One GI string will not dim, none dim, or GI never turns off.

GI will not dim or will not turn off

Cause: One string not dimming: its triac, or the 74LS374 latch. None dimming: the latch or the zero-cross circuit, usually its LM339. GI never off: the zero-cross LM339. Rarely the processor.

Checks

• Compare strings in GI test to see whether one or all are affected.

Fix

• Replace the triac, latch or LM339 as indicated.

Systems

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

Related tips

• All matrix lamps stay on
• One GI string is out
• A lamp row very bright or dead
• GI strings won't dim in the GI test
• One GI string dead after a restoration: unkeyed J115 plugged in upside down
• None of the controlled lamps work, or they light in the wrong pattern
• GI string dead after connector replacement: broken traces
• A whole string of backbox lamps is out: blown GI fuse F110

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