High pitched whine from BACK of speaker

[dndemattia]

Good afternoon,

I have a Fender BXR Sixty that has come across by bench and have gone through the basic troubleshooting. I checked for grounding issues and replaced the main power supply filter caps. The amp still has a high pitched whine that seems to be coming from the back of the driver and I don't hear it in the headphones. Anyone have any ideas?

Thank you,

David

[52 Bill]

I thought that the BXR amps all had sealed back enclosures. How can you tell something is coming from the back of the driver?

The headphones are driven by the main power amp, so I'd guess that there is a parasitic oscillation happening related to the speaker output wiring. Also look for broken high frequency snubber caps all over the board.

[dndemattia]

52 Bill,

I thought it was odd as well, but the whine is coming from the back of the cabinet when I put my ear to it.. I really can't hear it from the front of the driver. Thanks for the diagram and the suggestion. I'll get to it.

[dndemattia]

Here are todays results. Hooked the scope to the speaker and I'm getting a 5 milli volt high frequency wave on the output with the volume at zero. Also, when I plug in my bass and leave the volume at ZERO, I see waveforms on the scope (but no sound) when I play the bass. Is this right? Also, sound the transistors be getting hot when volume at zero.

[dndemattia]

Just wanted to follow up on the repair. After weeks of troubleshooting, I found that the transformer was resonating against the case. I pulled the transformer, poured some black model paint in the coils of the Xfmr and that fixed the mysterious whine. Oddest repair I have ever had.

[J M Fahey]

Get used to it; you'll find stranger things.
Vibrations in bass cabinets are a pest, and sometimes impossible to solve.
What will you do when you find a buzzing improperly glued cabinet part?
Sometimes you can run (internally) a thick bead of glue along all internal edges , sometimes you do not have access to the buzzing part and rebuilding the cabinet is impo$$ible.
Oh well.