Yamaha PS-2

YAMAHA PS-2

analogue keyboard with accompaniment & carry case

This brown, ultra- compact keyboard from 1980 was one of the very first polyphonic portables with automatic accompaniment; it constitutes (together with Yamaha PS-1 and PS-3) the beginning of the famous Yamaha PortaSound series and came in a brown hard shell plastic carry case.

This is the instrument in its carry case.

The original German retail price of the PS-2 was 450DM (about 225€). A white fullsize variant of this instrument was released 1 year later as Yamaha PS-10.

main features:

37 midsize keys
small built-in speaker
8 note polyphony (only 4 notes with accompaniment)
4 note polyphonic chords
5 OBS preset sounds {organ, string, clarinet, piano, harpsichord}.
4 OBS preset rhythms {waltz, swing, rock, latin}
locking multi- contact push button switches to select sounds and rhythms (one switch per sound/ rhythm)
sustain switch
single finger chord
tempo knob
separate rhythm and accompaniment volume knobs
digital part is the ancient and expensive looking CPU "Yamaha YM1 104 C, 14 0455" (48 pins), which outputs raw waveforms and trigger pulses for analogue percussion. All main voice timbres are analogue filtered squarewaves with capacitor envelope.
analogue percussion {base, snare(?), hihat, low & high tabla(?)} sounds muffled and unspectacular. The hihat is mixed from 2 digital shift register feedback noise outputs at the CPU. The base is a dull popping noise.
jacks for power supply, sound output and "expression" volume pedal

eastereggs:

11 additional rhythms are selectable by simply activating multiple rhythm lines simultaneously after re-wiring the rhythm switches.
vibraphone sound envelope
some higher piano keys addable (like in Yamaha PS-3)
In the key matrix are 4 combinations (logically left next to the lowest piano key) to shift everything (also rhythm) one octave up and to make weird distorted program crash sounds. (The program doesn't hang and continues like normal afterward.)

modifications:

rhythm buttons rewired to get additional rhythms by pressing multiple switches together.
5 switches and 4 pots added to mix combinations of the 4 analogue filters (like organ drawbars) with any of the 5 available envelopes when none of the preset sound buttons are pressed.
4 DIP switch block added to the crash/ pitch shift pins.

Above the buttons you see the sound knobs and switches I added.
The blue thing to the right is the DIP switch block.

notes:

This instrument sounds very much like an analogue 1970th home organ. The CPU is an ancient and expensive looking IC in a grey ceramic package with a white rim, black label plate and golden pins. Another technical oddity is that the keyboard uses a plastic foil contact mat instead of the nowadays common silicone rubber contacts. All the main voice sound presets are based on squarewave with different envelopes, those are post- processed by analogue filters. When a key is trilled with sustain, each new note occupies a new sound channel, which produces a great phasing sound and volume increase effect although this eats up polyphony.

The analogue percussion sound quite dull, but at least it has an unusual timbre. The base is just a dull and quiet "pop" noise (like heard in the speaker when switching a radio on) consisting of 2 pulses muffled by a capacitor. The 2 tuned drums (tablas?, or congas??) have a strange timbre between drumming on empty plastic bottles and ceramic jugs. The semi- digital hihat was likely an innovation at its time and indeed sounds a bit more metallic than the analogue transistor noise ones found on other electronic percussion of that era.

When I bought my Yamaha PS-2 on a flea market, it made always a static beeping of varying intensity in the speaker. It turned out that a bad solder joint of an electrolytic capacitor at the voltage regulator had caused this. After fixing it the beep was gone.

circuit bending details

The following descriptions relate to the instrument opened and placed keys facing down with view at PCB solder side.

The CPU itself can outputs only 5 basic sounds made from a squarewave with volume envelope. They are a wind instrument sound (plain squarewave with short attack and release envelope, in 3 variants with different tremolo), a harpsichord sound (unequal pulse wave with ratio 1:7 and falling decay envelope) and a vibraphone sound (plain squarewave with falling decay envelope, integrated vibrato and sustain), those are post- processed by 4 switchable analogue filters to form all the final preset timbres. Apparently not all timbres are entirely squarewave, because in one of the filter settings(?) IC3 converts square waves into a coarse digital sawtooth wave (2 bit resolution?) that looks like stairs. For the envelopes of the 8 main voice channels there is a row of 8 capacitors near the CPU.

