CASIO
MT-750		   midsize ToneBank MIDI keyboard with simple synth

The Casio MT-750 of 1989 (service manual date) was the midsize version of typical midi capable ToneBank home keyboards made after MT-540. Despite similar hardware it focussed on having many instead of complex sounds.

For a midsize keyboard 61 keys and pitchbend was very unusual, but Casio made plenty of fullsize instruments based on this. Unfortunately all these midi keyboards lack the complex algorithmic "PCM" softsynth sounds from SA-series, so most sounds are a bit boring. But the MT-750 wavetable sounds (often mixed waveform samples with clever envelopes) still can contain reverb, echo and stereo effects and have more character than long sample based later CTK-series garbage from GM era. So it can do nice synth pads and strings and has a couple of drumkits. Unfortunately everything is very preset without sustain or vibrato buttons, but with the "tone edit" buttons it has a kind of simple synthesizer that can change 4 parameters of the preset sounds, but nothing can be saved or controlled by midi. The accompaniment is more versatile than average, but can not play non-chords nor change volume of its individual channels. Beside it has no velocity, the the tone quality is still as good as modern beginners keyboard and better than many noname tablehooters.

A fullsize version of MT-750 came out in 1990 (service manual date) as Casio CT-670.

main features:

eastereggs:

  • sustain pedal addable.

    • modifications:

    notes:

    For a midsize keyboard the MT-750 is rather bulky and takes almost as much space as a compact 61 fullsizes keys instrument. But so it also has nice internal speakers with enough bass. While the MT-750 itself is a bit rare, plenty of fullsize variants can be found easily. The timbre quality is more hifi and warmer than SA-series, but the sounds are less complex. Fortunately it still had no audible split zones, those distract in later sample based CTK-series tablehooters and made them sound so cheapish. It rather seems to smoothly blend between some waveforms, which is much more pleasant.

    The preset sounds and rhythms are sorted in an strange way. They have both their own row of cipher buttons. Type one digit to get a "basic" rhythm. Then press "beat bank" button and enter a  further digit for the listed "bank" variants. The same principle is used with preset sounds, those have an additional "select" button for 2 rows and a "tone bank" button. Apparently Casio tried to make the user interface act like (semi-) OBS buttons by placing on each cipher button what they proclaimed to be the most important preset rhythms and sounds (including "select" button for a 2nd row). But if you won't agree with their choice, this idiotic concept makes everything just awkward, because the bank mode behaves like submenus, so to change a sound you have to press the "tone bank" button again to exit the submenu, press a cipher button (and sometimes "select") for a new main sound, again "tone bank" to dive into its submenu and another cipher button to select the item. Yuck! - Most newer keyboards (and even their own SA-series) simply use multi-digit numbers.

    Instead of a proper keysplit mode, there is only a preset sound category "split" with 11 fixed combinations. Some synth effect sounds with same name like in SA-series are totally different. Instead of wicked program loop synthesis noises the most complex sounds here can only do siren envelopes, echo, panning, loop samples and can contain split points with different sounds on keyboard sections (like drumkit modes). This instrument tries hard to keep a hifi appeal and contains nothing distorting or glitchy. It sounds not bad, but does only things you would expect from a basic synth with many standard envelopes and no VCF, and nothing that in any way surprises or puzzles you how this effect could be done.

    Like in many "modern" home keyboards, the user interface feels somewhat intrusive. So the chord section stubbornly refuses to sound non-chords, and its behaviour (a chord sounds until all chord section keys are released) and sound can not be changed. The accompaniment styles have each a short and long intro and ending (long is "coda"), but you can not make it dynamically cancel an ending pattern by switching to break, intro or fill-in (a feature I remember from Yamaha). There are mute buttons for 3 parts {chord 1, chord 2, obligato}. Depending on rhythm, "chord 2" can be e.g. the arpeggio. But the mixing ratio of part volumes can not be changed. The MT-750 includes some nicely unusual accompaniments like oriental or japanese styles. Many are overorchestrated, but can be tamed by muting parts. "auto harmonize" plays main voice as trio matching the chord. Most spectacular is "echo line", which replies to your own melody notes delayed on a chord voice; this is an rudimentary kind of realtime composing algorithm, that I only knew from Technics K350.

    They "tone editor" synth buttons "detune" (honkytonk piano effect) and "delay" affect the subvoice of the right stereo channel. These panning gimmicks resemble Casiotone 7000. The "attack/decay" up button makes later attack and slower decay and down the opposite. "release" changes the attenuation. The values can be neither displayed nor saved nor controlled by midi, and get reset by selecting a preset sound.

