Dual pot issue

[cgmason1986]

Some days things work, some days they just don't. I am working a klone build, I finished it this morning and it worked great for about 15 minutes, then there was a quick fade in volume/gain and I've traced it back to the dual pot. Testing with my multi-meter, on the upper set of pins - instead of going from 0-100k, it goes from 0-20k-0. The other set of pins on the dual gang work fine. I thought maybe it was a bad pot so I swapped it out, checked the old one once out of the circuit and it worked fine. Sounds like some kind of grounding issue, but isolated to just this control?

These are the build docs:

https://aionfx.com/app/files/docs/refractor_documentation.pdf

Just a standard Klon KTR circuit

 

[Cybercow]

I had a similar issue with a Kliché build recently doing the exact same thing and it turned out to be the VR supply. Note how section "B" of the dual-ganged pot is directly connected to the VR line. Check your VR voltage level. I too, thought the dual-ganged Gain pot was bad and replaced it. Same thing happened. So I replaced all the caps around the supply section same result.

In my case, the zener diode was bad. It work perfectly for about 10 to 15 minutes and the Gain pot would stop working correctly. Once I isolated the bad zener and replaced it, all was well. I failed to do proper troubleshooting technique of first ensuring all the voltages were correct when the circuit was in failure mode.

Out of curiosity, because I'd never seen zener diode fail like that, I tested it on a meter and it checked fine. So on a whim I put it back into the circuit and sure enough after about 10 to 15 minutes it would fail again. So I put the known good zener back in and it was good again. That 12v zener is important.

Hope this helps. Good luck.

 

[cgmason1986]

I had a similar issue with a Kliché build recently doing the exact same thing and it turned out to be the VR supply. Note how section "B" of the dual-ganged pot is directly connected to the VR line. Check your VR voltage level. I too, thought the dual-ganged Gain pot was bad and replaced it. Same thing happened. So I replaced all the caps around the supply section same result.

In my case, the zener diode was bad. It work perfectly for about 10 to 15 minutes and the Gain pot would stop working correctly. Once I isolated the bad zener and replaced it, all was well. I failed to do proper troubleshooting technique of first ensuring all the voltages were correct when the circuit was in failure mode.

Out of curiosity, because I'd never seen zener diode fail like that, I tested it on a meter and it checked fine. So on a whim I put it back into the circuit and sure enough after about 10 to 15 minutes it would fail again. So I put the known good zener back in and it was good again. That 12v zener is important.

Hope this helps. Good luck.

Wow that did it, thank you so much! You have no idea how much this was bothering me. Is there any kind of pinned post somewhere for proper troubleshooting technique?

 

[Cybercow]

Wow that did it, thank you so much! You have no idea how much this was bothering me. Is there any kind of pinned post somewhere for proper troubleshooting technique?

So it was a bad 12v zener? Or something else in the VR or other part of the power supply?

 

[cgmason1986]

So it was a bad 12v zener? Or something else in the VR or other part of the power supply?

It was a bad 12v zener!

 

[Cybercow]

It was a bad 12v zener!

Glad you sussed it! Rock-on!

 

[giovanni]

Where did you guys purchase the zener diodes? Any other lemons from the same place?

 

[Cybercow]

Where did you guys purchase the zener diodes? Any other lemons from the same place?

I get zener diodes and other such parts from Mouser, SmallBear, Tayda, etc . . . . (in order of descending reliability), the usual vendors. A Zener, like any other part can just turn up bad. In manufacturing passive parts like resistors, caps, diodes, they rarely test them for more than a few seconds under load or by other measurements means. Under load, they fail after a time - but that is rare. CGMason and I just happened to bump into one of those rare instances. The key is knowing how to troubleshoot once a build is completed and doesn't work correctly.

 

[Feral Feline]

A mechanic once told me 1 in every million brand-new spark plugs is a dud.
This after putting in a fresh set of plugs and re-kajiggering the top end many times attempting to get rid of #3 cylinder misfire...
Another new plug, not a dud, and the engine purred.