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 1.21):
    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).

Note: The circuit was tested only on Trident mkII and I don't know if the original Trident uses same keyboard scanning circuit board (KLM-377). If someone out there owns a Trident mkI or has schematics for it, please mail me... thanks!

Schematics

PCB - component side
PCB - solder side

Zipped Program (.hex) file (version 1.21)
 

Part list:

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

Capacitors:
1x 10nF ceramic multilayer (C1)
2x 33pF ceramic (C2,C3)

Semiconductors
7x 1N4148
1x 6N137 (IC1)
1x PIC16F84 (IC2)

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 Trident's keyboard circuit. First of all unscrew screws at the sides of front panels, two small screws on back panel and four screws at the back of the bottom of a synth. Lift front panel. Don't get intimidated by the circuits complexity, you'll have to deal with just one board- the KLM-377, which resides at the left, above the pitch/modulation joystick. Now you'll have to find the chip labeled IC 12 (4514 1-of-16 demultiplexer, lower left corner of the board), 6 pin connector 02 (left edge), and power lines (ground and +5V).

    When you located all that, prepare the PCB connectors: solder wires to 2 (2 pieces), 4, and 6 pin connectors. Now solder the wires of one 2 pin connector to GND and +5V, wires of 4 pin connector to IC 12 (pins 2[LSB], 3, 21, and 22[MSB]), and wires of 6 pin connector to connector 02. Now solder a 5 pin female DIN connector (pins 4 and 5) to the wires of the other 2 pin connector. OK, most of the work is done now, relax a little...

    Now for a little mechanical engineering - you'll have to drill hole for DIN connector on the back side of the synth. There is enough place under the jack connectors to do that, but first move all that wires out of the way. Now 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 (pay attention to wires that run at the back, below the jacks). Now screw DIN connector to the back side.

    Next thing is placing the retrofit board somewhere inside the synth. There are quite a few options since there is a lot of place inside. I placed mine on top of the KLM-377 board and fix it with the already existing screw in the middle of the board (I've put a spacer between KLM-377 and retrofit board to make sure the components on KLM board are not touching the retrofit board). Second option is screwing a board on left, wooden side of the synth.

    The last thing is to connect all the connectors to retrofit board. The most important is to connect the power connector the right way around (GND pin is marked with line on the component side of PCB layout). Other connection should be made as follows:
    MIDI connector:
        DIN pin 4 -> J1 pin 1 (marked pin)
                5 -> J1 pin 2

    DeMUX connector:
        4514 pin 22 (Data 4) -> J2 pin 1 (marked pin)
                 21 (Data 3) -> J2 pin 2
                  3 (Data 2) -> J2 pin 3
                  2 (Data 1) -> J2 pin 4

    Output connector:
        02 pin 1 -> J4 pin 1 (marked pin)
               2 -> J4 pin 2
               3 -> J4 pin 3
               4 -> J4 pin 4
               5 -> J4 pin 5
               6 -> J4 pin 6

    THAT'S IT! Enjoy...

Picture of installed retrofit prototype in my Trident

System Exclusive message:

F0h      - SysEx start
7Dh      - non-commercial devices ID
42h      - ASCII 'B' character (my initials)
42h      - ASCII 'B' character (my initials)
01h      - 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,01h,01h,00h,F7h