Update;
I worked on this all day, 10 hours straight. This makes the third day in a row when I've done nothing but Ampzilla boards, but I am picking up speed.
The last one I worked over had some bad burns and many trashed parts. I just removed and replaced all 22 transistors, 16 resistors, all of the electrolytics that hadn't already exploded, and about 20 diodes. It was a pretty thorough overhaul and took a few hours to get done.
Amazingly, when I dropped the board into the amp and slowly brought up the power not only did it not re-explode, it biased up, balanced out and put out 20% over rated power, a full 240W. I'll run it some more and make sure it's not a time bomb, but it looks like this one's a runner. It's amazing that this worked, considering how little I understand about these circuits.
The next set of boards is a set of the 121F model, the second to the last version. These don't appear to have any more than a few scorches. I might get away with a few parts swaps to get them running.
OTOH, they're 30 years old and who knows what they've been through. It can't cost $25/board and a couple additional hours to swap out all of the semiconductors. Then I'd have confidence that the board wasn't handicapped by a weak part tha twas getting ready to fail and spit magic smoke.
Anyway, I'm happy. I now have 3 working Ampzillas and another probably just a day's work away from done.
I worked on this all day, 10 hours straight. This makes the third day in a row when I've done nothing but Ampzilla boards, but I am picking up speed.
The last one I worked over had some bad burns and many trashed parts. I just removed and replaced all 22 transistors, 16 resistors, all of the electrolytics that hadn't already exploded, and about 20 diodes. It was a pretty thorough overhaul and took a few hours to get done.
Amazingly, when I dropped the board into the amp and slowly brought up the power not only did it not re-explode, it biased up, balanced out and put out 20% over rated power, a full 240W. I'll run it some more and make sure it's not a time bomb, but it looks like this one's a runner. It's amazing that this worked, considering how little I understand about these circuits.
The next set of boards is a set of the 121F model, the second to the last version. These don't appear to have any more than a few scorches. I might get away with a few parts swaps to get them running.
OTOH, they're 30 years old and who knows what they've been through. It can't cost $25/board and a couple additional hours to swap out all of the semiconductors. Then I'd have confidence that the board wasn't handicapped by a weak part tha twas getting ready to fail and spit magic smoke.
Anyway, I'm happy. I now have 3 working Ampzillas and another probably just a day's work away from done.
djk said:
Looking at the voltages stated in the GAS schematic for the 121F driver boards, the driver transistors Q117, Q118, Q103, Q104 each have 1/2 of one side of the B+ supply from C-E, 40V out of the +/- 80V supply, so they are within 100V ratings in static conditions. At the 44VRMS output voltages that I observed, they would experience peak voltages of as much as 88V C-E unless I'm wrong. That would also be within 100V ratings.
That's not to say that TIP41/42 would be ideal components for this purpose, but the application appears to be within their C-E voltage rating.
Please clarify if I've misinterpreted.
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