    The demo song is a funky pop track "The Way that you Love Me" by Paula Abdul.
     

    hardware details

    The Casio MT-750 is built around the CPU "NEC D937GD 003" with DAC "Sanyo LC7880", external 1MB ROM "Panasonic MN238000CUB" and 8KB SRAM "OKI M5165AL-12".
    The hardware is technically similar like MT-540 (see there) but the ROM samples are scrambled by swapped address and data pins. It is unknown whether Casio did this as a crude copy protection or to simplify PCB layout. My MT-750 of july 1991 has one 1MB 16-bit ROM "Panasonic MN238000CUB" (and empty holes for a 2nd ROM with traces for the upper half ending nowhere), but its service manual (november 1989, even in part list) depicts in 8-bit mode 2x ROM "uPD23C4000A" with software numbers 65 and 66. Also the almost identical fullsize version CT-670 has those 2x 8-bit ROMs (told by e-mail and seen in service manual).

    The tone editor synth is described in patents US5044251 and US5177314.

    keyboard matrix

    This keyboard matrix is based on the Casio MT-750 and CT-670 service manual. I haven't analyzed it by myself, so there may be still unknown eastereggs. Particularly low-end keyboards of this hardware class (e.g. CT-655, CT-657) have less features by omitted buttons, which perhaps was the main reason for designing the awkward user interface. The CT-680 (CT-670 variant with reverb) has no separate rhythm volume switch, so it likely has its accomp volume slider connected through diodes to KO15 and KO16, which may be undone to control both separately.

    Like with MT-540, the hair thin CPU pins are not safe to mess with, because they tend to short by debris. Thus measure at PCB solder joints instead.
     
    30 KI1
    29 KI2
    28 KI3
    27 KI4
    26 KI5
    25 KI6
    24 KI7
    23 KI8
     
    CPU pin
    in 1
    in 2
    in 3
    in 4
    in 5
    in 6
    in 7
    in 8
    in / out
     
    tempo
    -
    tempo
    +
    o
    C1
    o
    C#1
    o
    D1
    o
    D#1
    o
    E1
    o
    F1
    out 0
    19 KO0 
    SY.
    detune
    +
    SY.
    detune
    -
    o
    F#1
    o
    G1
    o
    G#1
    o
    A1
    o
    A#1
    o
    B1
    out 1
    20 KO1
    SY.
    delay
    +
    SY.
    delay
    -
    o
    C2
    o
    C#2
    o
    D2
    o
    D#2
    o
    E2
    o
    F2
    out 2
    21 KO2
    SY.
    attack/decay
    +
    SY.
    attack/decay
    -
    o
    F#2
    o
    G2
    o
    G#2
    o
    A2
    o
    A#2
    o
    B2
    out 3
    22 KO3
    SY.
    release
    +
    SY.
    release
    -
    o
    C3
    o
    C#3
    o
    D3
    o
    D#3
    o
    E3
    o
    F3
    out 4
    49 KO4
    S.
    start/stop 
    S.
    record
    o
    F#3
    o
    G3
    o
    G#3
    o
    A3
    o
    A#3
    o
    B3
    out 5
    48 KO5
    R.
    synchro
    R.
    start/stop
    o
    C4
    o
    C#4
    o
    D4
    o
    D#4
    o
    E4
    o
    F4
    out 6
    47 KO6
    C.
    obligato
    demo
    o
    F#4
    o
    G4
    o
    G#4
    o
    A4
    o
    A#4
    o
    B4
    out 7
    46 KO7
    C.
    chord 2
    C.
    chord 1
    o
    C5
    o
    C#5
    o
    D5
    o
    D#5
    o
    E5
    o
    F5
    out 8
    45 KO8
    C.
    echo line
    C.
    auto harmonize
    o
    F#5
    o
    G5
    o
    G#5
    o
    A5
    o
    A#5
    o
    B5
    out 9
    44 KO9
    C.
    fill-in
    C.
    break
    o
    C6
    O.
    select
    O.
    tone bank
    R.
    beat bank
    C.
    ending
    C.
    coda 
    out 10
    43 KO10
    R.
    '5'
    R.
    '4'
    R.
    '3'
    R.
    '2'
    R.
    '1'
    R.
    '0'
    C.
    intro (long)
    C.
    intro
    out 11
    42 KO11
    O.
    '3'
    O.
    '2'
    O.
    '1'
    O.
    '0'
    R.
    '9'
    R.
    '8'
    R.
    '7'
    R.
    '6'
    out 12
    41 KO12
       
