Adcom GFA 555-II- Didn't like short on output :-( - diyAudio
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Old 17th November 2004, 12:15 AM   #1
Magura is offline Magura  Denmark
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Location: Denmark, Viborg
Default Adcom GFA 555-II- Didn't like short on output :-(

I have promised to help a friend of mine fix his Adcom GFA 555-II.

I have the service manual, but it dosn't seem to do me much good.

Anybody got any experience with this amp?


Everything is do the impossible just takes a little while longer.
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Old 17th November 2004, 01:05 AM   #2
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Why doesn't the manual help?

If it's a case of shorted output, then you probably have
blown output devices. Disconnect each one and look for
the blown one(s) which are usually shorted CE.
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Old 17th November 2004, 02:10 AM   #3
anatech is online now anatech  Canada
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Hi Magura,
Fixed a few of these. They aren't too bad. Keep testing transistors and resistors until they are testing good as you go back. Keep an open mind and make notes. Then relax and disassemble the unit, clean everything on the heatsink and reassemble with new parts. Do not overtighten the output transistors or drivers.
Generally speaking, I replace all outputs if a few short. Use On Semi parts if possible, ECG's and NTE's are not a good choice. If you do a search, I posted the factory bias setting proceedure earlier.
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Old 17th November 2004, 02:17 AM   #4
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I started this thread, got tons of help, and now have a great amp.

Looking for (info) Adcom 555 Mk2 Factory output trannys

edit: BTW, ON Semi was nice enough to send me some beautiful samples.
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