Casiotone MT-45 pretty semi-analogue keyboard with arpeggio accompaniment

This beautiful white Casio keyboard of 1982 (service manual date) has wonderful warm semi-analogue sounds, arpeggio accompaniment and sonorous droning multipulse squarewave bass tones resembling 'organ' on Hing-Hon EK-001.

Despite its age, the MT-45 control panel design with its blue lines, pastel coloured softline shaped knobs and switches looks still noble and modern and resembles a bit the "Aqua" user interface theme of Apple Macintosh computers.

This instrument was re-released as Casio MT-46 with red and less pretty control panel ornaments.

main features:

Note the elegant control panel design.

eastereggs:

This instrument employs the same very versatile accompaniment CPU like the Casio CT-410V, thus there are lots of keyboard matrix eastereggs available.  (For technical details see there)
  • 4 additional rhythms (rock, latin swing, bossa nova, slow rock)
  • 2 additional bass variations (III, IV)
  • 3 additional accompaniment variations (II, III, IV)
  • 3 additional arpeggio variations (II, III, IV)
  • 1 lower & 5 higher additional main voice keys
    • On the PCB with the sound output jack are empty component holes for an additional jack (volume pedal?). I didn't upgrade my MT-45 because I already own the CT-410V, which had most of these features and I enabled all the hidden ones.

    modifications:

    notes:

    I bought this instrument in very dirty state, and during transport(?) the plastic tab/ hook under one key cracked off from accidentally bending it too far up. (I replaced the tab with a piece of insulated copper wire and hotglue.) Someone had written the key names above the keys with blue Edding; my attempt to remove them with isopropanol also damaged the blue and black stripe pattern printed on the case there. Unusual with this keyboard is, that the keys are made from separate plastic parts and not hanging on one common, bendable plastic strip; thus I assume that it must be very old. The silicone contacts below are also separate parts and were stained with dirt and a sticky stuff (likely adhesive tape glue from the felt strip above); although I cleaned all these parts thoroughly, the keys still don't respond perfectly.

    The MT-45 hardware employs the same very versatile D930G accompaniment CPU like the Casio CT-410V (and many other Casio instruments), although here only few of its features are used. Unfortunately it was combined with a foreign main voice CPU that is too stupid to communicate with it (using its own keyboard matrix), so the accompaniment section at the left half of the keyboard can not be switched off, i.e. these keys can not be used for main voice play when neither chords nor manual bass is wanted. The only benefit is that the polyphony is not reduced by accompaniment.

    This instrument plays high quality analogue timbres, those although not always natural, reproduce a warm and very pleasant sound; it does not sound typically C64- like thin, but resembles rather full- size home organs of that era. Like with Casio VL-1, the release phase of main voice envelopes seems to be linear and thus sounds unrealistic since it fades silent too soon with audible end click. When sustain is switched off, all sounds stop almost immediately after releasing the key. With sustain on, sounds with decay envelope (piano, elec. piano, harp) ignore the key press duration and sound always with a fixed duration instead. The "elec. piano" sounds like a banjo, and also the normal piano resembles more a picked string. The "harp" and "organ" sound like adding a bit of analogue distortion during attack phase (it turned out to be indeed a suboscillator with very short independent envelope).

    The 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". The available styles on the MT-45 correspond to chord variation "I", bass variations "I" & "II" and the default arpeggio (= variation "I"?) of my CT-410V. The percussion of the MT-45 are analogue and resemble the CT-410V, although it has 2 drums less than the latter. Interesting is that the MT-45 produces a much softer, duller and more pressureful bass than the CT-410V, which timbre resembles a triangular wave or Roland TB303 (without resonance), while the CT-410V bass is a more sonorous droning ordinary squarewave tone. The MT-45 sounds also brighter than the initially quite muffled CT-410V, and generally I find its timbre somehow fresher and clearer and thus like it a bit better, despite the great CT-410V has far more capabilities. By sound and functions the Casio MT-45 instrument is mostly comparable with my Yamaha PS-2, but unlike the latter it is not that extremely compact and features more rhythms, sounds and nice sounding arpeggios. Even the internal speaker is of very reasonable quality. I only yearningly miss independent volume control knobs for the individual accompaniment voices (fortunately my CT-410V has them).
     