    O.
    '9'
    O.
    '8'
    O.
    '7'
    O.
    '6'
    O.
    '5'
    O.
    '4'
    out 13
    40 KO13
    C.
    off
    C.
    fingered
    C.
    single finger
    C.
    midi
     
    power
    on
    power
    off
     
    out 14
    39 KO14
    R.
    volume 0
    R.
    volume 1
    R.
    volume 2
    R.
    volume 3
    R.
    volume 4
    		  
    sustain pedal
    		  
    out 15
    38 KO15
    C.
    volume 0
    C.
    volume 1
    C.
    volume 2
    C.
    volume 3
    C.
    volume 4
    		      
    out 16
    37 KO16

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

    legend:
    "o"
    		 = keyboard key
    underlined
    		 = function needs locking switch (i.e. stays active only so long the switch is closed)
    R.
    		 = preset rhythm
    O.
    		 = preset sound ('orchestra')
    C.
    		 = chord/accompaniment
    S.
    		 = sequencer
    SY.
    		 = synthesizer
    orange
    background    		 = easteregg (unconnected feature)
    grey
    background    		 = unconnected doublet

    The sustain pedal exists in CT-670, where it is wired through an inverter and transistor. You may wire a switch here to hold notes.

    The panel LEDs are latched from pins KO0..KO5 by 74HC174-2 during a pulse on KO18  and by 74HC174-3 during a pulse on KO17. The LED display is demuxed through HC4511AP from pins KO0..3 during a pulse on KO19 and the segments are latched by 74HC174-1 from KO0..KO3 during a pulse on KO20.

    pinout D937GD

    The "NEC D938GD xxx" CPU (120 pin SMD, pins count counterclockwise, xxx = software number of internal ROM) was used in many early Casio ToneBank keyboards with midi. It has fully digital sound envelope processing and mixing. All volume controls of individual sound channels (rhythm, accompaniment etc.) are digital. Polyphony is 12 notes. The digital audio channels are output as multiplexed serial data for left & right channel to an external 16 bit DAC (Sanyo LC7880). The CPU has 3 ADC inputs for reading analogue signals (e.g. pitchbend). An external SRAM "NEC D4364C" is used for sequencer. Sequencer RAM and LED lines are multiplexed with the keyboard matrix. The CPU needs an external 16bit ROM (or 2x 8-bit) for wavetable samples and data. Despite this it also has also internal ROM, which seems to contain things like basic key matrix and user interface behaviour (those stay present even with external ROM removed).

    The MT-750 and CT-670 have software number "003".

    This pinout is based on service manuals of Casio MT-750 and CT-670.
     
