For this "episode" I received two Sony MZ-E70s (1999) from a friend in the MiniDisc community that no longer worked. He was hoping that I could end up with one working one out of two sets of parts.
One of them was in great condition, but wouldn't power on while the other one was in OK condition, but internally was pretty bad (corrosion, corrosion crystals, rusty screws and parts).
Check out the video for more details, but this was a very interesting takedown with just one screw on the bottom casing (inside the battery compartment).
TL;DW
There was no copper left on the internal negative terminal where the gumstick would have touched, so I tested a fix by bending a copper multi-stranded wire and fixed it there with copper tape. It worked fine after that.
Interestingly, there's a small PCB inside that says MZ-E55 which was a previous model.
This was an interesting disassembly overall!