The dot matrix display is blank, shows garbage or diagonal lines.

Blank display, garbage or diagonal lines: reseat ribbon cables

Cause: A blank display with good fuses and voltages is usually the controller-to-display ribbon fitted backwards. Garbage or diagonal lines usually come from the long ribbon running CPU board to Fliptronics board to sound board to display controller, or the CPU to driver ribbon, needing a reseat, fitted a row off, or torn away from its connector.

Checks

• Check every ribbon is on its header correctly, with the red stripe (pin 1) at the board's arrow or "1 2" mark.
• Check no ribbon is shifted one pin or one row off.
• Check the ribbon has not been pulled out of its connector.

Fix

• Reseat each ribbon: ease it about 1/8 inch off and push it back so it can't land a pin off.
• The ribbon connectors are gold plated, so reseating cleans them. On ordinary non-gold connectors, a reseat that fixes things means the pins need replacing.

Systems

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

Related tips

• Dot matrix display blank or faulty
• Dot matrix display faults
• Dot matrix display supply voltages
• Dot matrix display power connector pinout and voltages
• One or two display columns stuck on
• Vertical lines and garbage on the DMD mid-game: press the ASIC back into its socket
• Moving vertical bars across the DMD with known-good display parts: CPU board fault
• Resets caused by a failing dot matrix display

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