal levels ("pulse" and "pause") with a very short rise and fall time between them. This is the easiest method of digital sound generation because already an alternating bit sequence of "1" and "0" with equal switching frequency produces such a signal.

plain squarewave

+-----+     +-----+
|     |     |     |
+     +-----+     +----->t
In the simplest form a period of the signal consists of exactly one pulse and one pause with fixed duration, those are repeated again and again to produce the signal. Also multivibrator circuits output this kind of signal, which is called "plain" squarewave. When the duration ratio between pulse and pause is 1:1, it produces a timbre resembling a clarinet. 
+-+         +-+
| |         | |
+ +---------+ +--------->t
When one of them turns shorter, the timbre turns harsher, resembling more bagpipes.

E.g. a typical cash register beep or melody greeting card sound is plain squarewave. These tones sound very electronic and don't resemble much acoustic instruments (beside perhaps clarinet, flutes or harpsichord). Particularly they can not imitate well a trumpet due to wrong overtone structure. In analogue instruments this is sometimes compensated by a resonance filter, but with a (cheaper) normal low- pass filter the result is always either too dull or too accordion- like for a trumpet. Typical for all squarewave based sounds is that the bass range is purring in a buzzy way because the individual pulses become audible at very low frequencies.

(note: When no different pulse width ratio is mentioned, I mean with "plain" squarewave usually the ratio 1:1. Some people call square waveforms with unequal pulse ratio also "pulse wave", but I do not use this term.)

multipulse squarewave

Multipulse squarewave was beside FM and samples once the most widespread digital sound synthesis technologies in music keyboards, videogames and homecomputers. But although it was already used in home keyboard instruments long before the 2 others, it has nowadays become almost unknown although it can make a lot of great characteristic timbres. Multipulse squarewave is also the mysterious stuff the early Casio keyboard sounds were made of.

The trick of this technology is simply to repeat instead of a single pulse/ pause pair a longer sequence of multiple pulses to create additional overtones. Typically the bits of 1 or 2 bytes are outputted in a loop to form a pulse sequence with 8 or 16 steps; each bit can be either 0 or 1, which permits a lot of different timbres. The term "a multipulse" can be also used for an individual bit sequence pattern that determines such a timbre.
+-+ +-+         +-+ +-+
| | | |         | | | |
+ +-+ +---------+ +-+ +--------->t
This multipulse squarewave consists e.g. of 2 separate pulses per period
+-+   +-+       +-+   +-+
| |   | |       | |   | |
+ +---+ +-------+ +---+ +------->t
This one has a different pulse distance and thus different overtones....
+-+   +---+     +-+   +---+
| |   |   |     | |   |   |
+ +---+   +-----+ +---+   +----->t
... and again a different timbre.

Multipulse squarewaves can do a lot of interesting timbres. Short multipulse patterns can sound more sonorous and resonant than plain squarewave and partly resemble pure chords or typically the timbre of layering the same plain squarewave note from different octaves. (I am not sure how far this principle is similar to the concept of "subharmonic mixtures" employed in Oscar Sala's Mixtur Trautonium.) This can e.g. make a grainy synthetic metal organ pipe timbre with a wonderful "black", massive and sonorous droning bass range. Also a halfway realistic trumpet is possible with it. When sent through analogue filters, the sound drones even greater and makes wonderful warm bass timbres. I really can't understand why most modern analogue synthesizers only have oscillators with plain squarewave; multipulses could make them much better.
+-+   +---+ +-+ +-----+     +-+   +---+ +-+ +-----+
| |   |   | | | |     |     | |   |   | | | |     |
+ +---+   +-+ +-+     +-----+ +---+   +-+ +-+     +----->t
The longer the pulse sequence gets and the more irregular the pulses are placed, the more the timbre turns from a tonal sound into a buzzy noise or with a very long sequence even into almost white noise. 

