I bought this machine recently, and this was a pre-existing problem. Machine had been unplugged for at least 5 years. Game will occasionally play perfectly for one, or several games in a row.
Occasional error messages:
--Check FU-1 or Field Fuse
--Too many balls found (only? when a ball is already in lock)
--Occasionally serves a second ball onto shooter lane, sometimes right after serving a new ball, sometimes while a ball is already in play.
--Sometimes when multi-ball starts, locked ball fails to release. Then when only ball in play drains, triggers "too many balls" error.
--Occasionally a drained ball is not recognized and ball in play does not change, But there is already a ball in shooter lane, so same ball continues.
-- sometimes(only? on first ball of game), flippers do not get power until ball makes a score.
--FU1(on power board) and Field fuse(Under playfield, near lower left flipper) both test good.
--Opto boards removed, lenses cleaned and closely inspected. LEDs light(Digicam), all solder joints look good, pin connectors seem good.
--All playfield switches test good. Tested Optos by blocking beam manually, and with ball in place.
--On-board battery had been replaced by previous owner, looks and works good. Original battery was found in bottom of machine. I'll be moving battery off board asap.
--Voltages on all board test points have been checked and found to be in spec.