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Fix Adcom GFA 535 / 545 II - channels not working and/or "thermal protection" light is on

The Adcom GFA 545 Mark II - an iconic vintage amplifier, and my #1 recommendation to new audiophiles.

The Adcom GFA 545 Mark II - an iconic vintage amplifier, and my #1 recommendation to new audiophiles.

Your amp isn't badly broken!!

Last week, after adding a Dynaco ST-70 tube amp and a DBX active crossover to my home theater (for the purpose of tri-amping my from main speakers), my living room's 15-amp circuit had had enough. 

The result: on Friday my Adcom GFA 545 II amplifier - which was powering the mid- and woofer-cones of my Paradigm Studio Monitors, stopped playing out of the left channel.  And the next day, one of my 535 II amps blew its right channel, and the right-hand "thermal protection" light came on.

When I went to the AudioKarma forums, I found some threads that made my heart sink: according to them, if the fuses on the back of my amps weren't broken, then I had a serious problem in the amp, and I would probably not be able to fix it.  I searched around on Google, looking for a second opinion, but I found NO HELP, only bad news. 

But you know I wasn'y goint to stop there!  Read on to see how to fix your Adcom amp :) for just a few bucks.

Thermal Protection & Secret Fuses in the Adcom GFA Series

The Adcom GFA series has number of safety features built-in, to ensure that neither the amp nor your speakers suffer any serious damage from power issues of both the power supply and power output variety.

Here they are:

  • The "AC Power Fuse". this is the only fuse accessible from the outside of the amp. You get at it with a philips-head screwdriver. According to Adcom, this is the only fuse you should mess with.
  • "Thermal protection". This thermostat turns the amp off if it gets too hot. Normally, that happens when the speaker impedance is too low or when the volume is too high. Also, it is meant to close (and re-connect the circuit) when the amp cools down. But if you blow on of the internal fuses, the termal protection will remain on, even after the amp is room-temp.
  • Secret Fuses: Two fuses on each channel inside the amp offer second and third levels of protection from power-supply problems to the amp. You can only get at these if you open your amp. Of course, Adcom says "don't do this".

Opening the Adcom GFA Series

As if to drive home the point that Adcom does not want you to open their amp, the GFA series is sealed shut with rivets instead of screws. Adcom wants you to send your broken amp to a service contractor. According to the research I did on Google, fixing a broken channel on a GFA amp will cost between $80 and $180, depending on shipping and the price quoted by the service shop or Adcom itself.

But I had a feeling there were more fuses inside the amp (I describe the fuses i found in the list above), so I went to Home Depot with my friend K.V. and purchased a titanium bit for my drill, so I could drill out the rivets on my two Adcom GFA amps. We also got a rivet gun and some 1/8" rivets.

To open the GFA 535 or 545 amps ( and any other Adcom amp with rivets), drill through the rivets with a metal-drilling bit. Use a bit the same size as the little dent in the middle of the rivet. This should be enough to "pop" the rivet apart. If the rivet doesn't pop, use a bit the next size up. Once all the rivets are popped out, you can remove the top, exposing the guts.

I am SO SORRY we forgot to take pictures of our work. But I found a pic of the the insides of the GFA 535 II (to the right, below). Notice the two fuses on the bottom right, and two on the top left. There are the ones I replaced (even though only one was blown).

It was illuminating to see inside. Right off the bat, the 535 II looks like the slightly better-made amp, with individual power transformers for each channel. Otherwise, they looked like the same design and parts - except the 545 II was bigger all around. My 545 was made in 1986, and my 535 in 1990.

Inside the GFA 535 II


Finding and Replacing the Secret Fuses

Inside, it is easy to see that there are two fuses for each channel, nestled right on the circuit boards of both the GFA 535 and GFA 545 amps. They are rated 250v, 4A, and are barrell-shaped (see pic at right).

I bought replacements at Radio Shack, and switched out all four fuses in each amp. In my case, only one fuse was blown. I don't really know what these fuses do, but once I replaced them, and put the amps back together, my amps worked fine :)

Price of fixing Adcom GFA 535 or 545 II: $1.20 if you have the tools, $19.00 if you have to buy the tools

So there you have it! Instead of sending off two amps to Adcom for repair (at a total cost of somewhere between $160 and $360 for both amps), and waiting a month or two to listen to them again, my friend K.V. and I fixed both amps in less than 3 hours, including travel to Home Depot and the Shack.  Actual time on the amps was less than 5 minutes of work....TOTAL

Is this a money-maker for Adcom and the service business, or what?! 

The actual cost of fixing these amps was around $1.00 for the fuses (a 4-pack is $1.99) and $0.20 for the rivets used to put both amps back together. 

In total, I spent around $19.00: $12.00 for the rivet gun, $1.99 for a box of rivets, and $4.98 for eight fuses. 

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Sambone on June 04, 2017:

DON'T - I repeat DON'T drill out the "rivets": they're actually hex (allen) head screws. Nobody rivets audio components together! And no rivets require a titanium drill bit - rivets are soft malleable metal. That should have been a big clue.

Insert the correct size allen wrench and easily unscrew them or you'll wind up with some gnarly, ugly big holes if you apply a drill. Trust me, I just got an Adcom that had been butchered because of this bad advice.

Tom on February 07, 2017:

The description here is inaccurate.Adcom 545 has hex screw and can be opened without any issue with a hex key (I purchased from home depot).I screwed up one by drilling out screws to figure out they are screws.

