All matrix lamps are dark, or behave confused.

None of the controlled lamps work, or they light in the wrong pattern

Cause: Missing +18 V (BR1, F114, LED6, TP8; on WPC-95 D11-D14, F106, LED102, TP102), or dirty contacts on the short CPU-to-driver ribbon cable.

Checks

• Check the fuse, that the 18 V LED is lit and that the test point reads 18 V DC.

Fix

• If power is good, turn off and reseat the short ribbon cable between the CPU and driver boards to clean its contacts.

Systems

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

Related tips

• No lamp matrix lamps light
• All matrix lamps stay on
• 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
• One GI string is out
• A lamp row very bright or dead

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