The home run opto stops registering; the LED on its opto board stays on all the time, and swapping the game's two opto boards changes nothing.

Home run opto not registering: replace the opto emitter and receiver

Cause: The opto transmitter and receiver themselves had failed. On this game they sit under the back ramp assembly.

Checks

• Check the transmitter with a phone camera (a front camera often shows infrared when the rear one does not; try a known-good opto first).
• Look for cracked resistor leads and cracked joints on the opto boards.

Fix

• Remove the back assembly to reach the home run optos and replace the transmitter and receiver (or reflow their joints if they test good).
• Allow a couple of hours; the assembly has several layers to remove.

Systems

• Gottlieb System 3

Games

• Frank Thomas' Big Hurt

Related tips

• Boiler door cycles then stops halfway: cracked joints on the quad opto board

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