Casio CT-410V (MT-400V)

analogue synth keyboard with complex accompaniment
& real analogue filter

This is definitely one of the most interesting Casio keyboards, because it features a genuine analogue synthesizer filter with filter envelope and cut- off/ resonance control sliders, lots of accompaniment variations and separate volume sliders. Additionally it has a wonderful "stereo chorus" rotary speaker/ leslie simulator that produces a great sort of "hemi- sync" mind machine effect with stepless adjustable speed, which makes the thing perfect for brain wave synchronization features in psychedelic meditation or tekkno trance music. A smaller version of this instrument with midsize keys and no built-in speakers was released as Casio MT-400V.

All sliders of this instrument are real analogue potentiometers and thus stepless. The stereo chorus LFO can also modulate the filter in "wah wah" mode, which makes the typical "wokachika" sound known from certain 1970th funk (?) musics. The great accompaniment unit has 4 bass and 4 chord variations and the same unique, dark and sonorously droning squarewave bass tones like the "organ" sound of the Hing-Hon EK-001. In fingered chord mode the accompaniment plays the more notes the more keys are pressed, and this works even perfectly with all non- chord key combinations, which permits very versatile accompaniment sound patterns. (This flexible behaviour is absolute no matter of course, see Yamaha PSS-390 for an annoyingly stubborn example.) And despite this instrument has already many built-in features, there are even still lots of additional keyboard matrix eastereggs to discover, which makes it perfect for circuit bending.

main features:

49 fullsize keys
2 built-in speakers (stereo, reasonable big but sound a bit dull)
8 note polyphony (only 4 notes with accompaniment)
4 note polyphony in chord section
separate analogue sliders for main, rhythm, accompaniment and bass volume
12 semi- OBS preset rhythms {rock, pops, disco, 16beat, swing, latin swing | bossa nova, samba, beguine, tango, waltz, slow rock} selected by locking buttons + select button.
20 semi- OBS preset sounds {organ, piano, harpsichord, flute, oboe, trumpet, violin, celesta, mandolin, funny | pipe organ, elec. piano, vibraphone, clarinet, accordion, horn, cello, harp, elec. guitar, cosmic tone} selected by locking buttons + select button.
chord switch {off, fingered, casio chord}
4 bass variations switch
4 chord variations switch
tempo slider
rhythm fill-in and synchro button
vibrato switch {off, vibrato, delayed vibrato}
sustain switch {off, sustain, reverb.}
analogue resonance filter (12dB?) with analogue sliders for {attack, decay, sustain level, cut off, resonance}
filter input source switch {off, tone, rhythm, bass/ chord, noise} ("noise" = white noise hiss triggered by rhythm)
"stereo chorus" rotary speaker/ leslie simulator (based on a combination of panning and pitch shifting)
analogue slider for "stereo chorus/ wah speed"
main voice sound IC= "NEC D931C 011, 8431KY" (42 pin DIL) with timbres based on 2 mixed multipulse squarewave tones with different digital envelopes, those are differently low pass filtered through capacitors.
accompaniment CPU= "NEC D930G 011, 8434XK" (80 pins SMD) which produces different multipulse squarewave tones (partly with capacitor envelope) and trigger pulses for external analogue drums.
analogue percussion {base, snare, open cymbal, close cymbal, low tom, high tom, woodblock} which apparently uses no transistor noise. The cymbal timbre sounds half- way metallic and resembles the one of Yamaha PS-2, thus it may be mixed from 2 shift register feedback noises.
tuning knob
filter envelope control jack
jacks for sustain and "foot volume" (main voice volume) pedals
jacks for AC adapter, line out and headphone

eastereggs:

There are tons of great keyboard matrix eastereggs; for details see below.

arpeggio with 4 variants
3 additional main voice envelopes (for each preset sound!)
manual bass
octave switch (1 octave down during accompaniment)
key hold (simulates held down keys, likely for a pedal)
transpose
chord memory on/ off
sustain + reverb simultaneously

notes:

Although the Casio CT-410V and MT-400V are quite rare, there are many other old Casio keyboards without the synthesizer filter, those are more or less based on the same hardware class and easy to find at eBay. In the following I therefore will explain a bit more detailed than usual how to modify this instrument, because particularly the unique sounding squarewave accompaniment CPU is worth to be explored since many Casio instruments do not remotely make use of all its features. I conclude that the most complete featured specimen of this hardware class was the Casio MT-65/ MT-68, which has the same accompaniment CPU and main voice soundchip like the CT-410V/ MT-400V, but includes 4 arpeggio and 4 envelope variants (but no synth filter). This accompaniment is almost as flexible playable like the Testron one, and thus also accepts any disharmonic note combinations and not just those few ones that establishment has defined to be "chords". When rhythm is off, the rhythm select switches switch the bass and organ chord timbres among 4 squarewave multipulses, and also the chord and bass variation switches change them. Instruments of this hardware class can be recognized by the rhythm and sound preset names (the sound names "funny" and "cosmic tone" are easy to remember) and also the count of 12 rhythms and 20 sounds and the possible presence of locking switches for multiple chord, bass and/ or arpeggio variants hint to it.

The analogue percussion of the CT-410V sound rather unspectacular (no booming tekkno base or the like), but the decay time of all drums can be adjusted with trimmer potentiometers on the main PCB, and like all analogue drums they can be certainly modified in various ways.

The 20 main voices are fixed presets and not user- definable, but there are various mode switches (vibrato, sustain etc.) those modify them. The main voice generally sounds a bit dull when not sent through the synth filter; this has partly to do with dull built-in speakers, but there is also a 1nF capacitor which unnecessarily muffles all timbres - throwing it out is strictly recommended and will improve the sound significantly. (In opposite to this my Casio MT-45 sounds very bright by default, despite it has the same accompaniment CPU.) The main voice sounds of the CT-410V consist of 2 mixed squarewave tone suboscillators with different multipulses and different digital volume envelopes, those are (depending on the preset) muffled by 4 different filter capacitors. In the bass range many sounds turn into a more or less buzzy, sonorous purring drone, which is a characteristic style element of squarewave based instruments. These basses can resemble some of the famous POKEY sound effects on Atari XL homecomputers and are very different from the gradually duller and duller growing sine wave bass behaviour of average Yamaha FM keyboard sounds. When sustain is switched off, all sounds stop almost immediately after releasing the key, and the sound presets itself also contain neither vibrato nor tremolo.

Particularly the piano and the ringing "mandolin" timbres sound really dull, and also the too narrow pulse width of the mandolin makes it sound very unrealistic and buzzy at low notes. As usual with squarewave instruments, the dull "trumpet" sounds very artificial (but a much better trumpet timbre can be generated using the synth filter). Most realistic sound the organ, flute and clarinet presets. The "funny" preset is a sort of rough digital slap bass (creaky picked e-bass) which sounds like "ennng!" (possibly it was initially intended to be called "funky", but sounded to artificial for this?). The "cosmic tone" preset sounds simply like a plain saxophone tone, thus I guess that its strange name was initially rather related to its 3 switchable envelope variants (those are normally not available on this instrument but can be added by an easy modification), because they turn it into a slowly duller fading spacey synth lead "meow" timbre, which makes audible use of the crossfading between the 2 subvoices. Both sounds belong to the brightest and most interesting ones of this instrument.

Technically the CT-410V is completely mono, besides that the main voice is optionally post- processed by the "stereo chorus" circuit, which makes a pitch shifted version of it alternatingly pan left and right to approximate a rotary speaker effect. Either the main voice ("tone") or rhythm or bass + chord or a white noise source (chopped by