Omnichord CV trigger retrofit

Retrofitting an old Omnichord so it can be triggered by control voltage.

One of the projects I’ve been hankering to do for a while is retrofit my old omnichord for CV triggers.

It seemed like a prime candidate, all of the “play surfaces” are basically push buttons, so all I’d need to do is wire up some simple transistor/optocoupler CV controlled switches that on trigger short out the button pads.

Since there are 3 rows of 9 buttons for the chord selector I thought I’d first look at the strum panel. I wasn’t entirely sure how it operated, the top surface looks like a simple physical short between multiple tightly spaced exposed pads.

After cracking the case open I discovered it was nice and simple. There strum panel connects to a 16 pin header area. 14 of those pins are the notes in the “strum”. 1 pin is the common ground. 1 pin is the kill switch pad.

So all that was needed was to wire 14 jacks up to transistors in such a way that when a trigger CV is applied to it, it shorts 1 pin with the common ground.

I was able to achieve this with a single note, but when I wired up the second jack, I found that I was now getting 2 notes for a trigger at either jack, not 1 note from each.

Something is wrong with my assumptions or my transistor wiring. So I’ve reached out to the good folks at Muffwigglers and Electro-music forums to see what they think.

First published on nomeist.com.