GPM MC-5000 FM keyboard with accompaniment & programmable rhythm

This keyboard is another strange MC-3 successor. Despite it has the same case style and button layout like an MC-3, it has no simple squarewave sound anymore but a 2 operator FM sound chip with quite warm and noble sounding timbres.

Apparently this thing was technically a close predecessor of the great Fujitone 6A (aka MC-6A) because the sound is very similar. But the operating concept is much like the MC-3, and it still has the great programmable drum pattern ("custom drummer"), although this one is a bit more restricted than on MC-3. The percussion of the MC-5000 is made from 5 typical electronic sounding FM drums. The MC-5000 has 24 preset sounds and 16 preset rhythms (both selected through semi- OBS buttons those each alternatingly select between 2 sounds / rhythms when pressed twice), and the rhythms have intro, fill-in and ending, and each 2 accompaniment variations. The accompaniments are nicely arranged, although they sound fairly establishment and contain nothing really unusual. At least they accept also disharmonic key combinations and not only those few ones considered "chords" by the establishment. The sounds are 2 operator FM timbres those sound quite warm, smooth and noble, although not necessarily realistic. This way the sound generally reminds more to classic expensive home organ or e-piano timbres than to the rough and noisy sample stuff found on cheap modern tablehooters.

It was also released as PanToys MC-5000. Important is not to confuse this instrument with the MC-3000, which has the same case but contains normal MC-3 hardware.

main features:

eastereggs:

  • bass & chord voice select buttons addable (cycling through each 4 preset sounds).
  • 12 higher note keys addable.

    • notes:

    Like the Letron MC-3, also the GPM MC-5000 has a lot of red status LEDs to indicate the selected preset sound and rhythm, although these ones are round, small and quite dim in comparison to the rectangular Letron ones. But there is no LED to indicate which of the 2 sounds or rhythms per button is currently selected, which limits their use. Like with a classic Hammond organ, the keys are very responsive, thus you can easily play glissandos on them. But after years of storage the rubber contacts of most panel buttons worked poorly. Pushing them several times with more force or rotating the round buttons cleaned the contact sufficiently. Thanks to its separate FM sound IC, also this instrument can be easily modified into a simple FM synthesizer, like I did with the Fujitone 6A (and thanks to the programmable rhythm it may be even more interesting to change the percussion this way).

    The preset sounds of this instrument seem to be a real subset of the Fujitone 6A sounds (see there). They have the typical 2 operator FM timbres and most are not programmed really realistic but resemble more classic home organ or e-piano timbres since most have very simple envelopes or contain unrealistic vibrato. But they don't sound unpleasant but have quite warm and noble sounding timbres (less rough than my Yamaha PortaSound FM keyboards) with nice bass range. Unfortunately the MC-5000 features none of the really freaky tekkno noises of the 6A, but mainly acoustic instrument imitations. But at least there are some wahwah-like synth organ timbres ('metallic synth', some brass sounds); also "duck" is no water bird voice but a synth brass sound. The 'bagpipe' sounds not at all like expected, since its sonorous woody tone has a too slow attack rate and a strong tremolo (4Hz). The 'handsaw' is a spring- like resonant metallic timbre that slowly fades silent. The 'synth brass' has a slow attack phase and fades silent. The 'chimes' has a thin metal timbre (like a clock chime) with long sustain. The 'piano' resembles a Rhodes while the 'e. piano' has a brassy timbre, sounds duller and has vibrato. The vibrato of the "vibrato" button mixes with the given one of in some timbres (both have different speeds), and also intensity varies with the actual preset sound, which makes nice textures with some timbres. The "sustain" button sets the sustain (release phase) to a fixed duration of 1 second, which can be shorter or longer than the default sustain duration of that sound. Great is also that (like most FM keyboards) the timbres are time-dynamically playable, i.e. the timbre of notes changes depends on how long a key is pressed, which provides a relatively expressive playability despite the keyboard is not velocity sensitive. Unlike MC-3, the preset sound buttons of the MC-5000 are no really good realtime sound control, because they always stop held notes when sounds are switched, and only a very quiet sustain stays audible for a second.

