A bad vintage Ibanez pre-amp board is almost always due to a bad op-amp chip. You can purchase a replacement chip from Mouser for about 45 cents. Shipping will be more than the chip. Look at the chip and you'll see the part number. Search for it on the Mouser site and you'll find it.
If you can't replace it yourself, it shouldn't be that hard to find someone who can.
Of course, it could be a wiring problem. I have a wiring diagram for that guitar...
Or...maybe it's just the battery!