Switch 87 (gumball enter) never shows in switch test. The transmitter LED is lit, a new receiver makes no difference, and jumpering the row and column at the CPU board makes the switch register.

Gumball enter opto (switch 87) never registers: replace the LM339

Cause: A failed LM339 comparator on the 10-opto board that handles this opto.

Checks

• Look at the transmitter with a phone camera to confirm it is lit.
• Check continuity of the wiring from the receiver to the 10-opto board and inspect the board connectors for torn end pins.
• Confirm about 12 V on the opto board supply connector.
• If possible, swap in a known-good 10-opto board from the same game; if the switch then works, repair your board.

Fix

• Replace the LM339 at U3 on the 10-opto board (socket it while you are there).

Systems

• WPC Fliptronics

Games

• Twilight Zone

Related tips

• Confused switch matrix from LM339s on opto boards
• Phantom or erratic switches traced to an opto driver board
• Checking an under-playfield opto board for bad LM339s
• Opto switches dead all at once
• Clock minute optos miss now and then: repin the plug at the 10-opto board
• New lock optos fitted: those and other optos on the same plug stop working
• One switch does not work
• Testing CPU switch rows with the switch levels test

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