The keyboard matrix has 2 cable connectors. Connector "C1" contains 6 output lines labelled N7..N2. Connector "C2" contains 7 input lines labelled {B11, B12, B21, B22, B31, B32, B41}. The inputs react on + 9V. Not all functions are controlled by matrix lines; e.g. the single finger chord(?) is enabled by pulling CPU pin 46 against +9V.

additional keys

At CPU pin 38 there is an unused input where you can connect 6 higher note keys (wired through diodes against "C1").

timbre synthesizer & distortion

Originally each sound preset switch selects one of only 3 squarewave basic sounds (different pulse widths and envelopes) and by another contact one of 4 analogue filters to set its timbre. The sound preset switches are wired in series in a way that when multiple buttons are pressed, only the rightmost one will take effect because additional opener contacts in each switch disconnect all switches left from it.

filter controls:

Each switch has 6 pins. At the leftmost switch "PS7OR" (organ) the 2 top pins are unused. For manual filter controls, connect the left upper pin of switch "PS7OR" with 4x parallel [a 100 kOhm trimmer in series with a 10 kOhm potentiometer (wired as variable resistors: wipers as output, left ends unused) and its wiper connected with the anode of a diode]. Connect the other ends of these diodes with the filter control pins X1, X2, X3 and X5. These are located at the left end of the button PCB; there are 5 pins in a row those I referred in my scribbled schematics as X5..X1. (I don't remember well if these numbers are printed on the PCB or if they were by me, but I think they were printed.) These inputs select through a 4066(?) IC 4 different analogue filters those modify the basic sounds. By space reasons it is recommended to use trimmers with capstan instead of normal size 10 kOhm potentiometers.

basic sound switches:

The basic sounds are selected by connecting the "C1" outputs N2 to N6 through a diode with pins 2 of the CPU:

N2 = wind instrument, little tremolo
N3 = wind instrument, much tremolo
N4 = wind instrument, no tremolo
N5 = harpsichord/ piano
N6 = vibraphone
(N1 = sustain switch)

(Combinations of them don't create additional sounds.) To make them selectable, connect the right upper pin of switch "PS7OR" with in parallel 5x [a switch in series with a diode (cathode end)]. Connect the open ends of these diodes with {N4, N3, N2, N5, N6}.

This way it becomes possible to select the new timbre synthesizer feature by pressing none of the sound presets (push one button halfway in to release them). Now you can combine any of the 5 basic sounds with any combinations of the 4 filters. Adjust the 100 kOhm trimmers in a way that each filter can be as well fully enabled as fully disabled by its potentiometer. The analogue filter selection IC (IC5?, a 4066?) distorts greatly when pots are only halfway open, which is especially noticeable during main voice chords. By selecting any sound preset, the timbre synthesizer will be disabled automatically, because electrically it behaves now simply like another preset switch left to the "organ" one.

additional rhythms

The rhythm switches are wired the same way in series like the sound presets, thus when multiple buttons are pressed, only the rightmost one will activate a keyboard matrix line because opener contacts deactivate all switches left to it. By cutting the traces to the 2 center pins of each switch and connecting the center pins instead through a diode (anode at the switch) with the 2 center pins of the "start" button (= CPU pin 3), the limitation is overcome and thus 11 combination rhythms can be enabled by pressing multiple rhythm buttons.

tekkno noises

Connecting the keyboard matrix input B11 through a diode with outputs N5..N2 makes various strange and rough crash sounds. (I guess that here a 2nd CPU could communicate with the CPU in costlier keyboard instruments.) After releasing, the program always continues running as before (i.e. doesn't hang). One of these inputs even pitches the entire instrument (including with drums and accompaniment) by 1 octave up. I have wired a 4 DIP switch block to these.

(Attention: My hand- written schematics of this modification are quite old, messy and incomplete, thus I can not guarantee that the above instructions are fully correct.)

A longer 44 keys version with 9 sound presets was released as Yamaha PS-3 and a white 32 key version without accompaniment and only 4 sounds as Yamaha PS-1 (German retail price 300DM = about 150€). These all likely contain the same CPU and only differ in the count of switches and possibly of analogue timbre filters. Technically a direct successor of this instrument was the fullsize Yamaha PS-20 , which had a more complex accompaniment, arpeggio and 8 rhythms. Already in 1982 Yamaha abandoned the analogue technology entirely and thus made all following PortaSounds (e.g. PC-100) only with strictly digital sound generation, while Casio kept analogue percussion and filters in their instruments far longer until the mid of 1980th. Despite their age, the Yamaha PS-1, PS-2 and PS-3 are not really rare and still occur from time to time on eBay.

*removal of these screws voids warranty...*

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