The Sony MZ-R700 is a fun MDLP player from 2001 that came in some nice colors like a metallic yellow. It was a cheaper version of the MZ-R900 so it was plastic and used AA batteries.
I got one cheap on eBay that wouldn't power on and fixed that quickly by fixing the battery door contacts.
After that, it turns out that it wasn't recording at all!
Often with Sony recorders, that points to a write head failure (something along that delicate ribbon cable broke!) which can be a tricky fix.
To troubleshoot, I opened up the player and tried recording while pushing down the write head (GENTLY) closer to the disc. It worked! The disc was recording as long as I was holding the write head close to the disc, so it had to be some other mechanism - something to lower and raise the write head, perhaps?
After some research (and noticing a loose rod inside the device), it turns out I was missing a toothed metal gear that should be locking/unlocking the write head arm. This allows it to lower when recording, and raise when not recording.
