The fourth bookcase opto never registers, though every other opto works and the transmitter tests good. Fitting a new receiver changes nothing.

Bookcase opto still dead after a new receiver: the replacement part was bad

Cause: The replacement receivers came from a bad batch. They looked identical to good ones but didn't respond to the transmitter.

Checks

• Jumper the column and row pins at the opto board connector, then jumper across the receiver's own pins in place; if the switch fires both times, the board and wiring are fine.
• Compare voltages: a good receiver here read about 14.8 V when blocked and dropped to around 1 V when lit; the bad parts sat near 10.8 V and didn't change.
• Swap a known working receiver into the dead position (correct orientation) to prove the part.

Fix

• Fit a receiver from a known good source and discard the suspect batch. Many techs replace both the transmitter and receiver of a pair together.

Systems

• WPC Fliptronics

Games

• The Addams Family

Related tips

• Opto still dead after replacing both sides: the new transmitter can be bad
• Replacement opto transmitter and receiver pinouts
• Egg target optos dead after fitting replacement opto boards: refit original receiver
• Opto does not work: check 12 V at the transmitter connector
• Gumball enter opto (switch 87) never registers: replace the LM339
• Mounting a replacement opto receiver
• Telling an opto transmitter from a receiver
• Fitting a replacement U slot opto the right way round

Source: Pinball Repair help group!! (Facebook group)