Korg DW8000, Mono/Poly, MS-20...

I picked up the most beautiful and flawless korg poly-61 today off 
this guy. The only thing wrong is that the tones are jacked up. He 
was the original owner and told me that one day he decided to play it 
after a little while of inactivity and it powered up perfectly, but 
when he pressed a key it sounded messed up. He said that his friend 
told him that it could possibly be something wrong with an internal 
battery? I am not technical at all, but I've never heard of this. But 
if it's true it sounds easy and inexpensive to fix. I got it for 
nothing and it looks beautiful! The owner is an orchestra leader and 
he is in his sixties, i trust him, but still am clueless? If you have 
any info please fill me in. Do you know anything about internal 
batteries?

much respect,

aaron