Audio Research M100 Thumping

I just picked up a pair of ARC M100 mono blocks. When I turn them on, all is good. They operate fine for about 15-20 minutes, then the output cuts on one, and it begins a three-beat thump which sounds like a loud heartbeat. This occurs even if the volume is all the way down on the preamp. It also bleeds over to the other channel, but if I pull the speaker wires from the offending channel, the other channel is clear.

Ideas? Should I simply try replacing some of the output 6550s?


Not really...Looking at it on a scope but it could also be a faulty filter cap as well. I would suggest having a tech you trust look at it. If that preamp has a tube rectifier in it, make sure the tube is functioning and if need be replace it.
Thanks. I gave up and took it to a "guy." He is finding all kinds of problems, mostly poor workmanship by prior repair attempts. The person who owned it before me spent $1,000 on shoddy work. And it was not local, so he paid shipping too. I befuddles me why he would not just have shipped it to c-j and had it done right (and probably for less). It is so modified now c-j told me they would not touch it. But my guy is good (did fantastic work on my McIntosh MR-67), so I have high confidence.
Yup I have an ARC SP-10 that's been modified and the work was simply awful. It's taken me forever to find all the problems wrong with it but I think I'm finally in the home stretch.