Some potentiometers have a ground short when tightened

[Mr. Strypes]

I've built around 150 guitar pedals and run into this problem 4 or 5 times. Everything is working fine, so I box it up. It's still working until I tighten down the pots. Suddenly, there is a grounding issue. (The power draw goes way up, the LED won't light up, and there is no sound.) When I loosen the nut on the fussy pot, everything works again. This has happened with pots from Amplified Parts, Stomp Box Parts, and Tayda, so I think they aren't poorly made.
Have any of you experienced this issue?
Do you know what might cause it?

 

[jcpst]

Hrm. So let’s say an average of 3 knobs, giving us 450 pots, and 4-5 of them had that grounding issue.

I wonder if that’s within their manufacturing tolerances, ~1%

 

[Robert]

Do you know what might cause it?

I'd suspect a short somewhere else that isn't making contact until the hardware is tightened.

It's hard to say without knowing the circuits involved but shorting all three lugs of a pot to ground wouldn't necessarily cause a dead-short condition. (High current, no LED, etc)

Tightening the pot might be physically responsible for the short, but it might not necessarily be electrically related.

This is usually caused by an improperly insulated dual-gang pot... the back of the pot makes contact with component leads on the back of the PCB, then the pot is fastened down to a grounded metal enclosure.

 

[Mr. Strypes]

I'd suspect a short somewhere else that isn't making contact until the hardware is tightened.

It's hard to say without knowing the circuits involved but shorting all three lugs of a pot to ground wouldn't necessarily cause a dead-short condition. (High current, no LED, etc)

Tightening the pot might be physically responsible for the short, but it might not necessarily be electrically related.

This is usually caused by an improperly insulated dual-gang pot... the back of the pot makes contact with component leads on the back of the PCB, then the pot is fastened down to a grounded metal enclosure.

Great info! I've insulated the pots and still run into issues. "A short somewhere else that isn't making contact until the hardware is tightened" makes sense.
I have experienced it a couple of times with dual gangs (one reason why I purchased a handful of your potentiometer insulators), but the other occurrences happened with singles. In all cases (as far as I can remember), the problem has always happened in tight builds.
Anyhow, thanks for the info, Robert! I appreciate you!

 

[Mr. Strypes]

Hrm. So let’s say an average of 3 knobs, giving us 450 pots, and 4-5 of them had that grounding issue.

I wonder if that’s within their manufacturing tolerances, ~1%

I see your logic, but I'm pretty sure it's not bad pots, since the problem is solved by either loosening the nut, adding insulation, or shifting other components around within the enclosure. The pot still does its job without having to be replaced.

 

[jwin615]

Early on I ran into a similar issue and ultimately resolved to this logic/thought
If you over tighten a pot enough, it somehow torques the wiper/track to the point that they're no longer in proper alignment. there's no repair/fix, only replacement.
Now, after thatany build in highly doubt that is the issue is you over torquing the pot collar.

I guess a better question would be, if you replaced the pots, was all well and good after?

Also, if you are getting a shirt to ground, you may be able to identify roughly where via temperature. Poke around with your finger and find the hot spot.

 

[Lytt Effects]

LED bezel?

I had this exact issue with a build. When I would tighten down a specific pot, it would short. Drove me nuts. Turns out that once tightened down, the pot was contacting the nut of the LED bezel, which was in contact with the washer, which was in contact with the enclosure, which was grounded. Using a nylon washer helped, as the bezel itself was coated (black), thereby isolating the nut.

Tight builds can wreak havoc once everything gets boxed and tight ended down.

 

[Mr. Strypes]

I'm sure it's not the pots. I replaced one once early on in pedal building but that didn't solve the problem. I'm thinking it's something else that's close to shorting out that, for some reason, shorts out when the pot is secured tightly to the enclosure.
Thanks for the advice about searching for the problem by feeling around to discover what is overheating. I hadn't thought of that before. That's a practical and simple solution.
(Love it!)
Thanks for responding!