We've kind of been going on the presumption that it's a bias failure.
Since only the top output transistor blew (is blowing), might it be a DC balance failure going positive and dumping current into the speaker?
When it blew did it just go silent or was there a "whump" or bad hum for a second?
How about checking leakage in C5040?
When you remove it, you can check your output DC offset without it.
(With that cap removed you have reduced your amplifier for both DC and audio to a gain of 1.)
Still, even if C5040 was shorted it would only amplify the DC offset of the input pair by 28 times.
Maybe Q5040 current sink intermittent failing would nail this thing positive and kill Q6050 (top output).
Check across D5000 for 5.6 V
Something shorting there would make this to go all positive.
Yes I’ve been trying to cross-out the possible reasons for this failure. I had a feeling that it might be something to do with the biasing as I have checked all the other factors to my best knowledge initially.
The first time the NPN was shorted, but the second time the two transistors had shorted out.
The borrowed output transistors are holding up so far (like last time). This time I want to run it on my bench long enough till I am confident that there are no issues with the amp. I will obviously return the amp with the new components soldered in and after running it again.
I wasn’t there when the amp failed but the owner did mention that there was a ‘pop’ sound when it went out.
I have checked C5040 with an ESR meter and it seems to be in order. I could remove it to make sure. If there was an issue with the capacitor being leaky what should I expect to see when I power up without it like you suggested?
I did check across D5000 and it seems to show proper forward voltage drop. I’ll check it after powering up for the 5.6V drop across it.
Any suggestion on how to suss out the possible intermittent failing Q5040?
Also please note that I measured the 50mV at the right channel output the left channel output measured around 30mV - the left channel was what had failed before. What would be the acceptable DC on the outputs?
Thanks for the help.