So I spent 6 hours with my cable/internet provider to undo the mess they made with no success so i was now prepared to work on the problem child. I with back to the Mistery Amp , a dual 500 , to try to sort it out. After my last post I took the time to clean volume and bias pots. I further changed out the out drives on the Pl32 board to 2n3439 and 2n3516 pairs and changed out the slowing caps on these to a lesser .105 mica cap.
I fired up and and after some minor connect problems I ran the beast up to about 10 volts. DC offset was very good at 1 mv per channel
After running for some time on the supposedly good channel I found some minor crossover distortion as per photo. This should up at higher , 8k +, frequencies in both channels and more on the negotiate side. N+THD was ok though. I switched to the problem Channel and ran at 8 volts output into 8 ohms to check temps. All was well for a while then it started to pull some serious current and blew the rail fuse on the possitive leg. I had downsized to 4 amp so it must have put out some big number befo tilting. Unfortunately I was not looking and jumped to power it off when it heard it load up. All the transistors have high ohms between collector and emitters. Mistery???
does one driver short when warm?? Does ic go wild?? I do not think bias related. This was a sudden power surge
Ideas please
glen