Switch matrix chips on the CPU board

On every WPC generation the switch columns are driven by a ULN2803 at U20, the rows are read by LM339s at U18 (rows 1-4) and U19 (rows 5-8), and the dedicated switches by LM339s at U16 and U17. U20 is the usual failure; it is socketed only from WPC-S on, where it sits under the battery sub-board. ULN2803 crosses to NTE2018 and LM339 to NTE834. A hard U20 failure can also take out U14 (74LS374), which is U23 on WPC-S (74HC4514) and WPC-95 (74HC237).

Systems

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

Related tips

• Why CPU U20 fails and how to replace it
• Switch matrix dead after a screwdriver slip under the playfield
• One switch does not work
• Row or column "ground short" errors
• "Slam tilt stuck closed" at power-on
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• Testing CPU switch columns with the switch levels test
• Testing CPU switch rows with the switch levels test

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