Ampeg V4 bass pot problem...?

[acorkos]

i have an early V4 with a problem....when i turn the bass pot back and forth, there's a loud thumping, which corresponds to a pulsing blue glow in the power tubes. all tubes are new, and the amp seems to running fine otherwise. i've spray cleaned the bass pot, and even disassembled it and lightly steel-wooled the guts.....no change in the thump.

any ideas? i'm going to swap in a substitute pot...but if there's something i'm overlooking, i'd appreciate any advice. here's the schematic: http://www.freeinfosociety.com/elect...view.php?id=46

thanks

[stokes]

Check for DC on the pot.If there is,the cap from the previous plate is leaky.

[Enzo]

Remove the pot and see if the resistive element is open. Is this pop always at the same point in the pot rotation? DC will tend to make them noisy throughout their motion and is easy to check for too, like the man says.

ANy loud sound coming through the amp will blink those tubes blue.

[stokes]

As Enzo says,part of the element in the pot could be open at some point in the rotation.This could also be caused by dc voltage on the pot.I've seen it occur in bias supplies that have a normal volume type pot instead of a cermet pot.Since these pots are not moved much,the dc eventually burns out that portion of the resistive elemement that the wiper is sitting on.But like he says there should be a scratchy sound throughout the entire motion of the pot as well.

[acorkos]

thanks for the info. i swapped in a new pot...same noise. when measured, there was in fact 5vdc across the bass pot. i replaced the .01uF coupling cap and the noise is gone. thanks again.