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Topics - Overthruster

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Amplifier Discussion / Fender Frontman 65R - power amp intermittent noise
« on: January 10, 2021, 12:50:14 PM »
Hi folks. I'm new to this forum but an old hand at audio electronics repair (30+ years) and I find myself really up against a puzzler with this one.

I have a Fender Frontman 65R guitar amp that produces sputter/static-like noise — intermittently (of course).
Here's some details:
  • It's definitely the power amp: I've disconnected the preamp and shorted the input to the power amp and nothing changes.
  • It's not thermal: Sometimes it starts immediately on power-up, sometimes it takes several minutes; circuit cooler has no effect, neither making noise better or worse, no matter what components I hit.
  • At this point I've replaced every semiconductor device in the power amp (they were all very inexpensive and/or I already had them on hand) except the output transistors.

I have not tried opening the feedback loop to isolate where the noise is originating because I don't trust this amp's stability running at open-loop gain.  (If there's anyone who has a Frontman 65 and is willing to give it a go I'd be much obliged  ;))

I've done lots of web searching and haven't found anyone with this problem on this amp, so your input and experience would be much appreciated.

I'll attach a schematic for reference.

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