@mods'n'admins: Maybe stickify this.
I see posts every now and then on any forum about having shopped older gear not in full working condition.
Most often, some general things are discussed and suggested, like replacing the power supply capacitors to fix excess hum.
However, some general things should be done to any older electronics, and it doesn't matter if it's about pedals, preamps, poweramps, hifi gear or what..
Hell, even old motorcycles, cars, boats and gals'n'guyz most often needs a brush-up!
So, with ANY old electronics, not limited to our very Auntie ADA variants:
Carefully clean it thoroughly. Vacuum cleaner with thin hose attached lets you 'get in-between', combined with a soft brush. Use compressed air with care
Replace all electrolytic capacitors - not just those ín the PSU; they all loose the electrolyte, dropping capacitance value when older than some 12-15 years
Clean all potentiometers using contact spray, or better, replace with CTS conductive plastic dittos
Clean all jacks; in case of both front+rear-jack inputs, clean/polish, using very fine emerald cloth, the contact closing the other jack. Or replace this jack
Carefully open and clean all cable connections
I would extract all IC's and reinsert them to ensure good contact - do use a chip extractor tool not to damage the chip or pins
Remove the ac fuse and check for corrosion on it and it's mating surfaces
Reinstall tubes, if any; replace as needed
Apart from these general mechanisms, consult the relevant forum for specifics, like replacing R903 (IIRC) in the ADA MP-2
Assemble, step back, inspect, scratch the beard, power on with shaking hands, and enjoy the shizzle now closely resembling how it left factory!