HI...
After doing some work on one :
Remove all the screws in the bottom panel ( not the feet though)
Lift the panel away from the unit and you'll find the memory battery towards the rear of the main PCB . The battery type is : CR2032
3Volt and VERY available.
I took a bunch of hi-res pics of the innerds too.
Next to check is a Yamaha PD-250 stage-grand