I got this Action Replay MK-I module. According to the seller it was untested, and for that reason sold as defective. It was in a… let's say very used state. The case was dirty, to a point that it was almost revolting to touch it. A side of the case was cracked open, and a knob was missing. The module must have been dropped at some time.
I carefully opened the case. The top and bottom shells are just stuck together, there are no screws, so it was easy to pull them apart. Inside I found some kind of coating on the PCB, so perhaps a drink had been spilled on the module as well. I also found a lot of fine paper dust like from a cardboard, and a small dent at the corner of the PCB that was caused by the drop.
The first thing I did was to give the entire module a proper cleaning in an ultrasonic cleaner, just with warm water and a drop of dishwasher detergent. And yes, I also washed the PCB that way, then dried it off and sprayed it with IPA to remove the last traces of water. That bath did wonders.
I expected that the dirt also reached the inside of the mechanical parts, so I decided to replace them all. They were a bit hard to find as replacement parts, but still available. As the original knob was lost, I used a different potentiometer that came with a knob. Unfortunately the new one is white, while the original one has likely been black, so I couldn't fully restore the original outside look.
The case was cracked open at one side because two pins inside were broken off. I fixed the pins with superglue. After that I exposed the case to the sun for a day, which removed quite a bit of the yellowing. Then I could put everything together again. Compared to the original state, the Action Replay is now looking nice and clean.
I gave it a test run in my Amiga 500, and it was working fine! Now I have an Action Replay for my Amiga collection. The only sad thing is that it cannot be upgraded to an MK-II or MK-III, as these modules are constructed differently.