This technique with very long bit loops is also used by shift register feedback noise generators, those e.g. produce percussion waveforms (snare and hihats etc.) in many old music keyboards. But these old noise generators can do much more; they were e.g. also employed in the famous Atari POKEY synthesizer chip that was built into their 8 bit homecomputers and (as a simpler variant) the VCS2600 videogame console. The POKEY can do incredibly rough and fiery timbres and special noises between buzz, drone and hissing - I yet found no other synthesizer specialized on these incredible kind of sounds (especially Yamaha FM synthesizers (like the OPL3 chip) suck really badly when they shall do different hiss timbres). POKEY can e.g. do noises like crunching crusty cookies, like a bumblebee in a paper bag or like lighting a match on a matchbox, like scraping a butter knife on a sesame crisp bread, like pulling a pine cone over sand paper - it can sound like overrolling a dry scone with a bike, like peeing on burning sausages on a coal grill or like a leaky gas boiler almost about to explode.

(note: Multipulse squarewave sound generators must not be confused with bit stream digital/ analogue converters. Although bit stream DACs also output a long pulse sequence (that is accumulated in the capacitor of a low pass filter), their step frequency in the produced sound is considered an undesired disturbing component that is therefore set to a fixed frequency higher than the audio signal range to make it (ideally) inaudible (e.g. Sony's "Super Audio CD" uses this technique). When the fixed step frequency gets audible (like with some cheap toy keyboards), the result is just DAC aliasing noise which makes the sound rather harsh and digitallic. In the opposite to this, multipulse squarewave sound generators use their step frequency as an intended overtone of the generated tone frequency, thus it varies with the tone frequency and stays in the audio range. This way such generators can produce warm timbres without disharmonic components because the step frequency stays always in tune with the sound.)

A good basic set of keyboard for the squarewave lover consists of an MC-3, Yamaha PSS-100, Hing Hon EK-001 (preferingly circuit bent) and perhaps a Casio VL-Tone 1 as a synth. Also MC-38 is nice. (This set doesn't include rare instruments, but only ones those are easy to find on eBay and have nice squarewave sound.)

squarewave music

Squarewave is also a style of electronic music using such timbres, typically with rhythmically ticking zipper noise envelopes and low resolution sequencer patterns. Although such timbres may be also used in dark wave music, the term has nothing to do with it. (But this is a different topic...)

program loop synthesis

This is the most versatile form of digital controlled sound generation. In opposite to normal digital synthesizers, the individual preset sounds are here not fixed sets of input parameters for fixed synthesis algorithms (like ADSR values and waveform numbers), but they each constitute a piece of program code that is executed during sound generation to set the timbre, pitch, volume and other parameters of the sound generator directly.

Program loop synthesis was mainly used in historical pinball and videogames (e.g. by Williams), but also old digital sound bank keyboards and sound toys used these techniques to produce a great variety of very different and unusual sounds on cheap hardware. (The Casio SA-series is the likely best example for this.) Unfortunately this wonderful technology has been abandoned in modern keyboards and sound toys - likely because sample memory has become so cheap that nobody wants to take the effort anymore to program sounds directly.

zipper noise:

Unlike fully analogue instruments, the sound envelopes of many old or cheap digital controlled instruments are not smooth but consist of coarse square rasterization steps, those make the envelope curves resemble stairs. These steps produce additional overtones those can resemble the noise of moving the zipper of a zip (that's what the name comes from), but depending on step rate and resolution (and optional analogue filters) they can also sound from a quiet fast ticking to bee buzz or scratchy noises.

Zipper noise is a key element in the characteristic sound of most squarewave instruments and a main reason why old home keyboards sound different than the same waveforms on expensive analogue synths or simulations on modern high resolution software synthesizers.

