Fender Super Reverb (45W Master Vol) Reverb control causing squelching/oscillation

[bw1985]

Hi all,

I am currently working on a mid-late 70s SF Super Reverb (45W master volume) with a reverb issue I am totally stumped on!

When the Reverb pot is turned up past 4/5, I am getting some crazy squelching and oscillation. I’ve gone through some of the standard troubleshooting for this issue but still can’t figure it out. The tank has been replaced as well. I’m getting reverb sound as well.

Is there anything else that could be causing these oscillations? I noticed the NFB loop was disconnected, but connecting it again did not have any effect.

Here is a schematic:
https://el34world.com/charts/Schemat..._bmstr_rev.pdf

Thanks in advance,
Brian

[pdf64]

The V4 shared cathode bypass cap may be bad.

[vintagekiki]

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/39559-reverb-oscillation

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/30340-oscillation-in-70-s-fender-quad-reverb

https://music-electronics-forum.com/forum/amplification/guitar-amps/theory-design/19815-how-to-silence-reverb-tank-feedback

http://www.tdpri.com/threads/fender-reverb-feedback-oscillation.657122/

https://www.tdpri.com/threads/reverb-squeal-feedback.414806/

https://www.tdpri.com/threads/reverb-feedback-problem.159465/

[Chuck H]

You mentioned that the NFB loop was disconnected. Have there been other modifications? Do you have any history on the matter. When the problem started? Were any changes made immediately prior to the problem? Can the owner report on whether the amp ever didn't have the problem? That sort of thing.

I might also suspect the power supply filter that serves the preamp.

[vintagekiki]

... ...

Do you have crazy squelching and oscillation when reverb pot is turned up and the guitar is not connected to ch.2?
Is the reverb pedal working properly?
Check correctness cables from tank to amp whether the shield is not in interrupt.
Check the connector from the reverb jacks on the amp, cables and tank whether is not oxid.

[g1]

Also, did you replace the tank, or someone else? If it's not the exact correct type with regard to ground scheme, you may get weird problems like this.