Jx3p bank c&d don't work

I have a great jx3p w/pg200 which I’ve had since new. No road use. Been working great except that I’ve lost my saved sounds on Bank c and d. I’ve heard it might be a battery issue? If so which battery and how do I replace it?
Also, if the unit is on for a couple of hours, it seems to lose volume. Any help is greatly appreciated thanks, Jesse

Can you save sounds on bank C and D and jump between those sounds as long as the synth is switched on?
If yes: -> the battery is most likely the culprit.
If no: -> the SRAM chip is most likely the culprit

Syntaur has the battery and the chip on stock
battery:


sram:

regarding the volume: did you measure it, or is it only an impression?