Russ on October 18, 2016:

They are the rail fuses. There's a + and - voltage rail for each channel, each has a fuse, 4 fuses. The 535 uses the 545's boards, and is missing 2 output devices per side (vs the 545). Although it has separate transformers for each channel, which is nice, they don't have the va combined of the 545's transformer. Nor does it have enough heatsink to put out the power of the 545. Both are very good amps tho.

Steve on January 20, 2016:

Did changing the fuses really fix the problem. Why did the fuse blow in the first place?

Rob Levinson on August 04, 2014:

On my GFA-535, they are not rivets; they are 5/64" hex screws.

I was able to see the blown fuse with a flashlight, looking through the vents.

Thanks for this helpful write-up.

studioscam2008 on June 14, 2014:

cuanto voltaje tiene la salida del transformador sin retificar ?

As the output voltage of the transformer is?

BB on September 18, 2012:

They aren't rivets on my GFA 545II. They are allen head screws. No need for drilling, just a small allen head wrench and you won't be damaging the case.

Brian on May 25, 2012:

I bought my pair of 545's probably 18 years ago and they stay on 24/7. I had one channel go out about 11 years ago and I called adcom and the guy I talked to actually told me how to get to the internal fuses that I didn't know we're there. And bam, back in business.

Googled 545 fuses today because I blew both of mine again last night with a bad spike from a bad flacc file and couldn't remember the size. Glad to see others are saving money by doing it themselves.

Bob on May 19, 2012:

My GFA 545 ii has hex head screws on the top and sides.

Thanks for the helpful post -- $2.13 worth of parts.

manolo on February 27, 2012:

man i really love you! jaja jocking but i really like to thank you my amp wasn't working and i was about to cry but anyway thanks a lot is a heritage from my father :)

Michael on December 20, 2011:

Will this work for a GFA-2 and GSA-700 also. Great stuff, thanks

David S on December 10, 2011:

Thanks man! I love this amp and am so glad you've made this cheap and easy!

Jeff P on September 03, 2011:

$2.14 for four fuses and the amp is back in business. Thanks!

kaz on August 28, 2011:

Opened up my 535 1 fuse was bad changed it and saved 300 dollars. THANK YOU

Donna H on August 23, 2011:

I did this!! Will, with help from a Hardware geek friend. We drilled out the rivets and there were two blown fuses. Replaced them, put it back together and it works like it is supposed to. The thing we did differently was used screws instead of rivets. Now, when a fuse blows again if it does, I just use the screw gun and I'm done in an instant. This amp is driving 25 year old polk speakers and does an awesome job. Someday, I'll join the current technology world and buy a new amp. Meanwhile, this works great!! Thanks for this instruction set

Jay on February 19, 2011:

Havn't tried this yet but I will with my tech wiz of a friend soon~thanks for the article its what ive been looking for!!

Rob MacGregor on February 08, 2011:


You saved me a lot of time and money. For that I am grateful!


Ninesvnsicks on October 26, 2010:

Wow this write up saved my amp all the torx screwed were stripped and rounded out so I never opened it up but after reading this I drilled them out and looked inside. Sure enough on the left channel the first 4A fuse was bad. I only had a 2A fuse so I used that and it seems to be working fine.

Thank you so much.

koza on July 27, 2010:

i opened up my 535II but the 4 fuses are fine (and the ac fuse is fine). still one channel is dead. any advice?


Rivets? on July 07, 2010:

Did you try using a T3 screwdriver instead? That's the screws used for the top of my GFA-545.

Adcompoor on May 27, 2010:

Fantastic, thank you, thank you, thank you. I just REPAIR MY AMP!! I was using my Roomba robotic vacuum cleaner two days ago and the little devil got tangle in my speaker cable while my stereo was playing. My speaker wires cross and fried the left channel of my Adcom 545 (mk1). I was distraught, to say the least. I'm so glad I found you website. I always wondered how to access the guts of these things. Rivets.... brutal. That's the toughest part of this repair, removing those rivets. For the poster above, after much trial and error, the correct drill bit size is 7/64th of an inch. If you use too small of a size, you can't 'pop' off the heads of the rivets. The repair itself is a piece of cake. You were right Issac, just A BLOWN FUSE. A $2 repair! I'm not going to rivet it shut, just use some screws this time. Just in case. PS Make sure anyone doing this to be very careful. These things store a HUGE amount of electricity which will deliver a LETHAL shock, even after being unplugged for a couple days. Adults only! Fortunately there is a lot of room in there to work (on the 545) but because DONT TOUCH ANYTHING EXCEPT THOSE FUSES. I had to thread some electrical tape under my fuses to lift them up. THANKS AGAIN, ISAAC! You saved my life!

ADCOMPOOR on May 26, 2010:

Hey, great advice. I'm going to try this tonight. Much too poor to send it back for $200. I'll keep my fingers crossed. Thank you so much for your experimentation!

Joe M on April 10, 2010:

I have tried to pop the rivets but unsuccessfully. Does the bit have to be a titanium? Which bit size?thanks!

Isaac on February 18, 2010:

Yes, the drill will pop 'em right out. And a cheap "riveter" will put the rivets back in super-quick :)


T-Dizzle on February 17, 2010:

THanks a lot for the write up. Rivets cant stop my drill! Appreciate it!

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