    The percussion has the typical electronic FM timbre known from the OPL3 "MIDI synth" of early PC soundcards, which is different from the MC-3 percussion. But like the latter (see there), the MC-5000 has still the same great "custom drummer" feature to program your own drum patterns. Unfortunately a small but annoying flaw makes this one way less versatile, namely the user pattern always selects here the accompaniment of the preset rhythm it was programmed with, thus you can not intuitively switch to a different accompaniment anymore, but only select the preset rhythm with the desired new accompaniment and then manually re- enter the entire user pattern again, which is unacceptable for live performance. A strange detail is that without rhythm the polyphony reduces by 2 channels as soon any drumpad is pressed; to achieve full polyphony again, you have to press the "percussion off" button (same like "demo", plays demo when pressed twice). Such a "percussion off" button exists also on the Fujitone 6A.

    The accompaniments are nicely arranged, although they sound brave and fairly establishment and contain nothing really unusual. Interesting is only that there is a "variation" button, with that you can switch back and forward to a second (usually more complex) accompaniment pattern (e.g. with additional arpeggio). Such a button exists on the similar looking Penrod AJ-430, although the latter has squarewave sound (like MC-3) and different patterns. The Fujitone 6A otherwise has no variation button. The individual intro, fill-in and ending patterns (the latter 2 with accompaniment) are a nice detail, although also establishment. Fortunately the fingered accompaniment accepts also disharmonic key combinations and not only a few establishment chords.
     

    circuit bending details

    The GPM MC-5000 is based on the CPU "SC-MC-5" (crystal clocked @ 3.58 MHz) and the FM sound IC "MC9003".

    The CPU is very likely a software variant of the "MC-3DX" found in MC-3 hardware. The different clock rate of 3.58 MHz is also used for the FM IC.
    caution: When I bought my MC-5000 on a flea market, the power amp IC "Samsung KA2206" was toast, thus the instrument played only through 1 speaker while it hummed badly and the IC ran very hot. After replacing the thing, it played perfectly. Possibly a kid had just plugged the AC- adapter into one of the speaker jacks and fried the amp this way, but this instrument generally plays very loud already at only 1/3 of the volume slider range, and the amp IC is powered by the full voltage of the AC- adapter, thus I recommend to set the power supply voltage not higher than necessary (at medium volume only "3V" on my unregulated adapter) despite the instrument has a "DC 9-12V IN" label at the jack.
    pcb number: SOC 7.820.433 This was the broken amplifier IC KA2206.
    Like most FM keyboards, also here many freakish sounds can be produced by interrupting or shorting data lines to the sound IC with each other. If you don't already have one, I recommend to upgrade it with a DipShit synth like I did in Fujitone 6A.

    keyboard matrix

    The keyboard matrix layout has similarities with MC-3, but has more outputs to support 61 keys and 2 digital volume slide switches for chord and rhythm. (The sliders don't need locking switches. Volume 0 = mute.) Also here the matrix outs are multiplexed with panel LEDs. Empty matrix places do nothing. As eastereggs there are bass & chord voice select buttons and 12 higher note keys.
     
    34
    41
    40
    39
    38
    35
    36
    30
    31
    37
     
    CPU pin
    out 5
    out 4
    out 3
    out 2
    out 1
    out 6
    out 7
    out 8
    out 9
    out 10
    out / in
     
