Adcom GFA-555 Thread

mr_rye89

mr_rye89

Veteran and General Yakker
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Lost in the Ozone Again
So we start with an Adcom GFA-555 power amp (a big 'un!). I've owned it since I was 10 or 11, used on and off for that time until 2014 I was adjusting the bias voltage or something like that and the probe slipped sending 80 vdc to the left speaker. FF to earlier this year I tested the transistors and found the drivers were only bad, the big TO3 ones can safely pass 150 volts before they smoke. I have the new drivers here, thermal paste, mica insulators.....

Now the victims, amp, one NPN board removed, board getting prepped for cleanup/reassembly. Let the fun begin!

IMG_20200721_194640.jpgIMG_20200721_194615.jpg
 
The other 3 heatsinks look nice and black but the one you are working on looks purple. Maybe the photo does this but the patterns under the transistors look black.
 
The photos are correct color wise. That sink has always been purple which is strange. That sink didn't ever run hotter than the others so I dunno.....IMG_20200721_212801.jpgIMG_20200721_212741.jpg

Anyways the big transistors are mounted and soldered, the new driver transistor on top will have to wait as I'm short a mica insulator, a 6a fuse, and a machine screw or two.....
 
I was adjusting the bias voltage or something like that ... ? If you were adjusting the blue pots then yes, as far as I can see on the drawing those are the bias adjustments. I see no DC offset adjustments. Far as I recall my GFA 565 Monoblock had no DC pot for adjustment. And as far as probe slips, I used like to probe every slip I see. Love that you tore that bitch down right on the kitchen table....Or that what it looks like from the pics. Looks like a good project. I will be watching the thread so I can see how it comes up.
 
UV exposure can turn them purple too. Big problem with the Counterpoint amps, which were anodized on the front panel pieces. You might see the flat plate black, with the center raised piece purple, or vice-versa.

One of mine is purple-ish; finding face plates is impossible.
 
Back together again! And yes I'm working in the kitchen cuz theres too much TV crap on my work table. No smoke test tonight as I forgot 6A AGC fuses and I need to round up some throwaway drivers......and yes those are four 15000uf 100v capacitors. Recapping may be expensive.IMG_20200722_202215.jpgIMG_20200722_211059.jpg
 
Last 4 I bought for the B&K amp were 18000 uF 100v and I think they were around 120.00 . United Chem. Got them from Digikey best I recall. Good luck on firing that puppy uppy.
 
Shorted transistor found! Dead shorted in fact. Hence the DC on the output. One step closer........
 
Change the electrolytics on the input board. They are leaky in every GFA-555 we see. MrRye, if you need a variac, let me know. I know "smoke test" is a joke, but a variac makes life on the bench much easier.
 
I replaced the transistor in question but I still have DC, I'm going to unhook the driver board from the bad channel tonight to verify a hunch.

And yes I've been meaning to get a variac for a long time now, it would make this job easier, not having to worry about fat-probing 80v rails......though it's not the volts here that worry me, I regularly work on tube gear, it's the amperage.
 
Ok so this thing really shat the bed tonight. fuses on the output board blew and R21 blew up (rail to rail short) and I'm sure it wrecked everything on the driver board. Not even sure what happened. Anyone need anything off of this?
 
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