2. amps-preamps
  3. 1406857519

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?

Thanks,

Brian.
bkrpdx

Showing 6 responses by hifigeek1

hifigeek1
770 posts
 
You may have a bad Hexfred in the power supply.
hifigeek1
770 posts
 
The only other thing it may be is the coupling caps in the C/J are leaking DC voltage causing it to destabilize the M100's. It's also possible a regulated power supply in the C/J or caps in the C/J are defective. How old is your C/J preamp??
hifigeek1
770 posts
 
This is called motor-boating and it means the amp is oscillating. M100's run very hot. Make sure the driver tubes are okay. I believe (from memory) they are either 7119's or 7044's. Those could be problematic. I would suggest you find a place to get both your amps checked out. I would not run the damaged amp.
hifigeek1
770 posts
 
Run the amp without turning the C/J on, but leave it connected to the amp, and see if it motorboats. If after the 15-20 mins. the amp remains fine, it's not the amp.
hifigeek1
770 posts
 
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.
hifigeek1
770 posts
 
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.
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