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I have an audio amplifier (Peavey KB-100) which was being used and it lost power out of nowhere. I opened it up to take a look and it turned on again later, so I suspect the thermal switch which is between the fuse and the transformer primary probably opened. I noticed that the transformer and the heat sink for the output transistors were pretty warm. The amp was being used at a relatively low volume (low enough not to annoy any neighbors from our apartment), but the amp had been left on for about a day. I'm trying to figure out why it overheated.
Today I've been measuring the quiescent current through the output transistors by measuring the voltage across the emitter resistors. It slowly creeped up from about 30mA right after turning it on, to about 100mA an hour later and stabilized there. The output stage uses +/-42V rails, so this is wasting over 8W with no signal input and no speaker attached, and the heat sink is fairly warm. I simulated the output stage in LTSPICE but got even higher quiescent current so that's not much help.
I don't know if this slow drift up of the quiescent current is normal. There's a "stabistor" diode in the output stage which goes through the heat sink (circled in the picture below, and labeled CR25 in the schematic). I'm trying to figure out what I might be able to replace to fix the overheating issue, and it appears that stabistor diodes aren't made anymore. If I replaced it with two individual diodes in series, only one would be able to go through the hole in the heat sink so I don't know if that would make things worse since it's part of the bias circuit. The schematic lists some quiescent voltages and I measure roughly the same. Could another option be a faulty thermal switch? You can see it in the picture with the yellow wires attached, and also labeled "T.B." on the right side of the schematic. I also notice that it appears the heat sink is meant to be attached to the chassis, which would of course help the heat spread, but it is not attached.
The only other issues I see are some slight discoloration around two power resistors that are used along with zeners to produce the +/-15V power rails. And there's another 2k power resistor in the power supply circuit which is completely open, but appears to be used just to discharge the positive supply capacitor (it's in parallel with the +42V rail capacitor). I don't see how either of those things would contribute to overheating of the heat sink.
Any other thoughts on the overheating?
Today I've been measuring the quiescent current through the output transistors by measuring the voltage across the emitter resistors. It slowly creeped up from about 30mA right after turning it on, to about 100mA an hour later and stabilized there. The output stage uses +/-42V rails, so this is wasting over 8W with no signal input and no speaker attached, and the heat sink is fairly warm. I simulated the output stage in LTSPICE but got even higher quiescent current so that's not much help.
I don't know if this slow drift up of the quiescent current is normal. There's a "stabistor" diode in the output stage which goes through the heat sink (circled in the picture below, and labeled CR25 in the schematic). I'm trying to figure out what I might be able to replace to fix the overheating issue, and it appears that stabistor diodes aren't made anymore. If I replaced it with two individual diodes in series, only one would be able to go through the hole in the heat sink so I don't know if that would make things worse since it's part of the bias circuit. The schematic lists some quiescent voltages and I measure roughly the same. Could another option be a faulty thermal switch? You can see it in the picture with the yellow wires attached, and also labeled "T.B." on the right side of the schematic. I also notice that it appears the heat sink is meant to be attached to the chassis, which would of course help the heat spread, but it is not attached.
The only other issues I see are some slight discoloration around two power resistors that are used along with zeners to produce the +/-15V power rails. And there's another 2k power resistor in the power supply circuit which is completely open, but appears to be used just to discharge the positive supply capacitor (it's in parallel with the +42V rail capacitor). I don't see how either of those things would contribute to overheating of the heat sink.
Any other thoughts on the overheating?
