GFA-535: Looking for troubleshooting help - diyAudio
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Old 2nd May 2013, 12:19 AM   #1
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Default GFA-535: Looking for troubleshooting help

Hello all,
Let me preface by saying that this is the first amp I've really had a chance to get my hands dirty with; I've only got into audio since I graduated college.

I had just finished replacing most of the old caps and the amp was sounding great. DC offset was about 20 mV on both channels, which, as I understand it, isn't horrible. Everything else checked out.

The problem came as I was adjusting the bias. I wasn't using any clips (lesson learned ), and my roommate burst into my room, scaring the living hell out of me. Well, the probe slipped out of my hand as I jumped a foot and contacted something on the amp board. Both fuses blew, naturally.

I can replace the fuses, but B- continually blows. Seems like the B+ fuse holds just fine. I have a schematic; anybody have any recommendations on where to start? Could it be that the transistor was toasted? Is this really vague?

Thank you!
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Old 2nd May 2013, 02:08 AM   #2
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i would pull the transistors one by one and test.

i'd start with the ones in the bias cir first.
if you post a copy of the scematic it might help.
I'm not crazy, my mother had me tested
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Old 2nd May 2013, 04:37 AM   #3
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Here's what I have for the schematic. I have a very vague idea of how everything is coming together, so again, any suggestions would be much appreciated!

If it does turn out to be that the transistors are shot, does anyone have a good source for replacements? I've seen lots of 'generics' on the Bay and such, but is that one of those cases where it shouldn't be trusted?
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File Type: jpg GFA-535 schematic.JPG (621.3 KB, 53 views)
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Old 2nd May 2013, 06:08 AM   #4
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Check those high power transistors first,esp the output stage....

For the transistors,many no longer produce by original factory but still available through KEC or Fairchild.
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Old 3rd May 2013, 05:57 PM   #5
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Thanks for the help! After checking, the transistors appear to be shot.

I've been to both of those sites before, but never for transistors; I'll check it out!
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