Several or all opto switches stop working at once.

All optos dead: lost opto ground connection

Cause: The ground return for the opto 12 V circuit is open. It can come through an unexpected connector.

Checks

• Trace where the opto ground comes from in the schematics.
• On Indiana Jones, J905 on the Fliptronics II board is the ground return for both the flipper opto boards and the drop target board; make sure it is plugged in, aligned on the right pins and in good condition.

Fix

• Repair or reseat the connector that carries the opto ground.

Systems

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

Related tips

• Opto switches dead all at once
• Opto switches dead or intermittent: check the 12 volt supplies
• Testing optos in switch edge test
• Flipper cabinet opto switch boards
• Where WPC games use opto switches
• Every opto dead and opto board LED off: check the J118 ground
• How WPC opto switches work and which half fails
• Opto does not work: check 12 V at the transmitter connector

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