I fired up my 90s Yamaha SY77 synth, and was greeted with no or strange sounds and a “Change Internal Battery” message. I don’t think the battery has ever been changed, so after 25+ years I guess it’s ok to do some maintenance. If you had your own sounds and patches, that data is probably lost when you get to this point. Hopefully you have a backup. I had some sounds, but no backup that I know of.
The battery is a CR2032 coin cell that is soldered in place, so you need a battery with solder tabs, to be mounted horizontal on a pcb. Don’t try to solder on a standard coin cell battery, a battery with solder tabs isn’t so expensive.
So I turned the keyboard over and removed the necessary screws to remove the bottom cover. Note that three screws on each end holds the side covers, not the bottom cover. Leave them, and also leave the rubber feet.
With the bottom cover removed, you can see the battery on the large pcb next to the floppy disk drive. To unsolder the battery, you need to remove the cables on the floppy disk drive side of the pcb, and on the keyboard side of the pcb. You can leave the cables on the left side of the pcb. It can be tricky to remove some of the cables, but you only need to pull the connectors (hard). Be careful and do not pull the cables (you may rip out the crimps from the connectors). Remove the screws that hold the pcb, and then you can flip the board over.
Unsolder the old battery. My new battery had two tabs for the plus-side (original battery had one plus tab), but there were corresponding holes in the pcb that I could use. Make sure the tab connected to the top of the battery goes to the “+” hole on the pcb.
After re-assembling everything, you need to do a factory reset. This is done by:
– Turn on the power
– Press and hold the [VOICE] button, the press and hold the [BANK D] button and the  button. Now it will display the “Test Program Menu”. (I had to do several attempts to get it to work)
– Now press [COPY] button to reload the factory data.