This may be Data East related, not just BTTF specific.
I purchased a BTTF with some issues. I played fine, but the bottom pop bumper coil had melted, taking the driver and pre-driver transistors with it. I replaced the coil and the transistors, and also repinned J5 on the PPB board that had been done badly by the previous owner.
Now on startup, the top left, top right and left slings are locked on. All other driver transistors have been checked I'm sure there's a simple explanation but I can't seem to find it. I've linked another post about other issues with this game, maybe they're related.