Marshall avt50 unstable

[unparalleled]

I have a real nut-scratcher fault in my shop.
The output stage amp chip is totally unstable, I have replaced it and the driver op amp and all associated components to no avail.
I am at a loss now and thinking of discarding the whole PA module in favour of another type of 50W amplifier device.
When the output chip is unplugged a clean signal is present at pin #1 (from the preamp).
As soon as the signal connector is replaced the top half of the scope trace clips, even when the PS connector is not connected to the output module.

This amp is just out of warranty and has had problems before.
The zobel network 10E resistor was replaced with a 10K after the original fried because of HF.
Has anyone else struck this problem?

[Enzo]

Well there is your problem, right there. You replaced the 10 OHM resistor with a 10k. No wonder your zobel network doesn't do anything.

[unparalleled]

Well there is your problem, right there. You replaced the 10 OHM resistor with a 10k. No wonder your zobel network doesn't do anything.

You misunderstood, I fitted 10 instead of10k someone else fitted.
The fried original would have been caused by the amp's tendency to become a radio transmitter.
I have cured a lot of weird faults in my time but this design seems to have instability built in!

[Enzo]

If that were true, then they all would do this. We need to find your problem.

ANy chance the little 22pf cap across the TDA input pins is bad?

What is the health of C44 at the - input? And of bootstrap cap C15?

Leaving the TDA unpowered, but connected kills the + half waveform at the TDA input pin, is that right? Is anything going on on the output bus at that time? ANy DC leakage from anywhere?

And is 0.22 ohm R82 in the speaker return OK?

[unparalleled]

If that were true, then they all would do this. We need to find your problem.

ANy chance the little 22pf cap across the TDA input pins is bad?

What is the health of C44 at the - input? And of bootstrap cap C15?

Leaving the TDA unpowered, but connected kills the + half waveform at the TDA input pin, is that right? Is anything going on on the output bus at that time? ANy DC leakage from anywhere?

And is 0.22 ohm R82 in the speaker return OK?

C44 replaced, 0.22E in feedback is OK, 22pf appears OK but I will replace in case it is failing under dynamic conditions.
The top 2/3 of the sinewave at OP clips off when signal is increased even when clean sig. is injected at the TDA chip input.
Signal from preamp out is clean unless connected to TDA board, whether the TDA power plug is connected or not.
The TDA bypass caps were 10nF not the recommend minimum specified 100nF for stable operation per the TDA manufacturer's spec sheet.
Installing 100nF improved matters but did not fix the problem.

[unparalleled]

Avt50 unstable output stage

R58 burning out seems to be normal behaviour for this model, I am surprised the cure has not been posted on the Net. (Bump)

[Enzo]

That is a common problem in all solid state amps. ANything that causes continuous high freq oscillation, such as feedback or loss of ground or a variety of other circumstances, will mean that the poor resistor must conduct all that energy to ground through the cap C92.