I bought my Sony MZ-R2 as mostly-defective, but when I got it, it seemed to be completely defective. Any disc I inserted would cause some laser mistracking noises and "DISC ERR" messages. Normally, I would have suspected a flakey optical block. However, I suspected a different cause when I defeated all the safety switches and watched the focusing motion of the laser lens - nice and smooth, like a healthy optical block. Also, turning the unit sideways or upside-down made no difference.
I took the bottom cover off with the battery still connected and a disc still inside. When I touched the connector for the optical block's membrane cable, the R2 suddently read the TOC and displayed a disc name, but then shortly afterwards went back to "DISC ERR". Then I pushed the edge of a credit card against the little pins on the optical block connector's pins... again it read the TOC. I pressed PLAY and it worked! I removed the credit card and it mistracked and said "DISC ERR".
So suspecting a bad solder joint, I resoldered all the teeny tiny pins with a super-fine point soldering iron and now my MZ-R2 works fine all the time.
This might not be a common MZ-R2 problem, but I just thought I'd write about it just in case it is. Unless you have VERY steady hands and a pinpoint tip soldering iron, I don't recommend trying this. It's only too easy to accidentally bridge two adjacent pins with excess solder.