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Quad VCA issue
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author Quad VCA issue
Still a noobie to SDIY, though I've got 10 modules under my belt (8 on the bench!), and I only have this one tiny issue I've been trying to track down with the Oakley Quad VCA. 3 out of 4 VCAs work perfectly. The first VCA on the master board (#3, though on the Bridechamber panel, it's #2) is a little funny. Its gain pot doesn't work properly. All the way down, it's totally off (and CV controls it normally). If I turn the gain up past about 1, it's TOTALLY 100% at full gain, so I don't get any sweep of the gain at all on that one channel only. The rest works perfectly. I've double checked wiring, and can't seem to find any problems. Any ideas where to look next?

When I first built this board, i fried the 10V LM4040 , and I had to replace it. It now puts out -10V normally. Probably not related, but figured I'd throw that in there. Have meter and oscope. Thanks in advance for any help!
The first thing that comes to mind is that the gain pot is damaged. The conductive track in the pot could be broken right next to the CCW pin. Throw a scope probe on the wiper of the pot and have a look at the voltage coming from the pot as you turn it. It should move in a graceful sweep from 0V to -10V. If it slams into -10V the moment you start to move it you know to replace the pot.

Duh! Yep, that was it, new 50K pot solved the issue. I just thought it couldn't be the pot for some reason, as I've never had a problem with a pot before. Thanks so much Tony!
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