    pin name purpose
    1 VDD2 supply voltage +5V (when power on)
    2 /POFF power off signal out (not used)
    3 TEST1 test (wired to VDD1)
    4 TEST2 test (wired to VDD1)
    5 RESET1 reset
    6 MI power on trigger in
    7 TCLK (wired to VDD1)
    8 COSI crystal in | clock in (21.7248 MHz)
    9 COSO crystal out
    10 VDD1 supply voltage +5V (always on)
    11 MPG 4 MHz clock test out (unbuffered)
    12 GND ground 0V
    13 MOSI midi & rhythm tempo crystal in | clock in (4MHz)
    14 MOSO midi & rhythm tempo crystal out
    15 INTF (not used)
    16 INT4 (wired to VDD1)
    17 MOUT midi-out
    18 MIN midi-in
    19 KO0 key & led matrix out
    20 KO1 key & led matrix out
    21 KO2 key & led matrix out
    22 KO3 key & led matrix out
    23 KI8 key matrix in
    24 KI7 key matrix in
    25 KI6 key matrix in
    26 KI5 key matrix in
    27 KI4 key matrix in
    28 KI3 key matrix in
    29 KI2 key matrix in
    30 KI1 key matrix in
    31 KO22 APO auto power-off /out
    32 KO21 (not used)
    33 KO20 led matrix latch out
    34 KO19 led matrix latch out
    35 KO18 led matrix latch out
    36 KO17 led matrix latch out
    37 KO16 key matrix out, ram address A12
    38 KO15 key matrix out, ram address A11
    39 KO14 key matrix out, ram address A10
    40 KO13 key matrix out, ram address A9
    41 KO12 key matrix out, ram address A8
    42 KO11 key matrix out, ram address A7
    43 KO10 key matrix out, ram address A6
    44 KO9 key matrix out, ram address A5
    45 KO8 key matrix out, ram address A4
    46 KO7 key matrix out, ram address A3
    47 KO6 key matrix out, ram address A2
    48 KO5 key & led matrix out, ram address A1
    49 KO4 key & led matrix out, ram address A0
    50 /WE1 ram write enable
    51 /OE1 ram output enable
    52 /CE1 ram chip enable
    53 D0 ram data bus
    54 D1 ram data bus
    55 D2 ram data bus
    56 D3 ram data bus
    57 D4 ram data bus
    58 D5 ram data bus
    59 D6 ram data bus
    60 D7 ram data bus
    pin name purpose
    61 IO0 rom1 data D0 in
    62 IO1 rom1 data D1 in
    63 IO2 rom1 data D2 in
    64 IO3 rom1 data D3 in
    65 IO4 rom1 data D4 in
    66 IO5 rom1 data D5 in
    67 IO6 rom1 data D6 in
    68 IO7 rom1 data D7 in
    69 IO8 rom2 data D4 in
    70 IO9 rom2 data D3 in
    71 IO10 rom2 data D5 in
    72 IO11 rom2 data D2 in
    73 IO12 rom2 data D6 in
    74 IO13 rom2 data D1 in
    75 IO14 rom2 data D7 in
    76 IO15 rom2 data D0 in
    77 BSEL (wired to GND)
    78 AR0 rom address A16 out
    79 AR1 rom address A0 out
    80 AR2 rom address A15 out
    81 AR3 rom address A1 out
    82 AR4 rom address A14 out
    83 AR5 rom address A2 out
    84 AR6 rom address A13 out
    85 AR7 rom address A3 out
    86 AR8 rom address A12 out
    87 AR9 rom address A4 out
    88 AR10 rom address A11 out
    89 AR11 rom address A5 out
    90 AR12 rom address A10 out
    91 AR13 rom address A6 out
    92 AR14 rom address A9 out
    93 AR15 rom address A7 out
    94 AR16 rom address A8 out
    95 AR17 rom address A17 out
    96 AR18 rom data D15 in
    97 AR19 rom chip enable /CE 
    98 AR20 (not used)
    99 AR21 (not used)
    100 /CE rom data out enable
    101 /OE (not used)
    102 /WE (not used)
    103 /CEP (not used)
    104 RFSH (not used)
    105 MCLK main clock test out (2.7156MHz = 1/8 of 2.17248MHz)
    106 SWD4 (not used)
    107 BCLK DAC bit clock out
    108 SWD3 (not used)
    109 SWD1 (DATA) serial digital audio data out (16 bit multiplexed left & right channel)
    110 SWD2 (not used)
    111 WCLK1 DAC word clock out
    112 LRCK DAC left /right channel separation signal
    113 STEST  test (wired to VDD1)
    114 VRT ADC high level reference voltage (connect to DVDD +5V)
    115 ADVDD ADC supply voltage +5V
    116 Vin0 ADC pitchbend in
    117 Vin1 ADC analogue in (wired to 116)
    118 Vin2 ADC analogue in (wired to 116)
    119 ADGND ADC ground 0V
    120 VRB ADC low level reference voltage (wired to ground 0V)

    Unlike MT-540, the rom pins are shuffled in a strange order. The data D# pins of rom2 (from MT-750 and CT-670 schematics) likely correspond to the upper bits of the single 16-bit ROM version used in later MT-750 models. The ROM contents is scrambled corresponding to the pin wiring, so the CPU likely sees its address space unscrambled.

    This CPU is a close relative (possibly software variant) of the D938GD used in Casio MT-540.

    A fullsize version of MT-750 came out as Casio CT-670. This keyboard was featured in "Casio Keyboard Lesson" VHS tapes with Richard Clayderman, where he e.g. played the classical piano piece "Ballade Pour Adeline". This keyboard was also released as Hohner PSK-75. A version with "digital reverb" instead of rhythm volume slider was Casio CT-680 (aka Hohner PSK-75R). A 49 keys variant (no pitchbend) was Casio CT-470 (aka Hohner PSK40 / PSK45). A variant with only 110 sounds was CT-655 / CT-656 (61 keys, no pitchbend, no coda, only 56 rhythms by fewer buttons, different sound order = changed rom? (demo is the same) | aka Hohner PSK60) and its even more crippled variant CT-657 (no midi, no detune, no break, no line echo, different demo | aka Hohner PSK65).

    attention: When a Casio keyboard has a "210 Sound Tone Bank" or "465 Sound Tone Bank"  label, then it generally seems to have only 20 or 30 preset sounds (of those only any 2 can be layered to form that many dual voice combinations) and thus has genuinely fewer individual preset sounds than the later Casio "100 ToneBank" instruments (of those e.g. my CT-840 still can layer any 2 sounds and thus can form even many more combinations).
     

     removal of these screws voids warranty...    
    WarrantyVoid
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