Casio MT-30

(photo taken from eBay)

main features:

• 37 midsize keys
• built-in speaker (with a slightly unpleasant midrange resonance)
• 8 note polyphony
• 22 preset sounds {electric piano, banjo, guitar, harpsichord, xylophone, celesta, glockenspiel, organ, accordion, pipe organ, oriental pipe, brass, cello, synthe fuzz, violin, trumpet, funny fuzz, st. ensemble, clarinet, flute, recorder, folk flute} selected by white keyboard keys + mode switch
• useless "tone memory" 4 step switch to assign 4 of the preset sounds for quick access
• vibrato and sustain switch
• main CPU "NEC D775G, E11046" (64 pins with strange zigzag layout) with timbres based on 2 mixed multipulse squarewave tones with different digital envelopes those are differently low pass filtered through capacitors.
• jacks for AC-adapter, headphone and cinch sound output

eastereggs:

• 13 lower note keys addable.
• likely switches for mild vibrato, faster vibrato, octave down and hold (sustain pedal) addable.
• when instead of a white a black key is pressed in sound selection mode, the instrument plays no sound at all.

modifications:

• polarity protection diode added, power supply jack polarity corrected.

notes:

This is definitely one of the very first midsize Casio keyboards, because unlike with later MT series instruments, this one contains a heavy sheet metal frame that holds the keys, and also the slide switches and potentiometers are screwed to it and not only soldered to the PCB or held by grooves in the plastic case like with later small Casio instruments. Also the black keys of this ancient instrument have perfectly plain sides and not slanted one like later midsize Casios. The case is held close not only by screws, but also by nasty snap-in plastic tabs those either crack off while opening or (worse) cause a lot of scratches on the case rim when you try to lever the thing open with a screw driver after successfully removing all screws. (Once it is open, I recommend to intentionally crack off most of the tabs at the rear side, because otherwise you would scratch the case rim again and again even more when you every time need to poke around with the screw driver to open it.) Interesting is that inside the case the loudspeaker compartment is horizontally separated by a pressboard wall and damped with foam rubber mats, which I never found in other small keyboards. Unlike many other early Casio MT instruments, the main PCB of this one is rather small and fills only 1/3 of the case.

The sound CPU D775G of the MT-30 was likely a very early predecessor of the sound chip D990G of the Casio MT-60, and the D931C of the Casio CT-410V, but unlike the latter it does not communicate with a host CPU but polls the keyboard matrix completely autonomously and only communicates with 3 demultiplexer ICs ("TC4049BP", 16 pins) to output the digital sound through a DAC made from a 12 pin resistor array and 7 individual resistors. Like with MT-60 and CT-410V the sounds consist of 2 mixed multipulse squarewave suboscillators with different pulse patterns and different digital volume envelopes, those are (depending on the preset) muffled by different filter capacitors. But while the latter use sophisticated envelope controls, the MT-30 uses for its 2 suboscillators only very simple attack- decay envelopes. 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. (For further technical details about this hardware family also see here.)

Like the MT-60, the sounds of the MT-30 are quite bright and use apparently only 2 or 3 different filter settings. But in comparison to the MT-60 most sound preset of the MT-30 are rather boring attempts of imitating acoustic instruments, those as usual with squarewave instruments don't sound natural anyway (except with flute- like instruments), and by the lack of complex envelopes there are also no gimmicks like ringing mandolins. But due to the 2 suboscillators and filters they sound at least less plain than e.g. the timbres of a Letron. Due to the muffling filters, the bass range of many sound presets is too loud in comparison to simultaneous high notes. Interesting is that Casio named the only piano sound here "electric piano" - likely because they knew that this hardware can not imitate a piano well - however it sounds more like a sonorous e-bass anyway. Unusual is that the "cello" and "trumpet" fade silent when keys are held down. The "trumpet" could be also called "violin", it only sounds slightly brighter. The "st. ensemble" should likely mean "string ensemble", but resembles rather a mellow horn. The "brass" is a sort of trombone and turns into a sonorous tuba in the bass range.  The "flute" and "recorder" only differ slightly at their beginning and the "folk flute" is a slightly brighter version. The "pipe organ" is quite harsh, the "organ" sounds a bit accordion- like and also the "oriental pipe" resembles high and bright accordion tones. The "guitar" resembles an electric one and the bass range of the "banjo" more a koto or harp. The "xylophone" and "glockenspiel" sound identical besides that the latter has a louder percussive start (more like a music box), while the "celesta" is slightly brighter, has no chorus effect and a longer decay phase, but all 3 sound quite similar. The only innovative sounds are "synthe fuzz" and "funny fuzz". The "synthe fuzz" resembles a rough bowed(?) sitar sound which fades brighter during the decay phase and is very dry and sonorous in the bass range. The "funny fuzz" is a sort of harp timbre with a mellower (less percussive) attack phase and consists of 2 notes (base note followed by 1 octave higher one) played with a very short delay.

