Fender Pro Reverb Problem

[TimeBandit]

It does seem to me that there's an issue with the signal essentially disappearing after the 500pF cap that's on the way to the grid of the reverb driver. I've traced it that far before it diminishes significantly.

Tom P: When you say "open wire probe" do you mean not connected to a meter or other device?

I have a scope, oscillator, multimeters. . . .

Helmtholz: I had come across that tip from Enzo a few days ago, and when I tried it, the sound produced was very very faint.

[Tom Phillips]

It does seem to me that there's an issue with the signal essentially disappearing after the 500pF cap that on the way to the grid of the reverb driver. I've traced it that far before it diminishes significantly.
That sounds like a potential fault. How about just replacing the 500pF cap. You could use any convenient small value cap such as 500pF to even 0.01uF for a quick evaluation. In the process you could also ohm out the wiring between that point and the grid of the 12AT7. That feed wire passes under the eyelet board and those out of sight leads can cause problems that are hell to find. While you are at it, I suggest that you verify the 1 Meg Ohm grid leak resistor is OK. I once found one that was 1 Ohm in place of the 1 Meg.

Tom P: When you say "open wire probe" do you mean not connected to a meter or other device?
Yes. Just something to use as a hum antenna without suggesting that you touch the pin or the lead wire with your finger.

I have a scope, oscillator, multimeters. . .
Great! I didn't realize that ...

My ideas above.

[TimeBandit]

Well, I THOUGHT that I had already swapped out that 500pF cap, but apparently not. I went ahead and did it, and lo and behold we have reverb! The 1 meg resistor checked out fine, as did the wiring. Thanks for the help!

[Tom Phillips]

Thanks for the news. There is nothing better than a report of a successful repair.

[Chuck H]

Well you said you HAD changed that cap. No reason to remember that if you hadn't. But then there's no reason to assume the cap you used as a replacement was good. I've seen this sort of coincidence happen repeadedly here on the forum and life in general (especially WRT auto repairs ). It can be very frustrating. But when you've ruled out the impossible then anything probable is all that remains. And all is well that ends well