Vintage Synth Repair

Open it up and take a look. Might be something obvious that you can see.

 From what I remember, it's easier to work on if you temporarily cut the 
leads to the batteries so you can completely remove the back cover. The 
ribbon cables are extremely delicate and cheap, so use great care not to 
lift/damage any traces. I wound up rewiring many of them on one of mine. 
I might have a complete power board that I could sell you, but I need to 
go look. Check for battery acid corrosion inside.