Well,I finally got back at the amp-Twenty transistors out,twenty transistors cleaned and checked and twenty transistors put back in with new silicon pads.What a time I had getting all the thermal paste cleaned out of the holes.This did not fix the amp but I can rule out bad mica gaskets. My next assault at fixing the amp will be new zener diodes and caps near the opamp. Cronie
I bought this amp off ebay. The guy who had it had it serviced to get the hum out. New isolators were put in with thermal paste.It didn't fix it so they gave up and decided to sell it as is on ebay.I bought it very cheaply knowing about the hum and decided to take on the challenge of fixing it. so far it is kicking my butt.lol. There is quite a lot of thermal paste around the transistors.could the thermal paste itself become conductive? I think I read somewhere it might. I also changed the opamp to no avail.
First,thanks for the reply.I also found out myself by looking at the dc300 type 2 schematic.Right now im pulling my hair out trying to eliminate 1/4 volt 120 cycle buzz equal in both channels. So far ive changed the bridge rectifier,the "beer can" caps with known good ones. Also changed the caps and diodes near the 42ac feed to the main board. Now im fiddling with the bias.I think Ive eliminated all the "easy" solutions-loose screws,broken ground straps,loose wires etc..Its funny-gains off it hums,gains in middle-no hum and gains full hums again. I can adjust where the hum appears and disappears using the input pot but cant get rid of it totally. But if the input pot is set according to spec it hums quite loudly. Any ideas?