Testing CPU switch rows with the switch levels test

Steps

• Same setup: Switch Levels test, J212 and J206-J209 unplugged.
• Clip a lead to J207 pin 1 (rightmost). Optionally hold a 1N4004 with the non-banded end out.
• Touch pins 1-9 of J209 in turn: switches 11 through 18 should show closed.
• A row that never shows, or shows with nothing connected, means its LM339 is bad: U18 for rows 1-4, U19 for rows 5-8.

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
• Row or column "ground short" errors
• One switch does not work
• A dedicated (direct) switch does not work
• "Slam tilt stuck closed" at power-on
• Optos test good but still don't work: the opto board
• Weak or dead flippers from failing LM339s on the Fliptronics board
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step

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