    circuit bending details

    The electronics of the MT-45 employs the same very versatile D930G accompaniment CPU like the Casio CT-410V (and many other Casio instruments), although here only few features of it are used. Unlike in many other Casio keyboards, here it is not combined with the versatile D931C main voice soundchip (that it was dedicated for?), but a foreign main voice CPU "Hitachi HD44140" (same like in the rhythmless Casio MT-11) with only 8 preset sounds.
    Unlike D931C, the HD44140 does not communicate at all with the D930G, but uses a completely independent keyboard matrix for all main voice related keys and switches. (Possibly Casio had not finished the D931C yet when the MT-45 went in production.) Unfortunately this results in that the accompaniment section at the left half of the keyboard can not be switched off, thus these keys can not be used for main voice play when neither chords nor manual bass is wanted. The only benefit is that so the polyphony is not reduced by accompaniment. But the HD44140 supports only 37 keys anyway, thus in a 49 keys instrument it would make little sense to circumvent this by complicated external logics.

    To the right of the HD44140 is a trimmer that seems to be the level adjustment for the highest main voice bit. Another trimmer to its left was omitted (empty solder holes), that was intended to tweak the level ratio between low and higher bits to minimize distortion. The trimmer even further left seems to set the main volume. The MT-45 has 2 strange black square crystal oscillators. That at the D930G has a "6596, KSS3B" (PCB mark "X1 (6.6M)"?) while that of HD44140 is printed "5752, KSS3A" (PCB mark "X2 (5.7M). So I guess that the upper number is the frequency in kHz. Casio likely used crystals here because else it would have been hard to keep D930G and HD44140 play in tune with each other, because their clock ratio of 1.14673157163:1 would be hard to produce by simple frequency dividers.
    On the amp PCB there is a section of unused solder holes those look like made for a sustain pedal jack (not supported by HD44140) or tuning trimmer (not used due to crystal clock); its 3 unused foil cable solder pads go into nowhere.

    rhythm & accompaniment

    About the rhythm and accompaniment section and the tons of hidden D930G eastereggs see Casio CT-410V.

    main voice keyboard matrix

    The main voice is produced by the self-contained sound CPU Hitachi HD44140, which has only few functions and runs completely independent from the accompaniment. Very unusual for Casio is its keyboard matrix layout with 16 output and only 3 input lines. Except 6 unused keys I found no eastereggs.
     
     6 KC1
    7 KC2 
    8 KC3
    9 KC4
    10 KC5
    11 KC6
    12 KC7
    14 KC8
    15 KC9
    16 KC10
    17 KC11
    18 KC12
    19 KC13
    20 KC14
    22 KC15
    23 KC16
     
    CPU pin
    out 1
    out 2
    out 3
    out 4
    out 5
    out 6
    out 7
    out 8
    out 9
    out 10
    out 11
    out 12
    out 13
    out 14
    out 15
    out 16
    out / in
     
    O.
    clarinet
    O.
    organ
     O.
    harp
    o
    F#0
    o
    A0
    o
    C1
    o
    D#1
    o
    F#1
    o
    A1
    o
    C2
    o
    D#2
    o
    F#2
    o
    A2
    o
    C3
    o
    D#3
    sustain
    in 1
    3 KI1
     O.
    violin
    O.
    pipe organ
     O.
    piano
    o
    G0
    o
    A#0
    o
    C#1
    o
    E1
    o
    G1
    o
    A#1
    o
    C#2
    o
    E2
    o
    G2
    o
    A#2
    o
    C#3
    o
    E3
    vibrato
    in 2
    4 KI2
     O.
    accordion
    O.
    elec. piano
    o
    F0
    o
    G#0
    o
    B0
    o
    D1
    o
    F1
    o
    G#1
    o
    B1
    o
    D2
    o
    F2
    o
    G#2
    o
    B2
    o
    D3
    o
    F3
     
    in 3
    5 KI3

    The input lines are active-low, i.e. react on GND. Any functions can be triggered by a non- locking switch in series to a diode from one "in" to one "out" pin.
     

    legend:
    "o"
    		 = keyboard key
    underlined
    		 = function needs locking switch (i.e. stays active only so long the switch is closed)
    O.
    		 = preset sound ('orchestra')
    orange
    background     		 = easteregg (unconnected feature)

    Despite the case shape and button layout of the MT-45 is identical with the (older?) Casio MT-60, the latter contains a much bigger PCB with totally different hardware, which features far more sound presets but thinner sounding accompaniment.
     

     removal of these screws voids warranty...    
    WarrantyVoid
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