A drop target that is down is not registered.

Drop target not sensed when down

Cause: A failed or dirty opto, almost always the transmitter side, or an opto broken off the board.

Checks

• Remove the three E-clips and lift the opto board off the target bank.
• Inspect for broken optos.
• Check the transmitter with an infrared detector card or a phone/digital camera; on old boards the red side is the transmitter.

Fix

• Clean the optos with a cotton swab and glass cleaner; this often cures it.
• Replace a dead transmitter.

Systems

• Williams System 11A
• Williams System 11B
• Williams System 11C

Related tips

• Glue new-style optos down
• Five drop target switches stop registering together: no ground at the opto board

Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Three, Clay Harrell