From the service manual for the Yamaha DX-100 FM synthesiser. The way this looks to me, both the 12v DC power supply, and 9v DC battery power get routed through exactly the same voltage regulator.
So it should be possible to just straight replace the 6x C-cells with 3x 18650s without needing to make any other changes, right?
I know exactly enough electronics to be dangerous, which is why I'm double checking here in case I've failed to account for something.
I'm aware that fresh 18650s put out a little over 4v, but I seriously doubt the vReg tops out at exactly 12v. I can disassemble the unit and check part numbers etc if necessary but I was really hoping to avoid that as it's quite a chore.
It looks to me like the 12v supply already isolates the batteries (some kind of barrel jack with integrated switch), but regardless I would definitely be charging them external to the device. I have a dedicated charger already so it would be quicker and easier to use that and just swap in a new set as needed. I'll be 3D printing adapter housings such as this, so adding a diode into that assembly should be easy enough. Thanks for your advice!