I have what I call a "bench-bomb" on my bench...in this case a piece of Hi-Fi gear that I took on in a moment of weakness (although, to be fair, it was a referral from an internationally respected studio acoustics colleague).
Enough preamble: The issue, with this 1970's vintage transistor pre-amp is; after it has been on, with test tones running, for about 20 minutes, one channel slowly starts to fade out (can take several minutes) and then go dead.
On the scope, the negative side of the waveform starts to flatten out first, with the positive side looking healthy, IIRC, right up until the signal disappears. This originally came back to me, after re-capping it with the fault in Ch-B. I traced that back to a shorted transistor in the line amp, Q56 (although I don't think that transistor was actually shorted to begin with...how could it work for 20 minutes if it was?). I replaced that transistor, a 2N24249 (although it's called out on the schemo as "2N4250") with a MPS92 (one is EBC and the other CBE so I just had to mount it the opposite way) and that side came back to life and hasn't failed in any subsequent tests. However, when I was burning it in, Ch-A decided to fail in exactly the same way, except that replacing Q6 does not solve the issue (nor was it shorted). NOTHING is getting hot, or otherwise showing signs of thermal stress at all. Before the failure sets in, both sides show essentially identical THD and N.
I have looked pretty thoroughly for cold solder joints and haven't found any. And, normally, such a joint would be more likely to NOT work until it got a blast of signal or was wiggled etc, not the other way round. I have never seen this behaviour before. Any ideas? I'm only including the relevant section of the schematic as I got the manual from another site and I'm not sure what their re-use permissions are.
Enough preamble: The issue, with this 1970's vintage transistor pre-amp is; after it has been on, with test tones running, for about 20 minutes, one channel slowly starts to fade out (can take several minutes) and then go dead.
On the scope, the negative side of the waveform starts to flatten out first, with the positive side looking healthy, IIRC, right up until the signal disappears. This originally came back to me, after re-capping it with the fault in Ch-B. I traced that back to a shorted transistor in the line amp, Q56 (although I don't think that transistor was actually shorted to begin with...how could it work for 20 minutes if it was?). I replaced that transistor, a 2N24249 (although it's called out on the schemo as "2N4250") with a MPS92 (one is EBC and the other CBE so I just had to mount it the opposite way) and that side came back to life and hasn't failed in any subsequent tests. However, when I was burning it in, Ch-A decided to fail in exactly the same way, except that replacing Q6 does not solve the issue (nor was it shorted). NOTHING is getting hot, or otherwise showing signs of thermal stress at all. Before the failure sets in, both sides show essentially identical THD and N.
I have looked pretty thoroughly for cold solder joints and haven't found any. And, normally, such a joint would be more likely to NOT work until it got a blast of signal or was wiggled etc, not the other way round. I have never seen this behaviour before. Any ideas? I'm only including the relevant section of the schematic as I got the manual from another site and I'm not sure what their re-use permissions are.
