Checking an under-playfield opto board for bad LM339s

Steps

• Most games from Twilight Zone on have at least one under-playfield opto board.
• In switch test, unplug the opto board's main plug (12 V plus the row and column wires).
• Opto switches read closed until blocked, so every opto should change from closed to open on screen.
• If a ground row short clears, an LM339 on that board has failed.
• Check the board has a full 12 V; 11 V or less gives erratic switches and points to BR1 or the LM7812.
• With good 12 V, replace all the LM339s on the board, in sockets.

Systems

• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• Confused switch matrix from LM339s on opto boards
• Phantom or erratic switches traced to an opto driver board
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• Row or column "ground short" errors
• Opto switches dead all at once
• Gumball enter opto (switch 87) never registers: replace the LM339
• Optos test good but still don't work: the opto board
• Two LM339s on the Fliptronics board read the flipper optos

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