ARP Omni 2 Synth Section

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ARP Omni 2 Synth Section

Postby error_trials » Fri Mar 28, 2014 8:22 am

Well, first post, hi all. I'm well into an ARP Omni 2 repair, funny I see the other 2 most recent posts in this help forum are ARP Omni questions - I'm seeing a trend and these Omni's are indeed living up to their reputation...

Anyway. I've fully re-capped the synth, all keys work, string section works, phaser/chorus works and up until I swapped the synth control IC's, the synth section worked... the 8' and 4' switches were rarely working so I thought it could be the 4013 flip-flop IC's, I swapped them with new 4013's, then realized it was just the switches themselves through some switch trading.

Now, the synth section has no output. I'm afraid something was shorted or a PCB trace was broken, etc during my "repair". I do have a scope and I've tried to trace the synth signal to see where it's stopped, and I'm a bit stumped:

The 4' and 8' waveforms come into the synth board as they should, on pin 16 and pin 2 respectively of J12 (referencing the schematic:, and they go away when I switch the 8' and 4' synth buttons to OFF (LED goes out too). They do appear as I trace the signal to the input of the VCF (pin 1 of M1), although with a significant decrease in the signal there, and a very very low (no?) signal at the VCF output.

What should I be seeing at the input and output of the VCF? Or better yet, where along the whole circuit should I look for signal to narrow this down? Or does my description alone suggest a certain component failure to anyone?

The bass section is also a bit funky, but that's an issue for another day... I don't believe it's affecting my current problem since the synth section worked previously and the bass section has always had the same issue.

Thanks for any advice!!
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Re: ARP Omni 2 Synth Section

Postby atkbg » Tue Apr 01, 2014 5:49 pm

I've had two Omni's that the filter IC died.
Replacing that fixed the issue for me.
Don't recall the part number right now.
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