Electro Harmonix – Mini Synthesizer

How to midify an Electro Harmonix – Mini Synthesizer. I’ve had a Mini Synthesizer for a long time before I found a way to midi-retrofit. Playing the Mini Synthesizer by hand might be fun but also could be frustrating. It’s got a membrane keyboard which plays awfully but on the other hand; it’s monophonic so who needs good keys anyway. Besides the keys it’s got an internal crystal mic glued on case which makes the synth velocity-sensitive. You can play notes with one hand and tap on the case rhythmicly with the other..very nice.

But! Without any external control inputs the synth can’t be sequenced so that’s a reason to midi-retrofit.

Putting in CV/Gate might look like a straight forward solution, but I and many others, have unsuccessfully tried. The keyboard uses stange voltages and it simply doesn’t react properly. I lack proper technical explanation for this phenomenon. (see Disclaimer).

I managed to mifi-retrofit successfully another method by using two parts:

A DTronics ( Midi to Trigger interface V4 ) has been used but I suppose every standard Midi note – > 5v gate converter board will do. I’ve used the Doepfer MTC64 in other (Solina String and ARP – Omni) units the same way. Buy this one is cheaper, maybe because it has fewer outputs. You don’t need 64 output gates for this little synth.

The DTronics needs 8V – 12V DC. The Mini Synthesizer needs 9V, so you can take power directly from the adapter’s input socket. I chose to solder on the PCB because the input socket already had many wires (batteries and PCB wires) which were wrapped in heat shrink tubing. PCB soldering seemed to be more efficient.

The Midi – Gate converter 5v outputs drives an optocoupler board which holds one optocoupler (and one resistor) for each key. When the Midi – Gate converter receives a midi note, the corresponding +5v gate output will drive (820ohm resitor in series) the optocoupler’s LED-side which causes the other side to shortcut and makes the corresponding Mini-Synthesizer’s key turn on.

Diagram and CNY74-4 and CNY74-2 integrated circuits (ICs) optocouplers pinout:

CNY74-2 and CNY74-4 optocoupler pinout

This has to be done 25 times (once for every key).

Don’t forget to wire ground from Optocoupler-board!

How to wire the Midi – Gate board’s power leads, midi connector(s), learn-switch and gate-output leads I’ll have to refer to the DTronics – Midi to Trigger interface manual since this wiring depends on which interface you choose using.

Have fun!

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