Trying to get my Doctor Who Mini-Playfield working again. When I hook up jumpers from a 12V battery the motor will move the MPF up and down. But when I run the MPF tests in the machine, it doesn't move at all. I've removed the old grease, applied a little silicone lube to the rails. I've aligned the disc, as best as I can tell, to the proper vertical plane of movement. What should the voltage to the motor be? I'm at a loss on what the problem can be.