After fitting four new optos in the centre/upper lock assembly, the lock optos don't work, and plugging the assembly in also kills the gumball entrance and magnet optos on the same connector. Unplugged, everything else works.

New lock optos fitted: those and other optos on the same plug stop working

Cause: The new receiver optos were faulty out of the box (internally shorted): their outputs sat at about 12 V even when blocked, which upset the other optos on that connector.

Checks

• Double check the emitter and receiver orientation against the board markings.
• Measure each receiver's output with the beam open and blocked; it must change.
• Try the assembly in another game, or swap in known good optos.

Fix

• Replace the receivers with optos from different stock and test each one before reassembly.

Systems

• WPC Fliptronics

Games

• Twilight Zone

Related tips

• Gumball enter opto (switch 87) never registers: replace the LM339
• Clock minute optos miss now and then: repin the plug at the 10-opto board
• Opto still dead after replacing both sides: the new transmitter can be bad
• Check opto power supply message with good fuses: reseat the opto board plug
• Eddy sensors: electronic ball switches under the playfield
• Mist multiball long opto board dead: check L1 and the MPS-A77 transistor

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