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.
Fortunately, this connector is still made, very common, and famously reliable. It’s a JST XH type. So I had 400 cable assemblies manufactured, so all GFA-565/585 and GFA-555 MK2 kits will now include new cables.