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Messages - Overthruster

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Thanks for all the suggestions, everyone. I'm pretty confident I can rule out the power supply — I've looked at the supply line voltages on a scope and there's no sign of any spikes or noise. I'll start investigating the possibility of bad traces and solder joints now. (I've already re-soldered dozens of joints "just in case" but I have many more that I haven't touched yet.)

Have you tried inserting a signal in to the Power Amp In jack?
By doing this you isolate the preamp from the power amp circuit.

Yep. Tried that and even with the input to the power amp shorted it keeps making noise.

Nice idea with the power switch but no go. I bypassed it and nothing's changed.

Had something similar years back,, after much swearing turned out to be failing contacts in the main power switch. they were arching intermittently

Now that's an idea! I'll check that next. I've seen it happen with old Pioneer stereo receivers, though I haven't worked on one of those for ages.

Oh yeah. The differential pair was the first thing I went for. That's a really common source of power amp noise issues.

Amplifier Discussion / Re: Krell amplifier repair
« on: January 10, 2021, 01:20:48 PM »
I'm trying to trace through the power amp section of a Krell hifi amp.

Cute bit of trivia: The Krell company was named after the alien race in the classic 1956 sci-fi movie Forbidden Planet (starring Leslie Nielsen!) Supposedly the founder was taken with the line "The fool, the meddling idiot! As though his ape's brain could contain the secrets of the Krell!"

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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