A replaced (or factory new) opto transmitter does not light.

New opto transmitter does nothing: leads may be reversed

Cause: The two leads are wired the wrong way round. Even factory parts have been seen built reversed, and some makers put the flat spot on the wrong side.

Checks

• Check the LED is not lit with an infrared card or camera.

Fix

• Swap the black and gray wires on the transmitter. Reversing the leads usually does not damage the LED.

Systems

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

Related tips

• Telling an opto transmitter from a receiver
• Replacement opto transmitter and receiver pinouts
• Bookcase opto still dead after a new receiver: the replacement part was bad
• Optos test good but still don't work: the opto board
• Opto parts to keep on hand
• Opto styles used in WPC games
• Cleaning dusty optos
• Testing optos in switch edge test

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