E.g. plain squarewave blips with zipper noise in a short decay envelope sound much like hitting a glass bottle, while without that noise it sounds rather like a dull xylophone. Due to these grainy stair distortions usually get particularly audible during fast attack phases of volume envelopes, they are also responsible for the seemingly astonishingly natural wind or bow noise at the begin of flute and violin sounds on so many old squarewave based instruments, and also the special creaky timbre of digital vibrato or tremolo on them is often caused by zipper noise.

digital aliasing noises:

Aliasing noise is the noise caused by the stair- like shape of digital signals. There are various sources of aliasing noise; this is a simple overview of what the most important ones do. (On the internet is plenty of more detailed info about the maths of sampling theory.)

sample bit resolution

The bit resolution determines the step height of the waveform; lower resolution signals (e.g. 8 instead of 16 bit) have higher steps and thus tend to contain a hissing component that resembles white noise. This noise is mostly audible during quiet sections of a sample, because waves with low amplitude are fewer stair steps high and thus their shape is approximated less accurate.

sample frequency

The sample frequency determines the stair width (in time) and thus the maximum recordable sound frequency; all overtones higher than half that frequency become distorted (transformed into wrong frequencies) when they are not filtered out before recording. Very important with digital sound generators is that the timbre of the distortion depends strongly on the ratio between the frequency of the recorded tone and the sample frequency (much like with carrier and modulator in FM synthesis), thus when both are in a harmonic ratio, the resulting sample can sound warm despite a low sample frequency, while with wrong ratio the distortion adds harsh glassy or metallic overtones. With old or cheap digital instruments instead of full length samples only short looped waveform samples are used, those volume is controlled by an envelope. Such instruments can even sound warmer than with longer samples of the same resolution, because here the waveforms can keep a perfect harmonic ratio to the sample frequency due to any vibratos and other frequency changes are not in the sample itself, but only generated by external pitch envelopes.

DAC aliasing noise

Usual musical instruments pitch a single sample up or down by changing its playback frequency to play different notes pitches. When this works perfectly, it causes no additional overtones and thus a warm sounding sample can keep its timbre over a wide pitch range. (That's why the Fairlight CMI synthesizer, Synclavier and Amiga computers had nicely clean and warm timbres despite fairly low sample resolution.) But normally a given sample is not just played exactly faster and slower, but the digital analogue converter (DAC) outputs the resulting sample with another fixed clock frequency, which with poorly designed instruments is too low to serve its purpose and thus has the same effect like resampling the sound with that frequency. And when the high overtones of high played notes would be above half that DAC output frequency, this again causes distortions those (unlike the original sample frequency of the tones) by the varying note pitch can not keep a fixed harmonic ratio to it and thus always causes that cold, harsh and glassy timbre that is known to sound "digitallic". The result resembles a ring modulator and can be heard e.g. in the (in-)famous "trumpet" sound of My Music Center. DAC aliasing noise exists not only with samples, but also in FM sound chips and even in poorly designed digital squarewave tone generators. That's the main reason why music of emulated historical videogames often sounds much colder and thinner than with the original sound hardware, despite squarewave tones normally are considered easy to emulate in software.

Advanced digital sound generators often also employ oversampling algorithms those compute intermediate values between the stair steps of the signal to reduce the digitallic harshness. But when they smoothen also low resolution samples itself and not only the DAC aliasing noise, they can also make the bass range of timbres too dull those were intended to have a rough and sonorous bass range.

sample resolution:

Because the audibility of digital noises or distortion caused by limited resolution strongly depends on the actual sampled sound (see digital aliasing noises), I rate the sample resolution not by absolute terms of bit resolution and sample frequency, but by the hearable sound quality. I consider a sample "low resolution" when it sounds so noisy or distorted by inappropriately low sample frequency that the intended timbre is hard to recognize (e.g. when a cymbal sounds like a shaker or a violin harsh like bagpipes). I call it "medium resolution" when the frequency and bit resolution is sufficient to easily identify the intended timbre, but there is still a small dose of glassy digital distortion or such noise audible when you listen closely. I call a sample "high resolution" when it is free of audible digital distortion or such noise (about like CD quality).

harsh & cold sound:

