Pan Toys MC-73 (digital squarewave keyboard with C64 sounds & accompaniment)

This rare squarewave keyboard from 1996(?, part of sound IC number) is yet another small member of the MC-3 hardware family. It has less features but the sound is very similar.

On the keyboard stands no manufacturer, but on eBay I saw that they were released by Pan Toys (and likely others).

main features:

eastereggs:

notes:

The case design (speakers & "voice/style/song" rotary knob) was obviously inspired by the Yamaha PSS-21 (see PSS-31), while the control panel buttons look much like MC-32. Like Pan Toys MC-7, the power LED flashes as tempo indicator. Unless volume is set extremely low, the sound of my specimen sounds rather bassless and somewhat distorted; possibly there is a design flaw in the amplifier.

The main voice sound is much like MC-3 but the "mandolin" rings and vibrato is slower. Also the accompaniment and rhythm sounds like with MC-3, but there is only a single finger accompaniment and no manual chord mode with rhythm off. During accompaniment, also the fill-in pattern contains accompaniment notes. There is no programmable drum pattern, but only the rightmost 5 keys can be switched into keyboard drumkit mode (sounds like MC-3).

There is a simple record/ playback sequencer which records all sounds (maximum 51 notes) and control panel events. Unlike most other such keyboards, the playback tempo can not be changed with the tempo buttons, but instead it even records tempo changes in the rhythm. (Pattern is erased by power off.)

The 8 demo melodies employ the currently selected preset sound. They play monophonic with standard accompaniment and repeat each in a loop.

The demo melodies are:

The misspelled "Unterlanders Heimweh" is not like the melody on Casio VL-1, but corresponds to the folk waltz on Elite MC2200 (see there) and Casio MT-36.

With the "1 key 1 note" button you enter any-key-play mode. For this press first "1 key 1 note" ("record" LED will flash), then "demo" and select a song. No you can step through the song note by note with keyboard keys. Press "1 key 1 note" again to exit. (My Thompsonic TS-37 accidentally came with MC-73 manual, which cleared up this detail.) This seems to be only monophonic with neither rhythm nor accompaniment.
 

circuit bending details

The Pan Toys MC-73 is based on the CPU "MC25-1" (crystal clocked @ 1MHz) with separate sound IC "DSG-MC-3". Unlike all other DSG based MC-series keyboards, this 42 pin CPU has SDIL package (narrower pins) and completely different from "MC-3DX". So it was likely one of the last instruments by Medeli with this sound IC.
When I switched it on after years of storage, it first hummed and distorted badly, which hints for a bad electrolytic capacitor in the power amplifier. But it quickly regenerated itself enough to be useable, so I didn't fix it. But I suspect that the amp is oscillating a bit by HF residues from the spiky DSG output, so some capacitor changes may help to remove distortion.

keyboard matrix

This keyboard matrix layout has nothing common with MC-3 hardware anymore, but keys are grouped by 4 and then divided in 2 horizontal halves which looks very messy. The rotary switch does not need locking contacts. The empty places seem to do nothing. As eastereggs there are 4 lower and 7 higher note keys addable, 5 drumpads and transpose down (different direction, same function).
 
28
27
26
25
12
11
10
9
 
CPU pin
out 1
out 2
out 3
out 4
out 5
out 6
out 7
out 8
out / in
 
 vibrato
tempo
-
R
synchro
P.
snare
o
C#1
o
F1
o
A1
o
C#2
in 1
16
 sustain
tempo
+
chord
on/off
P.
open hihat
o
D1
o
F#1
o
A#1
o
D2
in 2
15
 transpose
-
R.
fill-in
P.
base
P.
closed hihat
o
D#1
o
G1
o
B1
o
D#2
in 3
14
  transpose
+
R.
start/stop
P.
conga
o
C1
o
E1
o
G#1
o
C2
o
E2
in 4
13
o
F2
o
A2
o
C#3
o
F3
o
A3
o
C#4
o
F5
o
A5
in 5
4
o
F#2
o
A#2
o
D3
o
F#3
o
A#3
o
D4
o
F#5
o
A#5
in 6
3
o
G2
o
B2
o
D#3
o
G3
o
B3
o
D#4
o
G5
o
B5
in 7
2
o
G#2
o
C3
o
E3
o
G#3
o
C4
o
E4
o
G#5
o
C6
in 8
1
-
-
-
-
keyboard
percussion
'1'
any key play
-
in 9
20
-
-
-
-
- 
D.
song 
play
-
in 10
19
-
-
-
-
'3'
D.
style
record
-
in 11
18
-
-
-
-
'2'
 D.
voice
demo
-
in 12
17 

