A magnetic reed ball sensor under a ramp or playfield insert misses balls.

Reed switch ball sensor misses balls

Cause: The sensor must sit very close to the ball and the ball must roll right over it. A missing rubber mounting grommet lets the switch drop away from the playfield. Fast balls are also read less reliably, which is why many positions use two in parallel.

Checks

• Check both mounting grommets are in place and the switch is snug against the insert or ramp.
• A reed sensor can only see through a plastic insert or ramp, not through wood.

Fix

• Replace missing grommets and seat the switch tight against the playfield.

Systems

• WPC-95

Games

• Cirqus Voltaire
• NBA Fastbreak
• No Good Gofers
• Safe Cracker
• Star Wars Episode I

Related tips

• Second-generation auto-adjust eddy sensors (1996-1998)
• Auto-adjust eddy board LED flashes constantly
• Checking an under-playfield opto board for bad LM339s
• Testing optos in switch edge test

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Three, Clay Harrell (pinrepair.com)