Korg CX-3 (vintage) MIDI kit retrofit installation manual Vitalka Custom
This retrofit connecting directly to key contacts or SM304A shift register ICs. Required to solder 70 soldering joints.
Retrofit receives MIDI notes and triggering circuit boards the same as you playing on the keyboard with hands and triggering key contacts. It controls only the keyboard, all other controls in your hands and you still can play with the keyboard at the same time when it controls with MIDI.
Welcome to the metric system zone. Here we work with precision things and all dimensions are metric. 1" = 25.4mm.
Installation:
To your good usability select the friendly place to mount the LED, DIN5 MIDI connector, button and PCB. Mount connector, LED and button before soldering wires. PCB can be mounted under the keyboard on the left side. Drill holes and to mount it before soldering wires.
Drill dimensions:
MIDI jack (DIN5) = 18mm.
LED holder = 6mm.
Push button = 12.5-13mm.
Be careful with drilling hole for push button, it have so small range to get hole too big. See below dimension ranges from their datasheet. Drill a bit less and file it to fide fitting.
Board dimensions and distance between screws:
Dimensions 102x50mm. Height 14mm with inserted connectors.
Holes diameter 3.5mm.
Mounting outline and hole drills draft for board example sticker paper included.
Do not allow the board to bend, it can break the board.
This picture shown first (1) pin/wire on all connectors: 
All next wire should be connected to each key contact or to the main board:
The next picture shows the pinout for 14 and 8 wire ribbon cables. And also pin number of wires: Same orientation when board facing to you.
More visual wiring:
(7) V+ and (8) GND can be soldered to one of SM304A ICs. (Wire 7) to pin 19. (Wire 8) to pin 1. Or at power supply to +12V.
J1,J2,J3,J4 should be close.
All next wire should be connected to each key contact or to the main board:
Board pinout and MIDI note:
All note wires can be connected to every note key contact or directly to SM304A ICs (use CX-3 schematic to references and table below). Common bus wire should be soldered to ground or common bus for key contacts.
At this point your brain can blow. Let me explain:
Find key contacts for note (C) on the first octave and solder wires from the YELLOW connector. Starting from wire 1. When you finish it, your last wire will be 12 and note (B) on this octave.
On octave 5 RED, you also should start from wire 1 (C). Wire 13 solder to key contacts common bus. Wire 14 (C) should be soldered to last key note C.
One small important thing - use hot glue to fix inserted connectors to protect from disconnecting them during vibration when you ride on tour. Fix wires strong. Do not leave them holding only on soldering joints.
So, looks like installation finished. Now time to explain how set MIDI channel. This is what you need to know for starting use this retrofit.
Process for set MIDI channel: User operations
Stop all MIDI traffic.
To teach with which MIDI channel synth will work, push and hold the LEARN button for a 1-2sec. LED will blink fast four times and stay ON to wait for MIDI messages at one of 16 channels.
Send any note on the MIDI channel which you want for this synth.
When the synth receives MIDI messages, LED will blink fast four times and turn OFF. It indicates that channel was received and saved. Then will ON with every NOTE ON messages on the selected channel.
Now the channels saved to memory and will never change before you initiate the settings process again.
In case of PANIC, or you want to exit from settings - Short push the LEARN button.
PANIC - Short push
LEARN - Push for 1-2sec. LED blink fast.
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At moment when synth power on, all keys will be sound less a half second. If on this moment MIDI commands will received on saved channel it can be cause of panic.
This manual can be not so much clearly to understand, and may be edited for fix it without notification.
Last change: 28 May 2026
