To get rechargeable AA's to work in an 'R3, you need press the rechargeable batteries switch. To do so, open up your MZ-R3, holding it so the bottom is facing up and the battery bay is on the far side. Pop the battery door open. On the right-hand side, you will see near the bottom a little pin. Get a bit of newspaper or eraser to hold the pin shut, then put your AA rechargeable batteries in. They should charge normally. Do this at your own risk; it isn't likely to be a Sony-approved technique. - Gian-Paolo D Musumeci (and others)

– Content adapted from MiniDisc.org page

This also works with modern rechargeable (Eneloops and the like) NiMH AAs, no modification necessary. I have not attempted to charge these while installed in the 'R3.

When titling a track or disc, you can hit the play button to quickly go the first character of the different subcategories (foreign characters (when available), A-Z, a-z, 0-9).

Works with LCD remotes, but does not use the display. Controls work as intended as well.

Sony MZ-R3 User Manual (Scan)

Sony MZ-R3 Service Manual (1.1)

Sony MZ-R3 Circuit Diagrams

Basic Teardown