24-04-2012, 04:51 PM
The cct. diag.: got it, thanks.
So I see that is doesn't use diodes to bias the O/P transistor pair: it uses a transistor: O.K. Now reading through what you've reported so far, the following seems likely. On the ext. PSU, things seem reasonably O.K. - and there is no motor-boating. But when powered from the batteries, the trouble begins. You can adjust the quiescent to a certain value, and then that current jumps. All that, to me says this: Your ext. PSU is delivering a PSU voltage that is about 3v less than what the batteries are delivering - and that that 3v difference is critical (it shouldn't be: it's a symptom of the fault condition). When you adjust the bias pot., the current through the O/P pair correctly increases up to a point - then trouble begins. The link between that and the two sources of d.c. power is that either change (PSU source or bias adjustment) is increasing the gain of the amp. but to a point where it is going unstable - hence the motor-boating (low-freq. instability). I don't think you have any faulty semi-conductors now. So, on the basis of all that, I'd change all electrolytics in the feed-back path, one at a time, testing after each replacement, starting with C2 - then C3, C4, C6 and C9
Amendment!
I've just seen your recent post showing waveforms - I was about to suggest hooking up a 'scope to do just that! And now we have a much better idea of what is going on: it's gone unstable - at R.F.: high-frequency instability !
I thought that might have been a possibility: I was on the brink of suggesting that. So forget the above comment on electrolytics for now - change C9 first. If that doesn't fix it, change the following, testing after each replacement: C6, C4, C3, and C2. If none of those replacements fixes it, go back to my earlier comment about replacing the electrolytics.
Al.
So I see that is doesn't use diodes to bias the O/P transistor pair: it uses a transistor: O.K. Now reading through what you've reported so far, the following seems likely. On the ext. PSU, things seem reasonably O.K. - and there is no motor-boating. But when powered from the batteries, the trouble begins. You can adjust the quiescent to a certain value, and then that current jumps. All that, to me says this: Your ext. PSU is delivering a PSU voltage that is about 3v less than what the batteries are delivering - and that that 3v difference is critical (it shouldn't be: it's a symptom of the fault condition). When you adjust the bias pot., the current through the O/P pair correctly increases up to a point - then trouble begins. The link between that and the two sources of d.c. power is that either change (PSU source or bias adjustment) is increasing the gain of the amp. but to a point where it is going unstable - hence the motor-boating (low-freq. instability). I don't think you have any faulty semi-conductors now. So, on the basis of all that, I'd change all electrolytics in the feed-back path, one at a time, testing after each replacement, starting with C2 - then C3, C4, C6 and C9
Amendment!
I've just seen your recent post showing waveforms - I was about to suggest hooking up a 'scope to do just that! And now we have a much better idea of what is going on: it's gone unstable - at R.F.: high-frequency instability !
I thought that might have been a possibility: I was on the brink of suggesting that. So forget the above comment on electrolytics for now - change C9 first. If that doesn't fix it, change the following, testing after each replacement: C6, C4, C3, and C2. If none of those replacements fixes it, go back to my earlier comment about replacing the electrolytics.Al.
