Ampeg B2R Issues

[Cbc526]

I have a client that brought in an Ampeg B2R Amp for repair. The issue is that after 20-30 minutes of straight playing, it will start loudly popping through the cab its hooked up to. I ruled out the cabling and cab since other amps have no issue with them. I cleaned and replaced the various I/O on the amp to see if it was the jacks warming up. The issue persists.

Its a difficult issue to diagnose and measure since its intermittent. there the pops are definitely bursts of DC since there is a lot of cone movement on the speaker when it pops. I am hesitant to just replace the output transistors as it may be elsewhere. anybody here have any idea what the issue may be?

[Jon Snell]

It is highly unlikely to be the output transistors.
Have you got similar noise coming out of J2, (Effects send socket)?
If you have the noise issue, it is pre amp related, if not monitor Tp6 and Tp7 l& ook for the noise.
No noise on those, check the driver stage and work your way through until you find that noisy resistor or capacitor.
A quick way is to lift C36 and check for noise; no noise it is the pre driver stage.
Keep logical and you will find it.

[The Dude]

Seems like a heat related problem. I'd get a can of freeze spray and start hitting components when the amp fails to try to narrow the problem down to a particular component.