Why WPC coil diodes are on the driver board

Every computer-driven coil needs a flyback diode to absorb the voltage spike, up to twice the supply, when the coil switches off. WPC puts these diodes on the driver board next to each driver transistor for every coil except the flippers, so vibration cannot break them and the coil cannot be wired backwards. A shorted diode blows the coil fuse; an open or missing one causes odd game behavior and can kill the driver transistor.

Systems

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

Related tips

• Testing a coil diode on the driver board
• Testing the diode on a coil
• Testing lamp-row drivers with a test lamp
• Testing lamp-column drivers with a test lamp
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• +20 V filter capacitor C11 (C10 on WPC-95) blows apart
• Getaway: beacon runs nonstop in test and driver board C11 overheats
• Testing a coil diode

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