GEM/LEM PK4900 MIDI kit retrofit Installation manual A03.1
Welcome to the metric system zone. Here we work with precision things and all dimensions are metric. 1" = 25.4mm. And yes, you need schematic reading skills.
This manual for boards after 20th April 2025.
This kit is pretty universal and contains two different set of screws and full set of ribbon cables. For GEM PK4900 you will have unused 4 M3 screws and nuts for them.
Installation:
To your good usability select the friendly place to mount the LED, MIDI connector, button and PCB.
Drill all holes that required and mount connector, LED and button before soldering wires of coarse.
Drill dimensions:
MIDI jack (DIN5) = 18mm.
LED holder = 6mm.
Push button = 12.5-13mm.
Be careful with drilling hole for button, it have so small range to get hole too big.
Holes for wood screws is up to you, 1-1,5mm. Mostly in not needed and screws can be tighten without predrilled holes.
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.
All ribbon cables are 90cm. long, enough for placing all connections and you can cut extra length.
Panic button is not included in the kit. In case of panic - enough turn off and turn on the synth. If you prefer to install it part number is E-Switch RP3502MABLK
This picture shown:
First (1) pin/wire on all connectors. First wire will be start from this side.
J1, J2, J3, J4 jumpers that divide common rails for every octave separate or split.
For GEM PK4900 they should be open 
Bottom side jumper J5 should be open for GEM PK4900
The next picture shows the pinout for X6 connector ribbon cable and pin number of wires:
Same you can see on bottom side of PCB for more convenience.
(7) V+ and (8) GND should be soldered on the next points:
So, now time to soldering key contact wires: All next connectors should be soldered to each key contact
Solder wires to key contacts and common bus. Find key contacts for note C (Do) 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. Wire 13 should be soldered to common bus for this octave. Repeat all this on every octave.
On octave 5 (RED), also start from wire 1 - C (Do). Skip else except wire 13 that should be soldered to last key contact common bus.
One small important thing - use hot glue to fix inserted connectors to protect from disconnecting them during vibration when you ride on tour. Also fix wires strong. Do not leave them holding only on soldering joints.
Process for set MIDI channel: User operations
Stop all MIDI traffic.
To teach with which MIDI channel synth will work - push the black button (LEARN) LED will blink two times and stay ON to wait for MIDI messages at one of 16 channels. He will in OMNY mode.
Send any note on the MIDI channel which you want for this synth.
When the synth receives MIDI messages, LED will blink five 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 channel saved to memory and will never change before you push LEARN button again and starts settings process again.