Phenomenon or Easy Explanation?

[lowell]

Got a silverface Pro Reverb in. Low volume and distorted. Tried my bench speaker...sounds great. Back to pro speakers...sounds great now. Hmmm... decided to check the footswitch to see if vibrato and reverb work. When I moved the footswitch from the back of the amp the amp briefly went low volume and distorted again. I then pressed the footswitch case (ground) against the speaker FRAME and found that it caused the amp to mess up!!

I checked with my fingers to see if any of the speaker leads between the cone and terminals were touching the frame. And therefore shorting the speaker... Nope.

Thoughts? Some capacitive coupling back to the reverb circuit? Maybe a parasitic oscillation from speaker to reverb circuit? It's trippy...man.

[nosaj]

Got a silverface Pro Reverb in. Low volume and distorted. Tried my bench speaker...sounds great. Back to pro speakers...sounds great now. Hmmm... decided to check the footswitch to see if vibrato and reverb work. When I moved the footswitch from the back of the amp the amp briefly went low volume and distorted again. I then pressed the footswitch case (ground) against the speaker FRAME and found that it caused the amp to mess up!!

I checked with my fingers to see if any of the speaker leads between the cone and terminals were touching the frame. And therefore shorting the speaker... Nope.

Thoughts? Some capacitive coupling back to the reverb circuit? Maybe a parasitic oscillation from speaker to reverb circuit? It's trippy...man.

Try a meter on the reverb virato cables and the speaker cables.

I had a pro reverb when i was going through the speaker wiring I found at one junction there were only a few strands of wires making connection. Use alligator clips with meter on ohms and move the foot switch around. Could be an internal break in the cable that has rubbed through connecting ground and signal together? Just guessing here.

nosaj

[52 Bill]

Thoughts? Some capacitive coupling back to the reverb circuit? Maybe a parasitic oscillation from speaker to reverb circuit? It's trippy...man.

Turn down the reverb control and see if it stops.

Remember that the foot switch wiring has the reverb return signal running out to the pedal, so the shielding grounds need to be solid.

[Sowhat]

In my opinion It's the footswitch connector. Like Bill said, the actual signal goes to the footswitch, not just a control line, if the connection is not perfect all hell will break loose as you have been experiencing. I've had this problem with some jobs before and there was no method that would clean or repair the connector no matter what I tried. I had to replace the connector to fix the problem, maybe you'll luck out with some repair process but I rarely have such luck with these.

[lowell]

Turning down the reverb doesn't change it. However, turning up the reverb while the switch is touching the speaker frame adds a high pitch feeback. Not sure what you mean about the connector. I'd think the reverb footswitch wouldn't work if there was a connection issue. But maybe you can clarify what you mean. Thanks!

[Sowhat]

Turning down the reverb doesn't change it. However, turning up the reverb while the switch is touching the speaker frame adds a high pitch feeback. Not sure what you mean about the connector. I'd think the reverb footswitch wouldn't work if there was a connection issue. But maybe you can clarify what you mean. Thanks!

Not really saying you have a broken connection, I think it's a flakey connection where the signal circuit to the footswitch has developed some resistance when it requires it to be no resistance at all. I don't know if it's a corrosion problem or what but I have never been able to clean one with much success and have never seen any visible corrosion either. Try cleaning the connector with some Deoxit or something similar, maybe that will fix things, then again maybe not. I have replaced that connector however and that cleared things up... maybe I was just lucky.

[nosaj]

Not really saying you have a broken connection, I think it's a flakey connection where the signal circuit to the footswitch has developed some resistance when it requires it to be no resistance at all. I don't know if it's a corrosion problem or what but I have never been able to clean one with much success and have never seen any visible corrosion either. Try cleaning the connector with some Deoxit or something similar, maybe that will fix things, then again maybe not. I have replaced that connector however and that cleared things up... maybe I was just lucky.

You can find out for sure by bypassing the connector temporarily by doing a direct solder connection.

nosaj

[lowell]

Ok so I am investigating this. When I touch the footswitch to the speaker..specifically where the magnet and frame meet...I get an alarming spark! Yikes.

I realize now what's happening. One of the 2 speaker terminals are also the terminal bolt to the frame. The speakers were wired with HOT as the frame terminal. When the footswitch (common) touches the frame it shorts the output transformer secondary!! Wow. Wild I haven't seen this before since the footswitch in such an amp LIVES in the back. Rewired the speakers so frame is chassis - fixed.

And hey, bonus, the OT is still working.

[Sowhat]

Great find there, I'll have to remember this one.