monotron-E

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.


Schematics:

monotron Schematic (pdf)

monotron-E Schematic (pdf)

Ribbon Schematic (pdf)

 

BOMs:

monotron-E PCB Bill of Materials (pdf)

Ribbon PCB Bill of Materials (pdf)

 


Kit Includes:
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

Bag 1:
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

Bag 2:
1 14-pin SMD Header Male
1 14-pin SMD Header Female

Bag 3:
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)

Bag 4:
Q10 Washers for 3.5mm Jacks
Q10 Nuts for 3.5mm Jacks
Q6 Nuts for Pots
Q6 Pot Spacers

Bag 5:
Q6 Davies 1900H clone Knobs

 

ConversionKit


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.

Conversion PCB

 

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.

Back_Parts

 

Solder the 14-pin female header socket to the backside of the op-amp conversion board.

OPandTemco

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.

 

Screws

 

Disconnect the speaker cable from the monotron PCB.  Unlatch and disconnect the flat flex ribbon cable.

Unlatch

 

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.

monotron_3

 

Desolder and remove the 5 potentiometers and switch.

Pots

 

Remove components R8, R11, C9, LED1, R58, C11, R60, R62, R67, D1, D2 and IC4

SMD

 

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.

Cut1

Cut2

 

Solder conversion header to the pads of IC4

Conversion Header

 

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.

Tempco

 

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.

LFO_LED

 

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.

LFO_RST

 

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.

Harness

 

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.

Mate

 

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.

Sw_Wires

 

Insert and solder the orange and purple LFO LED wires to H2. The square pin is the anode (orange wire in picture below).

H2_LFO_LED

 

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.

OpAmp_Insertion

 

Connect the conversion harness between the op-amp board and the main PCB making sure to use the proper orientation.

HarnessConnect

 

Slide the potentiometer spacers over the pot shafts so that orientation key snaps in place.

Spacers

 

Install the front panel and secure with pot and jack hardware.  Mount the control knobs and tighten the setscrews.

 

 

Monotron_E

 

MW Thread