Several or all opto switches fail or work intermittently.

Opto switches dead or intermittent: check the 12 volt supplies

Cause: A blown 12 volt fuse, or low unregulated 12 volts (under about 11 V) from a failing bridge rectifier, filter capacitor or cracked solder joints. The unregulated 12 V powers the optos and the regulated 12 V powers the whole switch matrix.

Checks

• Check fuses F115 and F116 on the power driver board (F101 and F109 on WPC-95).
• Measure the unregulated 12 V; below about 11 V the optos can act up.
• Check bridge BR5 (WPC-95: diodes D3-D6) for the unregulated 12 V and BR1 (WPC-95: D11-D14) for the regulated 12 V.
• Inspect the 10,000 or 15,000 mfd filter capacitor C30 (C8 on WPC-95) for cracked solder on its leads.

Fix

• Replace the blown fuse.
• Replace the failed bridge or diodes, and reflow or jumper the capacitor and bridge connections as covered in the reset section.

Systems

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

Related tips

• Opto switches flicker open and closed in switch test
• Optos all read closed at power-on: blown opto supply fuse
• No playfield switches work: check the regulated 12 V
• All optos dead: lost opto ground connection
• Row or column "ground short" errors
• Every opto dead and the opto board LED off: failed bridge BR5 for the unregulated 12 V
• How WPC opto switches work and which half fails
• Watching the supplies while the game resets

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