Why CPU U20 fails and how to replace it

U20 (ULN2803) usually dies when coil voltage reaches a playfield switch. From WPC-S onward it is socketed for that reason. When replacing it on an older board, fit a socket. U14 (74LS374; U23 74HC237 on WPC-95) can occasionally fail on its own without U20.

Systems

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

Related tips

• Switch matrix chips on the CPU board
• Switch matrix dead after a screwdriver slip under the playfield
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• BR1 (D11-D14 on WPC-95) and the board around it scorched
• Lamp matrix faults with all lamp transistors good
• Switch columns to U20 pins
• Testing CPU switch columns with the switch levels test
• A whole switch column stops working: replace the ULN2803 column driver at U20

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