Yamaha TG55 module

[Milo]

Hey, experts;
I have a TG55 which I've had since the early nineties. Its been my workhorse on and off since then, but last time I turned it on, I just got exclamation marks on the screen.
I checked the internal 3v battery, whic read 2.98v - not too drastic, I thought.
Has anyone got a reset / fix for this condition?

Thanks