A group of opto switches misreads or the switch matrix acts confused, though the transmitters and receivers test good.

Optos test good but still don't work: the opto board

Cause: A fault on the under-playfield opto board: broken or pulled IDC connector wires, cracked solder on header pins, failed 1N4004 diodes, bad resistors, burnt or broken traces, or failed LM339 comparator chips.

Checks

• Rule out the CPU board first: in switch test T1, unplug the switch connectors from the CPU board and short each column to each row as described in the switch matrix section.
• Plug the CPU board back in, unplug every connector on the opto board and confirm switch test T1 runs clean apart from the missing optos.
• Confirm the red power LED on the opto board is lit.
• With the game off, ease each IDC connector slightly off its header and check continuity from each header pin to its wire.
• Look for cracked solder at the header pins on the back of the board.
• Test each 1N4004 diode on diode setting: about 0.4 to 0.6 one way and open the other. Check the resistors and the traces from the header pins.

Fix

• Repair broken IDC connections, reflow cracked header pins and repair broken traces.
• Replace bad diodes and resistors.
• If all of that is fine, replace all the LM339 chips (usually two to four), fitting sockets.

Systems

• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• Confused switch matrix from LM339s on opto boards
• Switch matrix chips on the CPU board
• Weak or dead flippers from failing LM339s on the Fliptronics board
• A dedicated (direct) switch does not work
• One switch does not work
• Testing CPU switch rows with the switch levels test
• Opto ball troughs from Indiana Jones on

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