Potentiometer Puzzle - Wasp VCF

[Strategy]

I built the EDP Wasp VCF as per this thread http://www.diystompboxes.com/smfforum/index.php?topic=102714.0 and I'm having a recurring issue with the cutoff frequency pot.

About 2 months into gigging out with my finished build, the cutoff frequency pot started sounding dirty and crunchy. Turning the pot gave this kind of squelchy jitter to the filter sound, not the smooth filter opening and closing you'd expect. I chalked it up to a faulty or low quality pot being turned too much; at the time I was using this as part of a desktop drum machine + synth/groovebox set up, so there was a lot of DJ style manual filter tweeking going on. I thought - I must have abused the pot somehow. This was a 16mm alpha from smallbear standard issue stuff. I've never had trouble with these.

I replaced it.  Now less than a year into it I am having the SAME ISSUE again. And it hasn't  gotten as much use. I've looked inside for any obvious shorts or other glitches, can't seem to find anything.

Sending a control voltage to cutoff frequency doesn't give any crunchiness, so it's definitely the pot. Like it has dirt in it.

Is there something in the circuit that could cause this? Do I really need to look at a more robust pot or is there an electronic problem I need to consider?

Any input appreciated.
STRATEGY

[GibsonGM]

If there was DC getting to the pot, that would definitely sound crunchy.  You may have a leaky cap or something in the circuit allowing DC onto the pot, would be my best suggestion. 

Did you use the old pot in any other circuit?  If you still have it, you could just try to use it as a potential divider (careful not to exceed its power rating or it will fry) to be sure it works ok.

Then input an audio signal (AC tone...) that's coupled via capacitor to your amp...you should get no crunch.   And if that's the case, you probably have DC on the pot in the effect, so open it up and measure for it at that pot.  If so, either a construction error or bad cap would be the culprit.

[Strategy]

Thanks!!! I'll see if I can find the culprit
Strategy

[GGBB]

I believe that pot is supposed to have DC as it's purpose is to control the CA3080 amp bias input. It is connected directly to +5V DC via a 1k resistor, so that shouldn't be the problem.

If you've gone through two pots now, might just be par for the course, but I can't imagine there's enough power flowing there to overstress the pot. Maybe you should just try a higher quality pot - maybe conductive plastic instead of carbon. Mouser has some TT Electronics pots that are very similar in footprint to the Alpha 16mm but conductive plastic - and they are quite affordable. http://ca.mouser.com/Passive-Components/Potentiometers-Trimmers-Rheostats/_/N-9q0yi?Keyword=p160&FS=True

[Strategy]

Thanks for the recommendation, I'll try these!
Strategy

[GibsonGM]

Won't just about ANY DC cause it to sound scratchy? 

Functioning ok is one thing, being annoying is another...I thought Strategy's pot still had function...I agree that it's not overstressed and didn't think it was (the suggesting to watch the power was for testing only)....5mA/.025W won't hurt anything, but shouldn't the DC cause it to have the scratchy sound, and thus any pot you put in will do the same eventually?

That's my understanding of DC on a pot, anyway. Like the Presence control on many amps - scratchy due to the presence of DC bias voltage.  Doesn't always make noise at first, but over time it's expected they will.

Maybe a 'better quality' pot will make it less "jittery" as you reported (?), or the conductive plastic might avoid noise better, anyway...

[GGBB]

Won't just about ANY DC cause it to sound scratchy? 

I think that's true when there is DC on a pot that is in the audio path, but this pot is merely control and doesn't sit in the audio path. I'm not entirely sure though.

[GibsonGM]

^   I know, huh?  I admit, I didn't check the schematic before replying, lol.  Looks like pin 5 connects to that pot, if we're talking the 10K log "Cutoff" pot (P2).   I looked up the 3080 pinout (don't use them)....amplifier bias?   So perhaps the pot IS just wearing, and shifting the bias around, resulting in annoying crackle?  It's like "not in the audio path", yet it is, in a way, as a sudden bias shift can also (I suppose) make noise.  Or, something is miswired and allowing DC from that subcircuit somewhere it doesn't belong (doubt it tho).

I'd have to go with 'put in the better pot, see what happens'...