Adcom GFA-555 Stupid Me... - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 12th November 2015, 01:48 PM   #1
Rob_K is offline Rob_K  Canada
diyAudio Member
Join Date: Dec 2007
Default Adcom GFA-555 Stupid Me...

Hi All,

New member here and am looking for any assistance.
Well, I purchased a used GFA-555 and the unit was working until I touched it.

I had opened it up to measure the bias. I was testing on the right channel resistor on the voltage amplifier board (not end board) when I accidentally shorted something.

The odd thing is the fuse on the output board blew. I borrowed the fuse from the left side and powered it up and the orange clipping light stays on and the speaker outputs on the right channel have 82 volts Dc on it.

I'm not a technician but can solder and measure using a DMM and have some very basic understanding of electronics.

I know this is a very old amp but was wondering if anyone can help me troubleshoot?

Now I was reading through the board last week and I came across a thread where someone did the exact same thing but I cant find it now.

I guess my other question is did I blow the Toshiba transistors as looking around the net, they don't make them anymore.

Any help would be greatly appreciated and any parts recommendations especially for the transistors would have me indebted to you if identified as faulty.

I really like the amp and the design looks very simple which is why I want to keep it. I guess this is my incentive to rebuild it from top to bottom.

Thanks everyone,

  Reply With Quote
Old 12th November 2015, 03:17 PM   #2
diyAudio Member
JonSnell Electronic's Avatar
Join Date: Dec 2012
Location: The Jurassic Coast, England. GB
Send a message via Skype™ to JonSnell Electronic
That is a job to determine what is damaged. With no power and discharged smoothing capacitors, check each transistor in turn and then diodes plus emitter resistors and anything else that looks damaged. Where you will get compatible transistors from depends on the originals of course.
Support for Flying Mole Class D, Allen & Heath, Valve amps, older designs & 2CVs.
  Reply With Quote
Old 12th November 2015, 03:36 PM   #3
Rob_K is offline Rob_K  Canada
diyAudio Member
Join Date: Dec 2007
Thanks, this is a great start!!
  Reply With Quote
Old 13th November 2015, 03:49 AM   #4
anatech is offline anatech  Canada
diyAudio Moderator
anatech's Avatar
Join Date: Jun 2004
Location: Georgetown, On
Hi Rob,
For completeness in case this turns up in a google search ..

You don't need to use the original transistors in this amplifier. What is important is that the entire set is the same and roughly close in beta to each other. I would tend to use either the MJ15022 and MJ15023, or the MJ21193 and MJ21194 for outputs. Be aware that with 80 VDC rails, the outputs and drivers will have to withstand the full 160 VDC if you get into clipping. That's some serious power. You will receive a shock if you touch both supplies, and it will not be pleasant!

If you find one output transistor blown, you need to replace all of them (and the drivers) on that channel. What you can get away with in a low powered amplifier will not fly with higher powered amplifiers.

"Just because you can, doesn't mean you should" my Wife
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Adcom GFA-555 davamp Pass Labs 11 11th August 2015 08:09 PM
Adcom gfa 555 help 406vette Solid State 34 15th October 2011 04:34 PM
Adcom GFA-555 eelekim Pass Labs 2 14th May 2009 04:42 PM
FS:Adcom GFA 555 and GFA 5300 trusound Swap Meet 0 13th December 2005 06:06 PM

New To Site? Need Help?

All times are GMT. The time now is 07:13 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright 1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2