Fender Deluxe Reverb

[goldtop5]

I have this Fender Deluxe Reverb amp on my bench for quite sometime. It's making a scratchy noise whenever you move the volume pot. It's not a dirty pot because I've used contact cleaner which usually takes care of the problem. The real issue I'm having is DC volts detected on the potentiometer leads. I'm probing through the circuit and still can't find out where this DC source is coming from. Any ideas? Thanks!

[chipaudette]

How much DC voltage are you measuring?

My first reaction is that the coupling cap that precedes the volume pot is problematic and not really doing its job.

But, in the Deluxe Reverb, there is no discrete coupling cap that precedes the volume pot. Instead, it's the caps in the tone stack that do the de-coupling. So, in this case, is it easy to replace all three caps in the tone stack?

Chip

[pdf64]

I came across some GT labelled chinese 12ax7s which actuallly generated their own grid voltage, creating scratchy pot syndrome. Sub in a different tube and the voltage went away. It's an easy one to check.
Is it an old eyelet board DR, or RI pcb? The eyelet boards are notorious for not being perfect insulators, causing odd dc voltages to appear where none should be.
Hope that helps - Peter

[52 Bill]

I've also seen bad tubes do this.

I would replace the tube first, because if any of the tone stack caps was leaky, I would think that the tone controls would have voltage on them as well and would make noise as well.

[goldtop5]

Update

I looked over the schematic and it seems like some of the components in the circuit were modified. I've replaced some of the caps with the correct values and the problem with the DC voltage went away. Before it was reading about 150V+, or depending on how wide open the pot is.

Now I'm stuck on trying to get the reverb circuit to work. The tank ohmed out ok and the reverb transformer tested good. I've swapped the 12AT7's with ECC81's and that didn't solve the problem. Cables checked fine. Also, the vibrato just shut down after fixing the first problem.

I'm not sure how much money this guy wants to spend, but we talked about restoring it back to factory specs.

[Alex R]

This sort of thing is generally not too hard to sort out; don't despair, it's a pretty simple amp really. Starting with the reverb - the return jack centre contact should give you a really healthy buzz through the speaker if you touch it with the reverb turned up even a little bit - does it?

The layout diagrams for these are all over the net - easy enough to check for modified components and return them to stock.