30-11-2016, 10:26 AM
This is most curious 
Generally, it's unusual for output transistors to be noisy. At the output stage, any noise they make won't be amplified by further stages, so for their noise contribution to be heard on the loudspeaker would require them to be very noisy indeed. Obviously, the converse is true for transistors in the earlier parts of the circuit, where - for example - the noise generated in TX1 will be amplified by everything following on from it.
It is possible for germanium transistors to fail with gross noise. I've seen this at least half a dozen times in the Hacker sets, where an AC128 in the role of driver (or "VAS", more correctly) intermittently breaks down between collector and emitter, producing large spikes at the output. I've yet to see this with the output devices - perhaps the breakdown mechanism is "weak", so is unable to pass enough current to affect the output (remembering that the opposing output transistor will "fight" it via negative feedback). Sadly, in each case, I simply throw away the offending transistor and move on (time is money on a repair job). Next time, I'll investigate further.
Anyway, while that might be the same mechanism that you're experiencing, what are the chances of it affecting 4 separate transistors? It's not impossible, but it does seem highly unlikely.
What happens when you swap TX3 with TX4? Could it be the diode? Or even the pre-set resistor? I'd want to monitor the DC voltage at the mid-point to see what changes with temperature occur, because it could be - as you imply - that freezing the transistor is effectively turning it off. For silicon, the Vbe temperature coefficient is around -2mV/C - without looking it up, I know that germanium is much greater than that. Certainly, I'd expect that voltage to fall as TX3 only is frozen.
Generally, it's unusual for output transistors to be noisy. At the output stage, any noise they make won't be amplified by further stages, so for their noise contribution to be heard on the loudspeaker would require them to be very noisy indeed. Obviously, the converse is true for transistors in the earlier parts of the circuit, where - for example - the noise generated in TX1 will be amplified by everything following on from it.
It is possible for germanium transistors to fail with gross noise. I've seen this at least half a dozen times in the Hacker sets, where an AC128 in the role of driver (or "VAS", more correctly) intermittently breaks down between collector and emitter, producing large spikes at the output. I've yet to see this with the output devices - perhaps the breakdown mechanism is "weak", so is unable to pass enough current to affect the output (remembering that the opposing output transistor will "fight" it via negative feedback). Sadly, in each case, I simply throw away the offending transistor and move on (time is money on a repair job). Next time, I'll investigate further.
Anyway, while that might be the same mechanism that you're experiencing, what are the chances of it affecting 4 separate transistors? It's not impossible, but it does seem highly unlikely.
What happens when you swap TX3 with TX4? Could it be the diode? Or even the pre-set resistor? I'd want to monitor the DC voltage at the mid-point to see what changes with temperature occur, because it could be - as you imply - that freezing the transistor is effectively turning it off. For silicon, the Vbe temperature coefficient is around -2mV/C - without looking it up, I know that germanium is much greater than that. Certainly, I'd expect that voltage to fall as TX3 only is frozen.
