My Counterpoint SA-100 broke. It started having problems shortly after I replaced the tubes in it, with Electro-Harmonix 6922 tubes. The left channel would make distortion and later the right channel too, especially at low frequencies. I just let it run like that for a year since this was just used for TV speakers. But, then things got worse and the left channel quit working altogether.
I see the gate stopper resistors are blown on the N-Channel devices. They smolder and smoke when turned on. So, the output transistors are toast as well.
I don't know what blew the transistors though, unless it happened when a tube died? Maybe a large transient blew the gates? But the gates are protected with diodes. It looks like the tubes are capacitively coupled to the gates with Wima 1uF caps. Those might need to be replaced as well.
Sooo ... I'd like to find a schematic for this, if it's out there. It would help, though I think I can do the repair without one. The left channel seems to work okay now with the new JJ 6922 tubes.
It seems to be just a matter of blown output transistors. They are the 75 watt IRF130 and I can get replacements at Mouser.
One other thing. Both channels have been running really cool (ice cold) for years. They used to run fairly hot. I didn't think MOSFETs need bias adjustment, but I guess they might after all this time. Or maybe Counterpoint adjusted the bias to run cooler after I stuffed it under a cabinet once and it over heated, and got damaged? Well, probably not.
The potentiometers for bias and DC offset are labeled, so that should be easy enough to fix. Got to these babies cooking again!
If I get the problem fixed I may also do some mods, but one thing at a time.
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