Fuzzdog OCD Oscillation

twebb6778

Well-known member
Hi folks, but of a long shot but hoping someone can help.

I recently finished a few of Fuzzdog's Disorder Drive's (their take on the OCD). All of them work perfectly except for one, which oscillates in the last 5% of the drive pot's range. The really interesting thing about the oscillation is that it doesn't seem to be the audio signal. It's pristine for the first 95% of the drive range, but then it seems to dump the signal and produce a high pitched oscillation. Playing in this last 5% of the drive pot changes the rate of oscillation.

I suspect either a bad op amp or pot, it's just a bit odd. I've never come across this behaviour before and thought I'd ask before investigating. Has this ever happened to anyone?
 
So my drive pot and IC seem ok, I'm now suspecting a bad capacitor somewhere.

On inspection all components are correct and there's no sign of any caps being visibly faulty. It's a little tricky to track down with an audio probe because the oscillation is permeating the whole circuit. I think I've isolated the problem to this area, does anyone have some tips for finding a bad cap in circuit short of just pulling and replacing them? The pads don't seem very durable so I'd like to avoid that if possible.

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I've tried a few opamp's but haven't tried another charge pump yet. I've got some spares so I'll give it a go.
I burned the pads for C1, which is why it's now a jumper. Replaced drive pot and still having the same issues.
 

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I was previously using an A1M, but swapped it out for an A500K because that's all I had and know earlier versions used it. The oscillation happens with both pots.
 
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