As Chuck (zmix) posted Those 2 wrong transistors were causing the oscillation and heat. He also said that the schem was wrong. Everything else was exact to the originals
I have no doubt that swapping the transistors takes care of the hum problem. What I'm not convinced of is that this is the best fix for the problem. You're talking about subbing in a fairly hard-to-find part with exceptionally good current handling capabilities that was hand-binned as being the best testing of all the silicon out of a production run. Those parts are the equivalent of the 3.33 GHz Core 2 Duo parts. They exist, but maybe one out of ten thousand parts passes at that level. If a circuit fails completely unless it contains a part that is binned at the very top grade, that circuit has something else wrong with it, and hand-picking parts to avoid failures, while an acceptable solution for one-off purposes (e.g. folks fixing their own devices), is not viable for mass manufacturing.
The question, then, is why the transistors are behaving in the way that they are. Those parts shouldn't be seeing nearly enough current from an audio signal to cause problems. They should be safe by two orders of magnitude. If they're seeing that much current, it may be an indication that something else is also wrong. By swapping the transistors, you may just be masking the symptom but leaving the root cause uncorrected. The root cause may also be easier to fix.
BTW, as best I understand it, it isn't two transistors. It is several sets of two transistors apiece, two each per EQ stage times... I think... four EQ stages.
And again, I question the analysis that the hum is in any way caused by oscillation. The noise that was originally described as oscillation did -not- go away even after subbing out the transistors.
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If the problem is truly caused by ultrasonic oscillation, it should be possible to use a capacitor to short everything over some reasonably high frequency limit to ground. That's a lot easier than subbing a hard-to-find transistor. That said, I don't think for a minute we're seeing only a single problem here, though. There have already been three or four posts (by the same author) that suggest other unrelated causes for hum, including positioning of the transformer relative to the inductor, etc. Heck, some of the oscillation could be something as trivial as the leads on one transistor being left too long.
I'm not saying that the proposed fix won't solve the problem---it almost certainly will. I'm saying that we should continue to look for the root cause and see if there is a simpler fix that will correct the circuit
without going to high-test parts.
