Checking LM339 comparators on opto boards

LM339 comparators are hard to test because they work on voltage levels. If the outputs are steady (not pulsing or drifting), you can check each comparator against its truth table using a meter on the inputs and a logic probe on the output.

Systems

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

Related tips

• Checking an under-playfield opto board for bad LM339s
• Testing an opto receiver
• Opto parts to keep on hand
• Opto styles used in WPC games
• Cleaning dusty optos
• Testing optos in switch edge test
• Flipper cabinet opto switch boards
• Where WPC games use opto switches

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