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Hello jimcrane. My first time replying to anyone on any issue.
I tinker with audio gear and amps are easiest to fix. Much more info is needed. This will take sometime. When channel cuts out is there a noise at that moment such as a boom, scratch noise? When channel goes quiet is there a noise from the speaker such as a low hum or hissing? When channel goes out stop playing music and listen to both speakers up close. Every amp has a quiet noise when it is idle with no music playing called temperature noise. If both speakers have identical sound the most likely problem is before amp stage and probably in amp input stage. If good channel is normal temperature noise and bad channel is dead quiet it is in final out put stage. I wonder if there is a rely on the end of out put stage with bad contact.
1-Unfortunately you have to install cheap pair of speakers instead of main speakers (protect them from load bursts), take the cover off and look for relays. Run your amp until bad channel goes quiet. With a plastic rod (break a plastic coat hanger to get a 8 inch or longer rod and slowly press on relays, connectors and boards that can flex looking for bad connection or poor solder joint. Sound might come back and try this process many times to zero in on a component.
2-If it is heat issue with out put transistors going bad you should see discoloration of circuit board at the part that is running over temp. Amp has protection to shut it down when temp is high. That is why power cycling amp does not help. How ever cooling it down might make it come back to normal. That is sign of bad output transistor.
There are some many ways this can happen that might fill 10 pages. Lets us start with simpler things.
Thanks to everyone for taking the time to reply.
Bsamooa, special thanks for your detailed questions and suggestions. To respond briefly: (1) there's no noise (e.g., boom or scratch) at sound cut out; (2) after sound cut out, no hum or hiss from the quiet channel; (3) I haven't tried listening after sound cut out to both speakers without music playing.
Bsamooa and noromance, I'm going to show your post to the repair tech. He said he couldn't duplicate the problem so he couldn't diagnose it, but perhaps your post will give him some ideas. If not, I'll bring the amp home, put it back into the system and when (and if) it fails again, I'll come back here and see what can be tried.
Ever since I've owned it, I've had the amp on a wood shelf nowhere near a heat vent, with 3-4 inches of open space around the amp's top and sides. The top (which has vents and which I've always kept clear) has always gotten warm to the touch during extensive use. Consequently, I don't think it's an overheating issue.
Not familiar with this unit, but looking at photos posted on this unit, does the design incorporate output relays ? The problem described, ime, sounds like this might be the issue, as relays have a notorious history of failing. Or an output current limiter circuit. Of course, it would need to happen during a test bench check, as experienced. Possibly, a weak solder joint, as well. Just some thoughts.
Jim, I've no helpful suggestions, but had that identical problem with my Phase Linear 4000 Preamp's right channel off and on for over forty years. I took it to three different repairman over that time, and even back to the Phase Linear factory, which was located outside Seattle back in the eighties, not too far from home then. None of them could find or engineer a permanent solution to whatever the problem was.
A bunch of replacement caps helped it keep going for a while the last time around. The final solution came last year in the form of a dumpster. I gave away the matching Phase 400 amp, which was still going strong and never a moments trouble. I wish you the best of luck finding a better solution in a much shorter amount of time.
Bring it to a good tech. Have him spray the circuit board with a spray made for identifying where the cold soldered joint(s) are. Glen Grue of Classe told me they had bad batches of circuit boards on the CAP 101 and CAP150. It is an easy and cheap fix. Don’t throw out, it could be given to someone just starting out after it is fixed.
Thanks to everyone for their help!
Stereo5, I'm going to try contacting Classe again. Who knows, maybe I'll get a response this time.
The local repair shop I went to is Audio Specialties here in Portland; the owner, Doug, is well-respected. I don't know specifically what he did, but I would trust him to have done a thorough inspection. Still, I may ask him about identifying possible cold solder joint issues.
Bigkidz, you're very generous to offer to look at it. But I'm in Portland, Oregon, and I think the shipping costs alone would be prohibitive relative to the probably fair market value of the amp, which I think is in the $400-500 range. When repair costs are factored in, I'd have that same consideration if Classe wants me to ship it to them.
Skyscraper and EBM, I feel ya.
The amp is back in my system now, working without problem. But tick, tick, tick.....
One final thought: If Classe knew that boards were defective, even it discovered the fact after manufacture, should it have an obligation to repair/replace the boards without charge? Now, it's been around 20 years (or more) since the amp was measured, so Classe probably doesn't have any legal obligation (statutes of limitation and ultimate repose and all that), but how about a moral or ethical obligation? If Classe responds, I'll ask, but I'm not hopeful.
Again, thanks to all! A great community here!