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.
 = rhythm
P.
 = drumpad
D.
 = dial knob
orange
background 		 = easteregg (unconnected feature)

pinout MC25-1

The "MC25-1" (42 pin SDIL) is the CPU of the Pan Toys MC-73 keyboard. Important is that this seems to be the only Medeli MC-series 42 pin CPU that is not based on MC-3DX. It has internal ROM, controls a squarewave sound IC "DSG-MC-3" and polls the keyboard matrix through 8 output and 12 input lines (active-low). There are outputs for 5 panel LEDs. It is crystal clocked @ 1MHz and likely a generic microcontroller. 

By lack of schematics, all pin names were chosen by me (partly inspired by Casio naming conventions), based on my own examination. So do not depend on them - they may change if I find schematics or an official datasheet.
 
pin name purpose
1 KI8  key matrix in
2 KI7 key matrix in
3 KI6  key matrix in
4 KI5 key matrix in
5 D3 data bus (to DSG pin 11)
6 D2 data bus (to DSG pin 10)
7 D1  data bus (to DSG pin 9)
8 D0 data bus (to DSG pin 8)
9 KO8 key matrix out
10 KO7 key matrix out
11 KO6 key matrix out
12 KO5 key matrix out
13 KI4 key matrix in
14 KI3 key matrix in
15 KI2 key matrix in
16 KI1 key matrix in
17 KI12 key matrix in
18 KI11 key matrix in
19 KI10 key matrix in
20 KI9 key matrix in
21 +Vs supply voltage +4.65V
pin name purpose
22 /SE sound ic enable (to DSG pin 17,18)
23 L1 vibrato led out (not used) 
24 L2 record led out
25 KO4 key matrix out
26 KO3 key matrix out
27 KO2 key matrix out
28 KO1 key matrix out
29 /RESET reset
30 D7 data bus (to DSG pin 15)
31 D6 data bus (to DSG pin 14)
32 D5 data bus (to DSG pin 13)
33 D4 data bus (to DSG pin 12)
34 X1 crystal in (1 MHz)
35 X2 crystal out
36 NC (high resistance)
37   (lo out)
38 L3 transpose led out 
39 L4 tempo led out
40 L5 chord led out
41 NMI (lo, test?)
42 GND ground 0V

The supply voltage pin 21 is connected through a 47 Ohm resistor. This may be done either to keep spikes out of the analogue circuitry, or to avoid overload damage if the CPU in crash state switches shorted matrix inputs as outputs and draws too high current. But during shitshot I saw no voltages output through key matrix inputs, which may hint that this CPU has dedicated input-only pins.

The unused pin 41 is either an NMI pin without supporting software and/or a test pin. It is lo, and pulling it high (tested with 1k resistor)  immediately locks up the CPU. This usually disables keys and buttons, and often plays accompaniments or demos with wrong timbres. The strangest is that when pulled hi during power-on, all matrix pins stay lo and data bus pins hi; when quickly releasing  pin 41 (resistor removed) and pull hi again, pin 12 outputs a clock divider frequency (4.6µs measured on oscilloscope = about 217 kHz, but sometimes also slower like e.g. 72µs = 13.9 kHz) as long pin 41 stays hi. Repeating this makes the amplitudes change a little, which suggests that the CPU attempts to output serial data over other lines and so differently burdens the supply voltage (fed through 47 Ohm resistor). Possibly the internal ROM can be dumped through a serial protocol here, similar like with Intel MCS-48 microcontrollers. (I haven't examined this further.)

By the CPU type label, also a keyboard variant named "MC-25" may exist, which appears to be the genuine name of this hardware class. The preset sound and rhythm list of the Pan Toys MC-73 resembles Bontempi ES3000 (aka MC-2100) although the latter has different hardware and less polyphony.
 

 removal of these screws voids warranty...    
WarrantyVoid
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