Phantom closures involve an opto switch, or a row or column acts erratically including normal switches.

Phantom or erratic switches traced to an opto driver board

Cause: A failed LM339 on the under-playfield opto driver board feeds bad signals to the CPU board, which look like a bad row or column. Games with many optos such as The Shadow can use an opto24 board (up to 24 optos) that also has a 555 timer which can fail and cause intermittent faults.

Checks

• With the game on, unplug the opto board power plug (usually next to its row of diodes); the board LED goes out. Retest the switches; a changed problem means a bad opto board.

Fix

• Replace all the LM339s on the board (usually two or three), in sockets; on an opto24 board also suspect the 555.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• Confused switch matrix from LM339s on opto boards
• Checking an under-playfield opto board for bad LM339s
• Row or column "ground short" errors
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• Gumball enter opto (switch 87) never registers: replace the LM339
• Optos test good but still don't work: the opto board
• Opto switches dead all at once
• Testing CPU switch rows with the switch levels test

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