DX7 Memory Corruption

Hi All -

I recently managed to get a semi-broken down DX7, first make and
model.  The battery needed to be replaced, which was not too much
difficulty.  The presets were wiped, but I ran the internal diagnostic
check and everything seemed to check out.

I have started programming new presets and storing them in the
internal memory...but the internal memory seems to "forget" after a
few hours of the machine being turned off. I'm wondering what might
cause this problem.

Thanks,
Geoff