    o
    B5
    o
    B4
    o
    B3
    o
    B2
    o
    B1
    -
    tempo
    -
    O.
    horn
    -
    C.
    chord select
    in 12
    4
    o
    A#5
    o
    A#4
    o
    A#3
    o
    A#2
    o
    A#1
    sustain
    tempo
    +
    O.
    bagpipe
    -
    C.
    bass select
    in 11
    3
    o
    A5
    o
    A4
    o
    A3
    o
    A2
    o
    A1
    vibrato
    demo
    O.
    duck
    -
    R.
    volume 0
    in 10
    2
    o
    G#5
    o
    G#4
    o
    G#3
    o
    G#2
    o
    G#1
    P.
    base
    transpose
    +
    O.
    bassoon
    -
    R.
    volume 1
    in 9
    1
    o
    G5
    o
    G4
    o
    G3
    o
    G2
    o
    G1
    P.
    snare
    C.
    off
    O.
    pipe organ
    R.
    bigband
    C.
    volume 0
    in 8
    8
    o
    F#5
    o
    F#4
    o
    F#3
    o
    F#2
    o
    F#1
    P.
    conga
    C.
    single finger
    O.
    jazz organ
    R.
    swing
    R.
    volume 4
    in 7
    7
    o
    F5
    o
    F4
    o
    F3
    o
    F2
    o
    F1
    P.
    open hihat
    C.
    fingered
    O.
    violin
    R.
    disco
    R.
    volume 3
    in 6
    6
    o
    E5
    o
    E4
    o
    E3
    o
    E2
    o
    E1
    P.
    closed hihat
    C.
    variation
    O.
    clarinet
    R.
    waltz
    R.
    volume 2
    in 5
    5
    o
    D#5
    o
    D#4
    o
    D#3
    o
    D#2
    o
    D#1
    R.
    play/space
    transpose
    -
    O.
    flute
    R.
    tango
    C.
    volume 4
    in 4
    16
    o
    D5
    o
    D4
    o
    D3
    o
    D2
    o
    D1
    R.
    program
    R.
    start/stop
    O.
    metallic synth
    R.
    pop
    C.
    volume 3
    in 3
    15
    o
    C#5
    o
    C#4
    o
    C#3
    o
    C#2
    o
    C#1
     -
    R.
    ending
    O.
    mute trumpet
    R.
    march
    C.
    volume 2
    in 2
    14
    o
    C5
    o
    C4
    o
    C3
    o
    C2
    o
    C1
    o
    C6
    R.
    synchro/fill-in
    O.
    piano
    R.
    rhumba
    C.
    volume 1
    in 1
    13

    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
    R.
    		 = preset rhythm
    O.
    		 = preset sound ('orchestra')
    C.
    		 = chord
    P.
    		 = drumpad
    orange
    background     		 = easteregg (unconnected feature)

    • bass & chord select button
      A button at 3->37 cycles the bass voice through 4 available timbres. A button at 4->37 does the same with the chord voice.
    The panel leds are connected through transistors at pins 26..28 (spike frequency 111 Hz?) multiplexed with keyboard matrix out pins 34..41. Also the unused pin 29 has LED demux signal. Strange is that the keyboard matrix inputs pins 1..8 all produce output spikes even when no contact is closed. They are 2 spikes + gap in normal mode and 8 spikes + gap during demo. The gap width fluctuates, so there may be internal bus data accessible, but I guess it is rather a side effect of internal functions (e.g. resetting the port into output mode at start of a routine) than meaningful data. Who knows if there may be 7 segment display or even velocity sensing control pulses embedded, but the pattern looks too regular to do anything complex.
     

    pinout MC9003

    The "MC9003" (24 pin DIL) is the FM sound IC used in some MC-series keyboards made by Medeli. The "MC-9003A" in Fujitone 6A is likely the same and only differs in a clock out pin. Although no exact specification is known and pinout differs, there are strong similarities with the low cost OPLL sound IC Yamaha YM2413B.

    These sound ICs are likely identical with the "UMC U3567" (same pinout) found in Korean slot machines. The similar "UMC UM3567" differs in its DAC output that needs no external 2.2k pulldown resistors (distorts if present), but instruments with MC9003 and MC-9003A both have those resistors installed. (Thanks Traktor for info. Pinout explained on www.msxpro.com. An MP3 demo song on that site seems to sound even a bit clearer when played through U3567 than through a genuine YM2413.)

