One issue that vintage Adcom amplifiers occasionally suffer from, is corrosion on the snap-in board headers and plugs that connect the bias compensation transistors to the input board on models GFA-565/585 and GFA-555 MK2. The corrosion can be caused by high humidity, salty sea air, or in the case of the GFA-565, leaked capacitor electrolyte. Failure of this connection can cause a DC offset to to appear at the output.
I have a matched pair of GFA-565 boards in stock! Fully tested. Near aerospace-grade construction. 😸
Some early models of the GFA-555 MK1 used these vertically-mounted input boards. My existing horizontal board kits could be adapted to work with these amps, but it required fabricating brackets, drilling holes, and adapting the wiring. It was kind of a pain and a bit confusing.
The biggest design challenge for the vertical board model, is the way the original bridge switch is mounted to a small circuit board that is held in place by the RCA jacks. These are not great RCA jacks, and most people will want to swap them for nice chassis-mount types. So I came up with this “Mezzanine board” arrangement with two PCBs stacked together. The first board holds the bridge switch in place, and houses connections for the RCA jacks, LEDs and thermal breakers. Then the input board stacks on top of the mezzanine board, and uses WAGO cage-clamp terminal blocks to connect to the output modules. The input board can be removed and serviced without soldering!
My GFA-555 MK2 boards are ready to go! Thanks to Jon Morris of Morris Audio in Wyoming for helping me work out the bugs and validate the design. He’s already restored a few customer’s GFA-555 MK2’s with my boards.
NEW – I am now offering these boards fully assembled and tested for $380. All you need to do is make sure your power supply and output section are good.
Do you have one of the first batch of ADCOM GFA-555 amps ever made? My GFA-555 MK1 boards work great in these amps, but they must be physically modified to work with the horizontal board.
Until now! I have designed new boards for these early “Vertical” 555’s.
I really like how this layout turned out. Very pleasing symmetry.
This is the original board:
Dang, I kinda want to keep this one, but here it is for sale!
Hoppe’s Brain Refurbished ADCOM GFA-555 MK1
This particular GFA-555 came to me as a customer trade-in, and was very well loved and cared for, so I thought it deserved a new life.
I went all-out on this one. It’s got all the options, including an entirely new input circuit board, and a soft-start power supply with (4) 27,000uF capacitors.
By popular demand, I have created new input circuit boards for the GFA-555 MK1! These boards integrate all of the improvements I normally make when I refurbish a GFA-555.
I’ve just built a batch of eight GFA-565 boards, and they turned out amazing! I wanted to share the improvements I’ve made to the boards, and my build process.
Because I’m terrible at self-promotion, here’s a video I should have posted last month!
YouTube blogger XRayTonyB did a three-part video series on repairing a GFA-585 using my boards!
As usual, he goes into great detail on the restoration process, and gives all kinds of generally helpful electronics troubleshooting advice along the way. Tony’s channel is loaded with videos about restoring some really interesting gear like vintage receivers, amplifiers, test gear, etc.
This was not a paid promotion, and there is no commercial affiliation. Tony just bought boards from me like any other customer.
I’ve just finished building this batch of GFA-565 input boards. More are on the way soon!