Every controlled lamp is on and never turns off.

All matrix lamps stay on

Cause: Usually a failed U9 on the driver board; U18 may have failed with it.

Checks

• Check U9 and U18 outputs with a logic probe.

Fix

• Replace U9, and U18 if it is also bad.

Systems

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

Related tips

• No lamp matrix lamps light
• How the WPC lamp matrix works
• How the WPC lamp matrix is switched
• How the lamp matrix drives a bulb
• WPC lamp matrix and GI driver parts
• None of the controlled lamps work, or they light in the wrong pattern
• Lamp matrix fuse F114 (F106 on WPC-95) keeps blowing
• A lamp row very bright or dead

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