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I've got this vintage synth called the Suzuki Omnichord OM-84. It's principally an electronic autoharp, with a metal ribbon you can strum or tap to make notes as though you're playing strings, but it also includes a separate voice for basslines/drones, a drum machine, and a melody keyboard function. The strumplate tone comprises two voices, so there's five total. The built-in speaker and the quarter-inch mono headphone jack output a signal with all five voices combined, mixed up and down by individual volume pots. There's also a main volume pot elsewhere on the board.

I'm interested in breaking these voices out into five individual line-out jacks, ideally independent of the volume attenuators. This is so I can capture them as different signals for recording purposes and put different effects on them for live performances. I don't know much about circuit design, but this strikes me as doable because the five signals are all produced upstream of the amplifier chip, which takes everything and routes it to the speaker and the headphone jack.

I was briefly thinking about looking for the signals on the board by soldering speaker cables to a pair of multimeter probes and touching points that look promising. Then I realized there's twelve volts DC on this board and I don't know what I'm doing well enough to blindly explore. I'd be worried about hurting the synth. I've got a schematic and some pictures. If anyone can identify likely places to tap off, I'd love the advice.Omnichord circuit board, backOmnichord circuit board, frontenter image description here

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You should be able to pick up the various signals at the top (clockwise, ungrounded) terminal of the individual volume controls.

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I've drawn some green wires and some red wires the green wires are the grounds and the red wires are the signal

This should be the right connections, to avoid damage after connecting the wires measure the resistance between the green wires, it should be a low number like the resistance of a piece of wire. also measure from the pin of the DC jack to the green wores - that should also be low.

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  • \$\begingroup\$ This looks like just what I need, I appreciate it. You're saying I should measure resistance between wires 1 and 2, wires 1 and 3, and wires 2 and 3, right? (As well as between each of them and the center pin of the power jack.) This makes sure that each of the connections is to ground, because otherwise it means the wrong voltage at the jack? \$\endgroup\$ Sep 27, 2022 at 21:11
  • \$\begingroup\$ Also, I was expecting the dual-gang pots to have two different ground pins, because there are enough pins to go around. Could you help my understanding and explain why they have shared grounds? Is it similar to how left and right audio signals on a TRS cable have a shared ground? \$\endgroup\$ Sep 27, 2022 at 21:11
  • \$\begingroup\$ One more newbie question. Are there any measurements I should take at the red marked terminals, just to be as safe as possible? I'd be pretty sad if I burnt this synth. \$\endgroup\$ Sep 27, 2022 at 21:13

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