There can be many reasons for that but if the machine blanks
and restarts again especially when the flippers are hit (as you
would switch it quickly "off" and back "on")
there are two common reasons:
1. DIODE ON FLIPPER COIL IS OPEN CIRCUIT OR BROKEN
OFF.
Check quickly all diodes on flipper coil. The diodes are
suppressing back EMF which can interfere with the computer
system. Broken off or open circuit diode is good explanation for
the problem. Grab the diodes by your fingers and pull them
(switch the machine off for your safety). They may look connected
well but come off when you pull them. Check whether they are
soldered well too. To check for open circuit you would need to
disconnect one end of the diode from the coil. I would not rush
to do it, they don't become open circuit that often and if the
machine has four flipper you would need to disconnect eight
diodes and this is bit of hassle. It is more common that the
fault is caused by defective bridge rectifier as described in the
following paragraph.
2.DEFECTIVE BRIDGE RECTIFIER BR2 ON THE POWER DRIVER
BOARD
If the flipper coil diodes are okay another common cause is a
defective bridge rectifier BR2 on the Power/Driver board. This
rectifier is part of the +5V power supply and one of its diodes
becomes intermittently open circuit. Instead of full wave
rectification we get half wave rectification and thus a lower
voltage for the input of the +5V regulator. The bridge rectifier
may appear fine when checked with your multimeter. but if the
machine does blank out and you checked all diodes on the flipper
coils then replace the rectifier, you have good chance that it
will fix the problem. It is easy to damage the tracks on the
circuit board if you are not sure how to do this, it's not worth
the damage. You are better off to get it done. In WPC 95 machines
the bridge rectifiers are created by individual diodes and we did
not experience this problem. This is what I like on
Williams/Bally they always try to improve.