Heathkit Power Amp Overheating

[melvin]

I have a Model AA-1640 stereo power amp that is overheating. One side gets hot after a while and trips the thermal protection circuit.

I tested it without a load and there's no dc on the output and the bias voltages at the input of the drivers are close to the numbers on the schematic. However, when I check the voltage across the 1ohm ballast resistors (R233-R236 etc.) , one of them has about 10 times that of the others. Should I assume this is the probem?
Furthermore, should I try replacing just that transistor?

An equivalent seems to be the MJ15024G which I'd have to order.

Any help or advice would be greatly appreciated.

[J M Fahey]

Nice beast of an amp.
For a quick test, just remove that one, and check that amp stopped overheating, obviously no load, no signal.
You *might* just have a leaky transistor.
Problem is: what caused it?
Maybe the amp was abused (repeatedly shorted, overheated, etc.) and that pack of transistors was degraded; obviously one was weaker than the others, it does not mean the others are safe and sound.
You'll probably have to replace its other 3 "brothers" too, the ones that are in parallel with it, if not for other reason, to share current evenly.
Good luck.

[Steve Conner]

That looks like quite a monster.

Now, do you mean that out of the 8 power transistors in a channel, just one of them is showing an abnormal reading on its ballast resistor and the rest are fine?

If so, I'd check that that transistor is properly in contact with the heatsink, the insulating washer and thermal compound are in good shape. Maybe it's going into runaway because it's not getting cooled as well as the others.

Or, more probably it's going bad as JM suggests. The MJ15024 would be a great substitute, and yes, replacing all four in the bank with MJ15024s from the same batch is advisable. Buy them from Mouser or similar, don't be tempted by the ones on Ebay that seem to be mostly fakes. (The G just means lead-free.)

Cheap option: The amp is so powerful that it would probably work fine if you just left the bad transistor out.

[melvin]

Thanks for all the advice. I got 4 MJ15024's from Mouser, installed them. Amp is working great, no overheating.