Built amp about a year ago. Initial startup good. Listened to it for few hours over several days. With a CD playing, I heard a loud crackle through speakers and then pop and went silent. Then the burnt electic smell. Opened it up and the left channel switch and both resistors were smoked. Bob asked me to check line voltage which was pretty high. Suggested a variac and sent me replacement DPDT switch and resistors. Replaced parts and checked everyhting else. Found one bad solder joint on speaker terminals but that was it. Reflowed a couple others and tried to go through startup again. Could not adjust bias voltage which was at 0. Original failure must have fried rectifier. Replaced it and went though startup again. Was fine for a minute but before I could even adjust first resistor, popped inside and blew fuse of variac. Open it up and the green and red wires on 1 & 2 of the left channel switch again show they were overheated. Collateral damage to the yellow wire that goes to the board. Looks like some arching issues. Switch was in linear mode (towards front ). the ST120 fuse never blew.
Bob referred it to Joe in Tucson who asked me to check pin 8 of recitfier to ground which was in 10-13 MOhm range.
This is where I am at.
I believe it was wired right as it worked for several hours. This is my first build but was not novice to soldering. Worked on C-130 auto pilot amps and compass systems and did fairshare of soldering then but that was a while ago. But still understand a good solder joint.
Am going to look at every connection for evidence of arching. Something has to be shorted to gound somewhere. Any common items to look at first? Quad cap?
Last edited by RogerRef on Sun Oct 24, 2021 2:25 pm; edited 1 time in total (Reason for editing : spell check)