Casio MT-36, MT-90

(digital squarewave keyboard with unusual percussion)

from http://weltenschule.de/TableHooters/Casio_MT-36.html

Casio MT-36

This simple Casio beginners keyboard has only 4 rhythms and 6 simple squarewave sounds, but features unusual semi- digital percussion (resembling Casio SK-1). The rhythms always insert a fill-in every 4th bar, and there is a single finger accompaniment (but no manual chord mode).

(picture taken from eBay)

Unusual is that the case plastic is not black but a very dark blue. A white version of this instrument was released as Casio MT-35 and Realistic Concertmate 400. The original German retail price of the Casio MT-35 in a German Conrad catalogue from 1986 was 299DM (about 150€).

main features:

• 44 midsize  keys
• polyphony 8 notes (only 4 notes with accompaniment)
• built-in speaker (with tinny resonance)
• 6 OBS preset sounds {piano, elec. piano, organ, oboe, clarinet, vibraphone}
• 4 OBS preset rhythms {rock, swing, bossa nova, waltz} with automatic fill-in every 4th bar
• tempo slider
• main & accompaniment (with rhythm) volume sliders
• single finger chord (only with accompaniment, no organ chord mode)
• 4 percussion sounds {base, snare, open & closed hihat} of unique electronic style.

base	= dull analogue drum
snare	= dull shift register(?) noise
open & closed hihat	= unique electronic metallic timbre (low- res waveform sample??)

• sound generator is multipulse squarewave. Its digital envelopes (with audible zipper noise) are linear and thus sounds unrealistic because they fade silent too soon.
• main voice CPU(?) "Toshiba TMP8049P, 3437  8523H" (40 pin DIL), accompaniment CPU "HD43720, 56 43" (55 pin SMD)
• demo button (claimed to be "Unterlanders Heimweh" but sounds different, cycles through all preset sounds)
• tuning trimmer
• headphone and power supply jack

eastereggs:

• at least each 2 additional preset rhythms and sounds addable (existing in Casio MT-90).
• sustain switch addable (exists in MT-90)
• switchable automatic fill-in (it is turned off in MT-90)

modifications:

 polarity protection diode added, power supply jack polarity corrected.

notes:

Although the rectangular OBS sound and rhythm select buttons look like locking switches, they don't lock. The main voice timbres resemble Casio VL-Tone 1 and thus are quite unrealistic. They are all made from multipulse squarewaves with different pulse patterns and none of them features vibrato (not even the vibraphone) and there is no button to enable vibrato separately.The percussion of the rhythm features an unusual metallically clicking hihat (made from a waveform sample with zipper noise envelope?) that strongly resembles Casio SK-1. The dull base drum sounds unspectacular and is likely semi- analogue. Also the snare is a bit dull but has a nicely grainy POKEY timbre (likely shift register noise). Unusual is that despite partly digital percussion, this instrument has still a real analogue tempo slider. (The rhythm speed can be adjusted from very low to quite high.) The combination of plain squarewave sounds with polyphony and digital percussion is quite unusual for Casio, because they normally used "Consonant Vowel Synthesis" (2 mixed squarewaves with independent envelopes) in their older polyphonic instruments (see Casio CT-410V for explanation) and timbres based on samples in their later ones.
Despite there is a single finger chord accompaniment, this instrument has no manual chord mode; when the "casio chord" slide switch is on, the thing always starts rhythm when any chord section keys are hit. (Likely this is simply a hardwired synchro start diode in the keyboard matrix; removing it would enable manual chord like with Casio MT-90.)  The rhythm always automatically inserts a fill-in (with accompaniment track when on) every 4th bar; this stupid feature also existed in some Yamaha keyboards (e.g. PS-20 and MP-1), but unlike there it can not even be turned off.
In the manual stands that the demo melody of the MT-36 would be the German folk song "Unterlanders Heimweh", but the tune it plays sounds very different from the wonderful music called "Unterlanders Heimweh" on the ROM-Pack RO-551 (which corresponds to the famous demo of Casio VL-Tone 1) - instead it sounds just like a rural folk waltz and resembles a bit "Little Brown Jug". (Read more about the unofficial Casio anthem "Unterlanders Heimweh" here.) The simple folk waltz tune is also just a very short monoto that cycles through all 6 preset sounds again and again without any complex accompaniment changes or the like, but at least you can play to it or change the preset sound and tempo.
An MT-36 variant with 49 fullsize keys was released as Casio CT-102 (seen on eBay, came out in a black and a white version). A competitive product to the MT-36 was likely the Yamaha PSS-150, which looks and sounds quite similar.

Casio MT-90

This is basically a longer version of the Casio MT-36 with 8 preset sounds, 6 rhythms, more controls and an additional stereo chorus. Unfortunately the demo button was omitted.This instrument was released earlier(?) as Casio MT-200 (seen on eBay), which had an additional slot for a computer interface Casio PA-1 and also came out  in a white version.

main differences:

 49 midsize keys

 2 built-in speakers (mono sound generator is routed through a stereo chorus)

 8 preset sounds {piano, elec. piano, organ, elec. organ, clarinet, oboe, vibraphone, strings}

 6 preset rhythms {rock, disco, swing, waltz, bossa nova, slow rock}

 no (automatic) fill-in

 separate chord & rhythm volume sliders

 sustain switch

 stereo chorus with on/off switch

 single finger chord has manual organ chord with rhythm off

 "bossa nova" rhythm has additional analogue conga drum (instead of snare)

 no demo button

 tuning knob

 jacks for power supply, headphones & line out

modifications:

 polarity protection diode added, power supply jack polarity corrected.

notes:

The outer case shape of this Casio instrument looks like their early keyboards (e.g. MT-60). On eBay I saw the Casio MT-200, which case and control panel text looks identical to my MT-90, but its manual mentions a slot for an optional computer interface Casio PA-1. The case of my MT-90 has no such slot. Next to the tuning knob there is an unpunched hole for an additional jack, which is not supported by the PCB. The MT-200 had the same unpunched hole. (I haven't analyzed the hardware further yet.)The stereo chorus adds a 6Hz chorus vibrato to the main and chord voice when enabled. But even when switched off, it still slowly pans the sound left and right with about 0.5Hz. Unlike my first expectation, there is no fingered chord mode on this instrument. But at least there is a manual organ chord mode with rhythm off (made from squarewave chord + dull bass voice with a little sustain). Unlike MT-36 the "bossa nova" rhythm employs an additional analogue conga drum instead of the snare, and the base drum knocks louder (possibly by better speakers). The percussion also sounds a bit duller, which makes the grainy digital hihats less spectacular. Also the automatic fill-in of the MT-36 is missing, which makes the accompaniment patterns simpler but more versatile.