All the optos on one opto board read wrong.

Opto switches dead all at once

Cause: Missing +12 V to the opto board, which feeds the LEDs and the LM339 comparator.

Checks

• Measure +12 V at the opto board.

Fix

• Restore the +12 V supply (fuse, connector, wiring).

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C
• 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
• Confused switch matrix from LM339s on opto boards
• Checking an under-playfield opto board for bad LM339s
• Phantom or erratic switches traced to an opto driver board
• Gumball enter opto (switch 87) never registers: replace the LM339
• System 11B opto board fails with good optos
• Two LM339s on the Fliptronics board read the flipper optos
• Weak or dead flippers from failing LM339s on the Fliptronics board

Source: Pinball Machines: How They Work & Troubleshooting, Norbert Snicer