After a while of warning me to check the internal battery, my Roland D-50 synthesizer came up recently with all its internal presets gone, and random garbage for settings. After nearly 25 years of service, that little lithium battery finally gave out.
Replacing the battery is fairly straightforward. It's a CR2032 button cell. To access it, you first have to remove the five screws across the bottom edge of the back panel, and every screw on the bottom panel (there are lots of them). After you do that and remove the bottom sheet metal cover, this is what you'll find, minus the big yellow arrow pointing at the battery:
Here's a closer view of the main board, where the battery resides:
Be sure to note the polarity orientation of the battery.
The only problem at this point is that there isn't physically enough room to pull the battery out of its holder. There isn't enough clearance between the holder and the outer shell of the keyboard to remove it. So, you have to remove all the screws holding the main board down. I didn't find it necessary to disconnect any of the cables from the main board; once the board is loose, there's enough play in the cables to move the board away from the edge, making room to substitute a fresh battery. Be sure to match the polarity of the old battery, which you noted before you removed it, right?
Once the new battery is installed, just screw everything back together as it was before. The internal settings will be garbage until you reload them, by either loading a MIDI dump, or copying them from a memory card. Also, check your global settings, as things like the master tuning may be randomized as well.
While I had the cover off, I noticed one of the reasons the D-50 has one of the best-feeling keyboards of any synth I've ever played:
That's a strip of felt that the keys rest on when they're not being pressed, just like the stuff they use on real pianos.