Megatron

Modified Korg Monotron

Schematics

Monotron

A Monotron is a small battery-powered monophonic synthesizer made by Korg. It has a ribbon control for playing notes and features the same filter used in the popular Korg MS-20 vintage analog synth. While a fun little toy for making weird noises, the playablility of the Monotron is limited. It is not something you will likely find in most professional studios. However, Korg were kind enough to add several easy solder points to the bottom of the Monotron‘s PBC, which makes it very easy to add modifications.

Goal

My goal with this project was to create a MIDI and voltage controlled bass synth similar to the Minitaur, Mopho and Microcon, while keeping the ribbon control from the Monotron.

Megatron - Modified Korg Monotron

Modifications

MIDI Control
I added midi control using the MIDI-IF kit.
Re-Enable the Ribbon
Part of the installation process for the MIDI-IF kit involved removing a resistor that disables the gate control on the ribbon. Motohiko Takeda was very helpful in trying to explain how to use a transistor to re-enable the ribbon, but I could not get it to work. Instead, I used a toggle switch to reconnect the resistor when the ribbon is in use.
CV Control
This is a very simple mod thanks to the easy solder points on the Monotron‘s PCB. I just added toggle switches to select between MIDI or CV for the control of gate, pitch and cutoff.
Pulse Wave
Using a comparator, I created a pulse wave from the Monotron‘s saw wave. The comparator has a comtrol for the width of the pulse. I also added a passive high-pass filter, followed by an active low-pass filter. This signal is then actively mixed with the sub-oscillators and sent into one side of the Monotron’s auxillary input.
Sub-Oscillator 1
A second comparator is used to generate a square wave that clocks a 4040 divider. The first output of the divider is an octave below the sound source, and is used as a sub-oscillator. I added an active low-pass filter to the sub-oscillator before actively mixing it with the pulse and second sub-oscillator.
Sub-Oscillator 2
The second output of the 4040 divider is two octaves below the sound source, and is used as a second sub-oscillator. I added an active low-pass filter, which I find is especially useful at this low frequency. Without it, there is often too much high harmonics or clicks in the sounds. The low-pass filter smooths it out so there is just a low rumble added to the mix. This is actively mixed with the pulse and first sub-oscillator.
Overdrive
The Monotron is a monophonic synth, yet it has a stereo output. Since I only really need one side of the output, I took the other side, passed it through a pot wired up as a voltage divider (volume control), and send it into one side of the auxillary input. This creates a feedback loop that overdrives the filter. The feedback resonates at a low frequency, so I also added a high-pass filter that can be switched into place to change the sound of the feedback. This is done by simply passing the feedback loop through a 1uF capacitor.
Envelope
I added a simple AD envelope control to the filter that can be toggled on or off. Thanks to JovianPyx from electro-music.com for giving me the schematic and helping me debug it.
Power
I added a 7805 power regulator that uses a 9V DC adapter. This eliminates the need for batteries, and hopefully helps to stabilize the tuning and opamps. I also added a rocker switch to turn the power off and on, and reused a power indicating LED in the mod case.
Tuning
Due to the way I mounted the Monotron in the case I used, the volt/octave tuning trimpot was not accessible. I unsoldered it, added some wires, and re-attached the trimpot at the back of my mod case.

Megatron - Modified Korg Monotron

Megatron - Modified Korg Monotron

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