Now and then the game scores a drop target that is still standing, mostly while the ball bounces between the slingshots. The target switches are clean and gapped.

Drop targets score without being hit: wrong or touching switch diodes

Cause: A previous owner had replaced the switch diodes with a different type and left the leads long; one diode lead was touching its neighbour, giving false switch closures in the matrix.

Checks

• Look at the switch diodes for long leads that can touch each other, and check every band points the same way.
• Tape paper between the suspect switch contacts and play; if the fault stops, the switch itself is closing from vibration.

Fix

• Fit the correct diodes, trim the leads and make sure none touch.

Systems

• Gottlieb System 1

Games

• Pinball Pool

Source: Pinball Repair help group!! (Facebook group)