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Old 11th June 2014, 11:41 PM   #1
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Default Ampeg B2R with hf oscillation

Howdy folks. I'm a bit of a solid state noob and I'm working on an Ampeg B2R that I got in non-working condition. The output relay was triggered, so I jumped it out of the circuit. I get a very high-pitched oscillation through the speaker. The oscillation is present with or without the preamp board connected. The preamp out works just fine into another amp, but the power amp is goin nuts.

I read a 150vac ripple on the +60 rail, a 32vac ripple on the +16 rail, and both negative rails are clean. DC voltages are within 10% of spec and are stable. The o-scope shows very high frequency interference on the + rails, can't determine the frequency exactly but there's plenty going on above the audible range. I read a 10mv dc offset on the output, and between 20-30mv across R37, R39, R41, R47, R48, and R50.

It's obvious to me that something is kicking back hf interference of some magnitude into the positive rail, but beyond that I am stumped. Can anybody help me out? Schematic attached.
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File Type: pdf 2546d1212338958-ampeg-b2.pdf (438.4 KB, 24 views)
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Old 12th June 2014, 12:59 AM   #2
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To make matters more confusing, it all of a sudden started working. I will assume that one of the solder connections that I re-flowed or one of the jumpers that I jostled has solved it. The ripple is still there(possibly from the fan motor?) but the amp functions flawlessly. Bad ground cnxn perhaps...

I don't like mystery problems and mystery solutions because I never actually know if I've fixed it or not. I didn't learn much either. Still haven't put the relay back into the circuit. Won't get back to it until later tonight.

Thoughts?
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