With digital hardware the most common reason for harsh, hissy and cold timbres is DAC aliasing noise (see digital aliasing noises), but another way too often overlooked reason for such digitallic sound is an oscillating power amplifier IC, which oscillation frequency intermodulates with sound frequencies or stair shaped digital waveform residues and thus causes similarly harsh overtones like DAC aliasing noise. Oscillating amp ICs are usually result of a poorly designed feedback loop, wrongly wired GND lines or an omitted zobel network (capacitor in series to a resistor) across the speaker output lines. (For more info ask a search engine about the theory of amplifier ICs.) Thus with bad sounding digital instruments often only the analogue output amplifier is the trouble source, and not the sound generator itself. The quality of internal power amplifiers varies extremely among cheap toy and beginners keyboards; especially the amplifiers of older Yongmei keyboards often have design flaws those make extremely harsh distortions.

Also a damaged loudspeaker can distort and make sounds unpleasantly harsh when e.g. the voice coil collides with the magnet due to a bent sheet metal chassis, a loose magnet or loose voice coil windings (by overheat damage). Especially cheap speakers in toy or no-name instruments tend to have bent chassis or loose magnets those usually can be manually bent or glued back into place. Also watch out for mechanically rumbling case parts (e.g. keyboard keys and buttons) and fix them with a strip of self- adhesive felt or window insulation foam rubber when necessary. (But with certain cheap tablehooters the rumbling case noises IMO even rather enhance their trashy sound style by adding personality to them and making their sound more vivid.)

If you want to build an IC based 2W stereo power amplifier without harsh sound, try to get a KA2206 amplifier IC. This chip seems to be (at least in Germany) hard to find, but at least in cheap keyboards it makes the definitely warmest and nicest timbre of all such amp ICs I ever heard yet. (That's likely why cheap Medeli/ MC- series FM keyboards sound so much warmer than similar Yamaha ones - see e.g. GPM MC-5000.) But be careful to use a proper heatsink and not to short the outputs of the KA2206; it seems to be prone to burn out when overloaded.

fixing poor bass response:

The power amplifier in sound toys and cheap beginners keyboards often contains too small coupling or output capacitors, which causes poor bass response independent from the speaker size. To cut manufacturing costs, especially electrolytic caps those are wired in series to the speaker or headphone jack tend to be too small; replacing them with bigger ones (e.g. 470µF instead of 47µF) can improve the bass sound a lot. The only negative side effect can be that in toys with single transistor amplifier the popping noise at the start and end of every sound can become louder when the CPU switches the amp power on and off to save batteries.

Another reason for poor bass with internal speakers is when they are too small, of poor quality or when there is an acoustical short circuit, i.e. that the pressure wave from the speaker front can travel through case openings to the back of the speaker diaphragm and thus cancel out itself. (There is much info on the internet how loudspeakers work.) Often it helps to seal gaps in the case or around the speaker rim with adhesive window sealing foam rubber strips (see e.g. Potex - Super Jam). Larger case holes (e.g. when there is a round speaker behind a square grill) can be sealed e.g. with hotglued cardboard or plastic. But be careful not to close case vent holes near heat producing electronics to avoid overheating.

1970th full polyphonic technology:

Extremely old electronic "full polyphonic" keyboards and such home organs have no keyboard matrix. Instead there are 12 squarewave tone generators those continuously produce all tones of the highest octave, and a set of frequency dividers (flip- flop ICs) derive from them the tones for all lower octaves. This way the instrument internally always outputs all tones for all keys simultaneously, even when no key is pressed. Each key contact switches the corresponding tone onto a common line which is connected with the input of an amplifier that makes the tones audible. The amp typically contains various filters (selectable by switches or controllable by potentiometers) to modify the sound timbre.

