New! Version 1.4 now available
 

Brief functional description:
    The retrofit circuit emulates key presses according the data it gets from MIDI and the state of the synth's keyboard matrix. It doesn't interfere with the analog circuitry of the synth and therefore is completely transparent: you can still play the synth even if MIDI data is received.

Features (version 1.4):
    Receives MIDI notes. MIDI channel which retrofit responds to is set by System Exclusive message and stored in non-volatile memory. Continuous controller number #120 or higher turns off all the voices (panic).

Zip file with schematics, layout and program code (.hex, v1.4) files
 

Part list:

Resistors (1/4W carbon or metal film):
1x 220R (R1)
1x 360R (R2)

Capacitors:
1x 100nF ceramic multilayer (C1)
2x 33pF ceramic (C2,C3)

Semiconductors
9x 1N4148
1x 6N137 (IC1)
1x PIC16F84 (IC2)
1x 74HC148 (IC3)

Cristal or ceramic resonator:
1x 8MHz (X1)

Other:
1x DIP 18pin IC socket
PCB connectors (2pcs 2pin, 1pc 3pin, 1pc 4pin) or
    break-a-way strip line connector
1x DIN 5pin/180degrees female panel-mount connector
Some cable (preferably flat and color-coded)

Retrofit installation:
    When you have assembled retrofit board, you'll have to connect it to Polysix' keyboard circuit. First of all unscrew screws at the sides of front panels and four screws at the back of the bottom of a synth. Lift front panel. Find a 16 pin keyboard connector with white and black wires in the middle of the synth, aprox. 10cm above the keyboard, and power lines (ground and +5V).

    When you located all that, prepare the PCB connectors: solder wires to two 2 pin, and to four 4 pin connectors. Now solder the wires of first 2 pin connector to GND and +5V and wires of 4 pin connectors to keyboard connector. Also solder a 5 pin female DIN connector (pins 4 and 5) to the wires of the other 2 pin connector.

    Now for a little mechanical engineering - you'll have to drill hole for DIN connector on the back side of the synth. Place the paper towels over the circuit to protect if from filings. When drilling use low RPM (you don't want filings all over the place) and start with small drill. If you don't have drill large enough you'll have to use round file to make hole big enough. When you are finished use some magnetized tool to collect all the fillings from paper towel, then remove the towel and examine the circuit again. Now screw DIN connector to the back side.

    Next thing is placing the retrofit board somewhere inside the synth. You are on your own here, since I haven't mounted mine yet.

    The last thing is to connect all the connectors to retrofit board. Connections should be made as follows:
    MIDI connector:
        DIN pin 4 -> J1 pin 1
                5 -> J1 pin 2
    Power
        GND -> J2 pin 1
        +5V -> J2 pin 2

    Keyboard connector:
        P10 -> J3 pin 1       DB0 -> J5 pin 1
        P11 -> J3 pin 2       DB1 -> J5 pin 2
        P12 -> J3 pin 3       DB2 -> J5 pin 3
        P13 -> J3 pin 4       DB3 -> J5 pin 4
        P14 -> J4 pin 1       DB4 -> J6 pin 1
        P15 -> J4 pin 2       DB5 -> J6 pin 2
        P16 -> J4 pin 3       DB6 -> J6 pin 3
        P17 -> J4 pin 4       DB7 -> J6 pin 4

System Exclusive message:

F0h      - SysEx start
7Dh      - non-commercial devices ID
42h      - ASCII 'B' character (my initials)
42h      - ASCII 'B' character (my initials)
02h      - device (project) number
01h      - parameter number
channel  - MIDI channel-1 [values: 00...0Fh]
F7h      - SysEx end

Example - unit should receive on MIDI Channel 1:
F0h,7Dh,42h,42h,02h,01h,00h,F7h