    The original YM2413B comes in a smaller package (either 18 pin DIL or in SMD as 24 pin SOP) and the position of NC pins differ, but the order of function pins is the same, so possibly the same die got only repackaged to enshroud what it was. Even the specified clock rate of 3.579545 MHz matches well. The YM2413B can do either 9 note polyphonic melodic timbres or 6 melody sounds + 5 rhythm sounds. The IC has 2 analogue outputs for melodic and percussion sounds. Like in the DSG YM2163, the internal 9 bit DAC outputs all channels as timeslices, i.e. the 9 polyphony channels are time-multiplexed analogue signals (an absent or mute channel stays zero) with high resistance sections in between. So the output looks like a comb-like HF pattern that is AM modulated with the audio signal. With monophonic sounds the "envelope" exactly corresponds the audio, but increasing polyphony does not increase the output voltage but the density of the tines (representing channels  "1, 2, 3,... , 8, 9, 1, 2, 3,... ,8, 9..." and so on) with fewer empty gaps in between. While it may look complicated to extract sound from this, like with PWM a simple capacitor against GND is sufficient to demodulate this into an audio signal. The reason for this strange time slicing was likely to avoid additional higher DAC bit hardware for polyphony, because the summing is simply done in the capacitor, so complicated DAC extensions and digital additions could be omitted (despite resulting quality like 12 bit). Theoretically another benefit is that (without capacitor) the polyphony channels can be separated by external sample & hold networks to add more individual outputs.

    The YM2413B was designed for systems with tiny memory, thus its internal ROM already contains 15 preset instrument sounds; only the 16th sound is in RAM and thus freely programmable by an external CPU. But even here most synth parameters have low bit depth (e.g. 4 bit per envelope ADSR value). A variant with changed preset sound set was Yamaha YMF281. I suspect that the actual MC9003 is more versatile, because experiments with its data lines reveal that it can redefine things like percussion pitch, those have no accessible register value in YM2413B specs.

    This pinout is based on my own examination and the Yamaha YM2413B datasheet; pin names were taken from there.
     
    pin name purpose
    1 GND ground 0V
    2 D2 data bus
    3 D3 data bus
    4 D4 data bus
    5 NC  
    6 D5 data bus
    7 D6 data bus
    8 D7 data bus
    9 NC UM3567: GND
    10 XIN clock in (3.579545 MHz) 
    11 XOUT clock reference voltage? | MC-9003A: clock out
    12 NC  
    13 AO register /address data select
    14 /WE write enable
    15 /CS chip select
    16 /IC reset
    17 NC  
    18 MO main audio out
    19 RO rhythm audio out
    20 NC UM3567: GND 
    21 VCC supply voltage +5V
    22 D0 data bus
    23 D1 data bus
    24 NC  

    All "NC" pins are high resistance (tested with 1k against GND and +Vs) and likely unconnected dummy pins. Pin 11 outputs 1.8V DC at high resistance. Touching it distorts tone (mains hum modulates clock on/off). When pulled hi (through 1k resistor) it stops clock; pulled lo (through 1k) does nothing. So it may be no actual clock out but rather a reference voltage. The "MC-9003A" IC in my Fujitone 6A has here a real clock out, showing a triangular wave of about 3 MHz (measured on analogue scope, which is not accurate). NC pins and wiring are the same.

    In GPM MC-5000 and Fujitone 6A pin 14 is connected with 15, i.e. data bus is high resistance while not written. I.e. the CPU can never read data from the sound IC.

    Traktor told me that his First MC-6A (labelled only "First" on instrument, "MC-6A" on the box) contains an "MC9003" (without the A).

    Regarding the CPU type label, also an MC-5000 variant called "MC-5" may exist, which appears to be the genuine name of this hardware class (e.g. MC-3 keyboards (like Letron MC-3) also have "MC-3" in their CPU name). A similar FM keyboard was the MC-55 (case resembling MC-3A with different drumpads. I own one.) A technically direct successor of the MC-5000 was released as Fujitone 6A (aka MC-6A), which has a spacey case design and 100 FM sounds with many great grainy tekkno noises. Another close successor was the fullsize Letron MC-103 (very similar accompaniment).
     

     removal of these screws voids warranty...    
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