Casio CZ/ VZ/ FZ - Pro Series

Hi all,

I bought a 2nd hand CZ1000 a few months ago. Nice machine, really. 
But this one forgets its user-sounds after a couple of days, when I 
do not turn on the machine for a while. It's about 5 days, then it 
starts to forget the user sounds.
-What might cause this problem.? 
-Is there an internal battery that has gone old, and if so, where is 
it located, can it be replaced, and how?
-If it is not an old battery, then what is causing this problem?

I do not have a manual of the CZ1000.

Kind regards

Ivo (Dramklukkel)