Tranny failure

[Mark Hennessy]

Well, one of the reports is that it suddenly drops in output level.

I've asked the question, but don't yet don't know if it's clipping when it drops (implying PSU or similar), but if it drops while still sounding OK, there's a chance this might be it.

But I think that he's testing it via a lamp limiter, so if it's oscillating, the cross-conduction in the output stage will bring down the rails while lighting the lamp. So if there's gross clipping when it's in "low output level" mode, then it's probably something else.

It's quite hard to piece it all together - a solid-state amp must be one of the hardest things to diagnose remotely...

[Diabolical Artificer]

Thanks chaps. Testing done without LL. The lamp lighting was a phenomenon I noticed weeks ago and thought I'd relate it as it might be pertinent . It's hard to keep an eye on all meters, scopes etc, but AFAIK, it's not clipping at all, I'll check today if it clips at low power.

I found it hard to count the peaks and was too lazy to work out what F it was with the oscillating OP on a scope, so cheated and stuck it on a F counter. I'll see if I can pick it up if I can find an old fashioned radio.

I've video'd my antics as usual and will pop it on Ut when my PC stops being silly. It might make it easier to see what's happening.

Thanks agian, Andy.

[pwdrive]

I would measure the current draw for each channel under fault conditions to get an idea as to what's what as well as measuring the B+ at the same time.

Lawrence.

[davegsm82]

As previously mentioned, measure the actual B+/- rails to see if they are sagging under high load.

I encountered many transistorised PA/Disco amplifiers during my time at Aitkens and one oddball fault which caused the symptoms you describe was the soft start circuit, usually encountered on big amps with toroid transformers but sometimes seen on small hifi gear too. The amp's would run quite happily through the startup resistors for quiet audio, but as soon as you cranked them up the rails would sag and the amp would hit protection mode. I can't see from the diagram but it doesn't appear to have a soft start, but you say this amp isn't the same as the one in the diagram?

Dave.