Optos change state rapidly in switch test and the game behaves erratically.

Opto switches flicker open and closed in switch test

Cause: The unregulated 12 V to the optos (F116 on WPC-S and earlier, F109 on WPC-95) is interrupted or low: cracked joints on the opto board connectors or driver board connectors, cracked joints on BR5 and C30 (C8 and D3-D6 on WPC-95). Many optos affected can also mean a failed CPU U20.

Checks

• In switch test, wiggle the opto board wires and connectors under the playfield and at the driver board.
• Check the fuse.

Fix

• Repair the connectors, add jumpers to C30 (C8 on WPC-95), or replace U20 or the 12 V bridge as needed.

Systems

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

Related tips

• Phantom or erratic switches traced to an opto driver board
• Opto switches dead or intermittent: check the 12 volt supplies
• Optos test good but still don't work: the opto board
• Confused switch matrix from LM339s on opto boards
• Optos all read closed at power-on: blown opto supply fuse
• Checking an under-playfield opto board for bad LM339s
• A whole switch column dead or all closed
• A whole switch row dead or all closed

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