Scratchy Pot

C Moore · Post by **C Moore** » Sat Oct 16, 2010 5:38 am

My volume pot is real scratchy. It is a sealed pot. I am not sure where I might be getting any DC from. Are there other things that cause a pot to be "scratchy" ?
Thank You

selloutrr · Post by **selloutrr** » Sat Oct 16, 2010 6:20 am

did you test it before you put it in? It's rare but possible it was bad from the start.

DR.Z · Post by **DR.Z** » Sat Oct 16, 2010 12:43 pm

My experience has been that a slight DC voltage across a pot will cause it to scratch. You may have a leaky coupling cap or more often a 12AX7 with DC grid leakage. Swap V1 with a known good one.

DR.Z

steviea55 · Post by **steviea55** » Sat Oct 16, 2010 1:45 pm

Nice looking build. I agree with DR. Z. Start with V1, if that's not it test your coupling caps for leaks. It's easy to build a tester.

Gerald Weber describes how this is done on page #339 of his book "TUBE AMP TALK FOR THE GUITARIST AND TECH" Looks like you got some of your parts from Nik. He has good stuff, however any cap can be bad. If they sit around and are not used they will start crystalizing.

C Moore · Post by **C Moore** » Sat Oct 16, 2010 2:39 pm

selloutrr -
Yeah, the pot has been in this amp for a year or so. It just started making noise.

Dr.Z and S55 -
I will try a different tube in V1.
As far as the coupling cap.....pardon my ignorance, but I do not see a cap immediately before the volume pot. I am looking at the "Kelly '90" schem. What cap would be leaking DC into the pot?
This is mostly a Ceriatone amp. I bought it from a guy on TGP who had a bunch of pieces. I put it together the best I can, but it has been a lesson in frustration so far......
Thank You

selloutrr · Post by **selloutrr** » Sat Oct 16, 2010 4:02 pm

keep your head up it's an easy fix

follow the suggestions in the above posts. test the 22mdf or 25mfd caps for leakage.

C Moore · Post by **C Moore** » Sat Oct 16, 2010 4:47 pm

selloutrr wrote:keep your head up it's an easy fix follow the suggestions in the above posts. test the 22mdf or 25mfd caps for leakage.

10-4.
I will check this stuff out.....
Thanks

C Moore · Post by **C Moore** » Sat Oct 16, 2010 10:53 pm

I cannot measure any DC on the pot, nothing seems to be "leaking" DC, I changed V1 twice, and the volume pot is still scratchy. It is real bad, all the way across the sweep, from start to finish.
I do not know what else to suspect but the twelve dollar, Alessandro branded, volume pot. I suppose there is only one way to find out.
Back to the operating room.......

Structo · Post by **Structo** » Sat Oct 16, 2010 11:29 pm

You have to disconnect one end of the cap to measure for leakage.
So the end towards the pots has to be un-hooked to measure the voltage.

smokin' tone · Post by **smokin' tone** » Sun Oct 17, 2010 12:33 am

I went through the same thing with a build one time. Volume worked fine for a couple of months then started getting scratchy. I eliminated everything else first since it was a sealed P.E.C. I ended up carefully drilling a small hole in the side and shot it with some cleaner and it cleared right up. This is one reason I don't like sealed pots.

v846 · Post by **v846** » Sun Oct 17, 2010 9:21 pm

smokin' tone wrote:I went through the same thing with a build one time. Volume worked fine for a couple of months then started getting scratchy. I eliminated everything else first since it was a sealed P.E.C. I ended up carefully drilling a small hole in the side and shot it with some cleaner and it cleared right up. This is one reason I don't like sealed pots.

I had the same problem with a PEC, was going to replace it but saw this thread so I decided to give it a try. Had at it with the Dremel followed by a shot of cleaner and no more scratchy pot

smokin' tone · Post by **smokin' tone** » Sun Oct 17, 2010 10:01 pm

Nice clean work David. I'm really surprised that this happens with sealed pots but it does. Glad you got it fixed.

C Moore · Post by **C Moore** » Sun Oct 17, 2010 11:50 pm

v846 -
Yeah, you did a nice job with that, and kept the pot free of Dremel debris.
Anyway........
The PEC pot was bad. It was only two years old. I have not been at this trade anywhere near as long as a lot of you guys. But I have never had a Alpha, Bourns, CTS pot go off like this PEC. It was way more than scratchy. From start to stop, all the way across, it was noisy everywhere. But it did not start out that way. This amp has set in storage (with all our other gear) for several months. Then I plug it in and the volume pot slagged.
Things happen, I know. It might not happen again in another Ten Thousand PEC pots. But I will never buy that many, so I will always wonder.
Thank You

v846 · Post by **v846** » Mon Oct 18, 2010 7:55 pm

Steve,
Yup works like a charm thanks for the tip. Good to use a punch to avoid skidding I found.
I did check for dc on the pot prior to drilling, have had this happen before but it was due to a conductive board.

hired hand,
This was easy enough to do so it might be worth a shot before replacing an expensive pot.
btw nice build

The Amp Garage

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