Dead CPU: isolate it from the other boards

Steps

• Game off: unplug the ribbons at J201, J202, J211 and J204 (on games that have it) and the switch connectors J205 to J209 and J212, leaving only J210 (power).
• Power up. A healthy board has the bottom (+5 V) LED on, the middle LED flashing and the top (blanking) LED off.
• If it boots, turn off and refit the cables one at a time, driver board ribbon first, powering up after each.
• If it stops booting when another ribbon is added, suspect the TTL chips U1, U2 and U3 across the top of the CPU board.

Systems

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

Related tips

• Reseating the U9 ASIC on a dead CPU board
• Dead CPU: the order to replace chips
• Signals a dead CPU board should show
• Dead CPU: checking address and data lines
• What is on the WPC CPU board
• Dead CPU: no clock signal at the 6809
• Which System 11 CPU boards fit which games
• The CPU must be a 68B09E

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