A game starts and scores, but the game over relay won't stay pulled in, so the flippers and pop bumpers are dead. Grounding the relay driver's tab makes everything work.

Game over relay won't hold in: poor 5 V pin from the CPU to the driver board

Cause: The 5 V supply coming from the CPU on pin 23 of the interconnect wasn't making good contact at the driver board, so the 74175 latch there didn't work and the relay driver (Q1) never held the relay on.

Checks

• Remember the relays are CPU-controlled through the short interconnect harness; check the 5 V at the driver board's chips, not just at the power supply.
• Grounding the transistor tab only proves the coil and wiring, not the transistor or logic.

Fix

• Repair the pin/connector carrying 5 V to the driver board (repin and reseat).

Systems

• Gottlieb System 1

Games

• Roller Disco

Related tips

• The CPU to driver board harness
• Rebound, rollover, outhole and drop target switches dead: corroded J6/J7 switch plugs
• System 1 driver board connectors
• Many lamp faults are driver board connectors
• Roller Disco: under-playfield coil transistor
• Testing a CPU lamp circuit by grounding its driver
• Testing a driver board with a Ni-Wumpf CPU
• Driver transistors Q1-Q4: relays and spare outputs

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