How GI dimming works

The CPU senses each zero crossing of the AC (120 times a second) and fires each GI triac a set delay later; the longer the delay, the dimmer the string. For full brightness the triac gate is held on. A 74LS374 latch and a 2N4403 per string drive the triac gates.

Systems

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

Related tips

• GI string still dead after replacing J115: broken through-hole, add a jumper
• One GI string dead after a restoration: unkeyed J115 plugged in upside down
• Several GI strings dead after a connector repair: lifted pads at J115
• All GI out (playfield and backbox) with good fuses and plugs: reseat the CPU ASIC and ROM
• All backbox lights out with good fuses: burned J120 GI connector
• A whole string of backbox lamps is out: blown GI fuse F110
• Two GI strings still dead after fixing a burnt J120: broken pads and traces
• One GI string dead though the board and fuse test fine: burnt connector in the cabinet

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