(12C/11C) Changing the firmware

[rprosperi]

No, it's not possible to change, these machines from the early 80's all use ROM and cannot be updated. The most recent 12C and 15C LE models can be re-flashed, but unless yours uses 2 x CR2032 batteries, it's not one of those, and it sounds like it isn't.

If you got that on eBay (from some unethical, scamming seller that replaced dead guts of an 11C with working guts of a cheap 12C) I would definitely open a case for a return/refund. 12Cs go for $20-50, 11Cs for much more, as you probably know.