The monotron is a super fun little analog synth and thanks to Korg’s open information policy it is a nice platform for modifications. Here are my mods for turning the monotron into a Eurorack module.
Monotron-E Bill of Materials A2
Q1 Conversion PCB set
Q1 monotron-E panel
Q1 Ribbon panel and Overlay
Q2 16-pin Eurorack ribbon cables
Q1 Conversion Harness
Bag 1: Headers and Trimmers
Bag 2: SMT Headers
Bag 3: Hookup Wire
Bag 4: Jack and pot mounting hardware
Bag 5: Knobs
Q1 1k Trim
Q1 50k Trim
Q5 3-pin 90deg Header
Q1 4-pin 90deg Header
Q1 3-pin Header
Q1 Shorting Jumper
Q2 16-pin Shrouded Headers
1 14-pin SMD Header Male
1 14-pin SMD Header Female
Q1 100mm Grey 30AWG (LFO reset)
Q2 20mm Purple 30AWG (Tempco)
Q1 45mm Orange/Purple 30AWG Pair (LFO LED)
Q2 20mm Blue 22AWG Solid Core (Switch Tap)
Q10 Washers for 3.5mm Jacks
Q10 Nuts for 3.5mm Jacks
Q6 Nuts for Pots
Q6 Pot Spacers
Q6 Davies 1900H clone Knobs
Prepare the monotron-E Conversion PCB set:
The monotron-E conversion PCB set is comprised of four PCBs. Three are used to build the monotron-E (mainboard, op-amp conversion, tempco) and one is the Ribbon conversion board. The boards are connected with PCB tabs that are design to be snapped apart. Carefully separate the four PCBs by snapping them along their tabbed edges. Remove the remaining tabs and clean up the edges.
The monotron-E main board is mostly assembled however a few parts will need to be soldered to the back side of the PCB. These include the 3-pin board connect headers, the 16-pin Eurorack power connector, the 3-Pin Bus CV enable header, the 4-pin conversion harness connector and the two trim pots. Trim the jack leads around the 4-pin header and angle the header slightly up so the connector can be easily attached.
Solder the 14-pin female header socket to the backside of the op-amp conversion board.
Set the PCBs aside while you prepare the montron for grafting.
Disassemble the monotron and prepare the PCB:
Remove the battery cover and unscrew the four screws from bottom of the monotron case.
Disconnect the speaker cable from the monotron PCB. Unlatch and disconnect the flat flex ribbon cable.
Remove the monotron PCB from the case. Then snap off the battery leads with a pair of pliers by wiggling back and forth so they don’t extent past the edge of the PCB.
Desolder and remove the 5 potentiometers and switch.
Remove components R8, R11, C9, LED1, R58, C11, R60, R62, R67, D1, D2 and IC4
Cut the trace between R58 and C17 with an A-ACTO hobby knife. This is will remove the input into the filter which will be replaced by the Op-Amp conversion board.
Solder conversion header to the pads of IC4
Glue Tempco board to Q1 using thermal epoxy or Cyanoacrylate (CA) glue commonly known as super glue. Wait for the adhesive to dry then solder the leads to the pads of R8.
Route the orange and purple LFO LED wires through the first and third potentiometer mounting holes and solder them to the LED1 pads with the orange wire to the anode and the purple wire to the cathode. Secure the wires by injecting a small amount of hot glue into the routing holes.
Insert the grey LFO reset wire through the second routing hole and solder it to the top pad of C9. Secure the wire by injecting a small amount of hot glue into the second routing hole then weave the wire through routing holes 5, 6, 7, 8, 9 & 10.
Route the unconnected white wire of the conversion harness through the pot mounting holes and solder it to both pads of R58. The signal connection is made on the top pad, but the wire will be more secure if you connect it to both pads. Use hot glue to secure the wire to the PCB.
Mate the monotron to the monotron-E conversion assembly by sliding the 3-pin headers into the monotron pot connection pads. Solder each pin of the 3-pin connectors from the back side of the monotron PCB.
Insert the grey LFO reset wire into the H3 and solder. Shape and insert the blue jumper wires between H1 and the two upper switch pads as shown in the picture below.
Insert and solder the orange and purple LFO LED wires to H2. The square pin is the anode (orange wire in picture below).
Plug the op-amp conversion board onto the 14-pin header making sure that pins are properly aligned. Reinforce the conversion board by injecting hot glue between the monotron pcb and the conversion PCB.
Connect the conversion harness between the op-amp board and the main PCB making sure to use the proper orientation.
Slide the potentiometer spacers over the pot shafts so that orientation key snaps in place.
Install the front panel and secure with pot and jack hardware. Mount the control knobs and tighten the setscrews.