When such an instrument has polyphonic (piano- like behaving) envelopes, then for each individual key there is an own analogue envelope circuit made from a capacitor, resistors and possibly transistors necessary (i.e. as many envelope circuits as keys). To make such an envelope circuit bank adjustable by a single potentiometer, a row of diodes (1 per key) connected with a single line can be added to modify the capacitor charge speed or level. Due to this sort of full polyphonic envelope hardware was quite expensive, cheaper keyboards had only monophonic envelope, i.e. when in piano mode a new key is pressed while others are still held down, the held down keys will sound with full volume again, because the envelope only controls the amplifier volume of all keys together, much like turning the volume control of an organ loud and then fade it silent while pressing multiple keys. Alternatively the newly pressed keys could be wired not to trigger the envelope again, i.e. they stayed as quiet as the silent fading old tone so far not all keys are intentionally released and then pressed again (old Hammond organs behave like this).

The 12 main oscillators of the highest octave can either derive their frequency from a common HF oscillator (=>it is easy to add a pitchbend wheel that changes this clock frequency) or they can be independently tuneable oscillators; in the latter case the tuning within the octave can be intentionally messed up, which is interesting for circuit- bending.

tube electronics:

Although tube sound electronics can be modified too, it must be strictly warned here that classic circuit- bending approaches must not be used on them. Electron tubes work with high voltage of multiple hundred volts, which is about as dangerous as mains voltage and can be lethal; tube electronics also typically uses higher energies than transistorized or IC based circuits, which makes expensive or irreplaceable components easily burn out (or even explode) when shorted or overloaded.

To modify tube based devices it is therefore strictly necessary to understand the basic working principles of tube electronics. (Explanations of them can be found on many tube amplifier sites). I therefore give here only a few general warning hints about things those are still not commonly known yet:

sound devices well and worse suited for circuit- bending:

When buying such plastic toys it needs to be reminded that they are often assembled in Chinese concentration camps or with children work or by sweatshop workers those get paid 3$ for a day of hard work (e. g. in the case of "Furby"), no matter how expensive these toys are sold in the rest of the world. To burden your karma less I therefore can only recommend to rather buy such toys used or get them from trash or at least don't buy them for high retail prices, because the profit makers of such toys are definitely not the ones those are urged to assemble these toys day after day anywhere in world- end factories.

(But to buy a whacky plastic sound toy for building an instrument from is at least a higher destination for it than to give it to a kid which will likely break it within few weeks (like the million others) and than junk it to the next waste incineration plant where it just gets transformed into a dioxin cloud to surround this planet. I also sometimes buy such chinese plastic toys in shops, but I avoid to buy expensive ones because I am aware that the profit with them is generally not made by the sweatshop workers urged to assemble them, and the more expensive the toys are, the more of that unfair profit got made by some cravat wearing criminals somewhere else. I also made a German poem about such a loudly screaming plastic toy from a Chinese concentration camp ("Noizz Toyz - Plastikspielzeug", see here).)

Information about actually sold toys and beginners keyboard models and their manufacturers (especially interesting with no-name models) can sometimes be found at the "Global Sources" homepage: http://www.globalsources.com/ . The manuals of most Yamaha and some Casio keyboards can be downloaded at the sites of their manufacturers (see links page), which can be very useful to find info about less known models.

Old keyboards and sound toys can be found on flea markets, but best buy source for such instruments is definitely eBay (ebay.com). In comparison to average flea markets you can not imagine how easy it is to find at this internet auction service all kinds of strange no-name and beginners keyboards and sound toys as well from the great classic era as also in brand new for very reasonable prices. (To bid there, you only need to sign up at a web form and within 2 days you will receive a letter with a password - that's all.)

general modifications:

When AC adapter input jacks came with no polarity protection diode ("stupidity diode") or digital ICs have no voltage regulator, I generally tend to upgrade the electronics with these crucial components. Particularly the stupidity diode I add to ANY device with ICs and standard power supply jack to prevent accidental destruction. I also modify my Casio instruments to standard polarity (center pin = +, outside = GND) to prevent confusion. (This kind of modification may be therefore omitted to mention in some descriptions of my modified instruments on this site.)
I                  CYBERYOGI Christian Oliver(=CO=) Windler                  I
I         (teachmaster of LOGOLOGIE - the first cyberage-religion!)          I
I                                      !                                     I
 removal of these screws voids warranty...