Vintage Synth Repair

I have recently purchased a defective Siel DK-600, hoping that it
might be an easy fixer-upper. Now.. what it does when I turn it on
is.. almost all leds are on, the number display in the "program
select" area is stuck at 00 no matter what you press and none of the
other selector buttons seem to work either. Also the led by the
"enter" button keeps blinking. So.. from what I told you what do you
think might be wrong? The battery? It doesn't seem to be just a loose
cable since all areas of the synth seem to be affected. Any hints and
suggestions are highly welcome!