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.
Master degree in EE here. It will almost certainly be fine to swap those batteries like that, I would be more cautious if it was some cheap Temu device but for a vintage Yamaha you'll be fine.
One thing I will caution you about though, the synth almost certainly lacks under-voltage protection and could drain your 18650s below their minimum voltage. Make sure your cells have built-in protection or add some of your own.