Adcom GFA 555-II- Didn't like short on output :-(

Magura · 17th November 2004, 12:15 AM

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?

Cheers

Magura

Nelson Pass · 17th November 2004, 01:05 AM

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.

anatech · 17th November 2004, 02:10 AM

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.
-Chris

mpmarino · 17th November 2004, 02:17 AM

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.

17th November 2004, 12:15 AM	#1
Magura diyAudio Member Join Date: Oct 2003 Location: Denmark, Viborg	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? Cheers Magura __________________ Everything is possible....to do the impossible just takes a little while longer. www.class-a-labs.com

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17th November 2004, 01:05 AM	#2
Nelson Pass The one and only Join Date: Mar 2001	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.

17th November 2004, 02:10 AM	#3
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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. -Chris

17th November 2004, 02:17 AM	#4
mpmarino diyAudio Member Join Date: Jun 2004 Location: Crunchville, where I don't fit in.	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.