Noise and then No sound when moving 80hz eq fader on c+

[JimAnsell]

I'm trying to diagnose the following problem without having to send this thing to boogie...
here is the problem:
every time i move the 80hz eq fader from the middle position up, it is extremely noisy. like wind on a microphone noise.
then when i move that fader up to the highest hash mark(but not all the way up), it pops really loud, and i get no sound at all until i
move the fader back down. It happens on all channels, but none of the other faders are acting up. just that fader in that range. i tried some contact cleaner, and "messaging" the dial, but nothing seems to stop it.

any ideas?

 

[McBarry]

Sounds like crap in the slider..
And at the upper reaches of travel it's dumping the signal to ground.
Spray cleaner, or disassemble and visual look and possibly replace.
The pop however makes me suspicious you have extra voltage there -check what AC and DC voltage, if any, is on the base of Q1 an Q2..

 

[JimAnsell]

deoxit did nothing. i might need to replace it to bring my c+ back to perfection.

 

[McBarry]

true, replace is sounding likely..
but checking for stray DC is also a good idea..
dep on how handy U are, also disassembly of the slider might reveal the cause, and if yr lucky might be fixable too..
just sayin'....

 

[JimAnsell]

quick update. Finally got around to calling mike b, who said its a common thing he sees on c's through early III's. He described the problem back to me better than i described it, and said that the culprit is probably the 3.3uf 35v cap on that slider. He says he replaces that cap on all of those particular amps when he gets them in for check ups regardless of whether or not they are a problem at the time.

I replaced it today, and it worked like a charm! Got that crushing low end thump back, and the amp is quieter than ever.