In the GI diagnostic test, the strings stay the same brightness from level 1 to 8. Random resets can go with it.

GI strings won't dim in the GI test

Cause: A fault in the zero-cross circuit. Diodes D3 and D38 (just below J109) take their feed from traces to bridge BR2, and those traces often break when BR2 is replaced. Next suspects are the zero-cross LM339 at U6 (U1 on WPC-95) and the 74LS374 at U1 (U2 on WPC-95).

Checks

• Buzz the two AC legs of BR2 to the non-banded ends of D3 and D38: the solder-side top-left BR2 leg goes to D38, the component-side bottom-left leg to D3.
• If only one string won't dim, start with that string's triac rather than the LM339.

Fix

• Repair the broken traces.
• Replace the LM339, then the 74LS374 if needed.

Systems

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

Related tips

• Lamp matrix faults with all lamp transistors good
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• Switch matrix chips on the CPU board
• BR1 (D11-D14 on WPC-95) and the board around it scorched
• Resets remain after rectifiers, caps and connectors are done
• Resets continue although the bridge and diodes test good
• How a WPC coil, flasher or lamp driver chain works
• A coil or flasher energizes as soon as the game is turned on

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