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. I like the MT-60 sounds more, but great is that also here 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 some polyphony.

Like with the ancient Casiotone 201, to select sound presets the "mode" switch has to be moved from "play" to "set", which will also assign the selected sound to the current position of the "tone memory" switch. The sounds are then selected by white keyboard keys. Very interesting is that the selected sound is played as a hint during the selection key press, but only when the key is held down longer than about 0.2s. (It's a pity that this trick is not used on average toy keyboards, because the delay would make it possible to rapidly select OBS sounds during live performance without hearing the selected sound, and despite easily test the sounds by pressing the buttons slightly longer.) When the "mode" switch is set back to "play", the keyboard behaves normal again (and the "tone memory" switch can select between 4 of these sound presets during play). Like with the Casiotone 201 only the white keys are used to select sounds; possibly an external ROM chip with additional sounds for the black keys was planned, because pressing any black keys apparently selects here an empty sound because it mutes the keyboard. With the Casiotone 202 and also the MT-60 all continuous tones (e.g. organs) are selected by one of  the black keys while decaying sounds (e.g. piano) are selected by the whites. After power on, the 4 "tone memories" of the MT-30 always contain the sound presets {electric piano, celesta, pipe organ, flute}.
 

Casio MT-40

different main features:

• 15 bass button keys
• monophonic bass accompaniment & manual bass
• 6 OBS rhythms {waltz, samba, swing, slow rock, pops, rock}
• rhythm fill-in button
• rhythm volume knob
• tempo knob
• main CPU "NEC D775G, E2406X" (64 pins zigzag)
• accompaniment CPU "NEC D8048C 316, X23186-211" (same like in Casio PT-7)
• monophonic dull bass tone (like triangular wave) with sustain, using a capacitor envelope.
• analogue percussion {base, snare, open cymbal, close cymbal, clave}; cymbals and snare use transistor noise.

eastereggs:

• one higher bass button key addable

modifications:

• polarity protection diode added, power supply jack polarity corrected.

Interesting is also the very odd way the "fill-in" button behaves; unlike all my other keyboards it does not play a complex pre- programmed drum solo pattern by itself, but only replaces the rest of the current rhythm bar with a simple drumroll pattern made from either 1/4 note base or 1/8 note snare hits (depending on the button press time) that extends to further bars so long the button is held. These fill-ins sound basically just like a fast going metronome and when the button is shortly released and pressed again, it toggles between the 2 patterns "b---b---b---b---" and "s-s-s-s-s-s-s-s-", thus less simple fill-ins can be played by pressing the button multiple times.
 

circuit bending details Unlike the Casio MT-30, the MT-40 contains an additional accompaniment CPU NEC D8048C, which outputs 5 trigger pulses for analogue drums, the bass tone and the bass envelope trigger pulse. It apparently does not communicate with the main voice CPU. There are 4 trimmers on the main PCB; 3 of them control base decay, clave decay and main volume bias(?). The keyboard matrix for the bass button keys and rhythm controls is multiplexed by an external IC "Toshiba TC 4049 BP" (16 pin DIL, marked "4049-1") next to the D8048C. A higher note bass key can be added from IC 4049-1 pin 5 through a diode to pin 19 of the D8048C, but I found no additional preset rhythms. Someone e-mailed me that an "8048" would be a microcontroller that can be equipped with an external 2kB program ROM. I don't know if the "NEC D8048C" accompaniment CPU of this instrument is possibly identical with it. accompaniment shitshot button A lot of wild random crash sounds can be generated by adding a shitshot button from the unused pin 3 of the D8048C (for safety through a 1 kOhm resistor) against GND. Possibly this pin was intended as a serial data input for expansions. In some crash states the thing plays on the bass buttons something like digital modem noises (no tone scale), in others it plays continuous irregular drum solos (like jungle grooves) those can be influenced by pressing bass button keys or switching the rhythm. Sometimes also chirping noises occur, those apparently abuse the drum trigger lines in odd ways.

Another keyboard series with simple monophonic accompaniment and analogue rhythm was the MC-2 hardware class (see Pan Toys MC-7).
 

removal of these screws voids warranty...
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