Switch readings make no sense, or a ground row short is reported, and the CPU board seems fine.

Confused switch matrix from LM339s on opto boards

Cause: A failed LM339 on the under-playfield opto board, or on the trough opto board in Indiana Jones, Judge Dredd, Star Trek: The Next Generation, Popeye and Demolition Man (World Cup Soccer and later trough boards have none).

Checks

• Unplug the opto boards and see if the switch problems change.

Fix

• Replace every LM339 on the suspect opto board, in sockets.

Systems

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

Related tips

• Optos test good but still don't work: the opto board
• Checking an under-playfield opto board for bad LM339s
• Phantom or erratic switches traced to an opto driver board
• Row or column "ground short" errors
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• Opto ball troughs from Indiana Jones on
• Gumball enter opto (switch 87) never registers: replace the LM339
• Opto switches dead all at once

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