ARP Omni 2 mono bass section weak output

Hi again, another ARP question. I’ve restored an old Omni 2 that was completely trashed, and got it all functional… except the bass section.

The bass strings work fine and sound good. But both the 8’ and 16’ bass mono synth sections sound pretty weak and low volume at the output (both the main output and the separate bass output). It looks like the bass volume slider works properly, as do both bass section switches and the staccato switch.

I’m poking around on the bass voice board and elsewhere to see what I can find but figured I’d ask the crowd here if anyone has worked on one of these and is familiar with the bass voice or has had a similar problem? I do have a scope and service manual and will be working on it tonight, and will follow up if I figure it out…

I guess I haven’t played any other Omni 2, so maybe the bass on these is just kind of weak - but I suspect that it should sound better than mine does.


Okay friends here’s a sort of anticlimactic update… spoiler alert it’s pretty boring: After some serious head scratching and some good learning I figured out what was wrong - the short answer (pun intended?) is that when I had re-capped the tantalums with electrolytics on the Omni 2 a couple years ago… I lifted a trace on the bass board, where the bass one-shot comes in, so the whole bass voice was messed up. Everything else worked fine, and at the time I just figured there were other (non-cap) problems on the bass board.

It took forever to figure this out because I was dead set on looking at the bass ADSR and VCF. I replaced IC’s and transistors blindly, mis-measured resistors (turns out old resistor legs aren’t the most reliable on their surface) and replaced erroneously… and went pretty crazy confirming components worked correctly, but the output was still muted. Once I got my head out of the ADSR and VCF circuits it didn’t take long to see that the bass one-shot circuit was busted.

On the bright side I got a lot better at reading / troubleshooting schematics and also learned the Omni bass voice inside and out… Super happy to have this thing out of my workshop and playing as it should. Case closed for now.

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Great update - these things are always like a detective story! And that’s a great keyboard to revive - congrats!