The MPU gives its normal 7 flashes and solenoid test works, but the score displays stay blank and none of the feature lamps light, even with a different MPU fitted and the supply voltages reading normal.

Boots with 7 flashes but no displays or feature lamps: MPU J1 connector wired wrong

Cause: The display and lamp data lines leave the MPU on connector J1. A previous owner had swapped or mis-pinned one of the lamp connectors on J1, so the data lines were scrambled and several pins were empty.

Checks

• Confirm the rectifier and solenoid/driver board test point voltages are in spec, so power is ruled out.
• Compare the wires in the J1 housing pin by pin against the schematic: pins for digit strobes, lamp strobes and lamp data should all be populated.
• Note that J1 sits the opposite way round to J4 below it; check the key pin is in the right place after any repinning.

Fix

• Put the swapped connector back on the correct header and correct the pinout at J1 so every wire matches the schematic.

Systems

• Stern MPU-100

Games

• Hot Hand

Related tips

• Game starts but won't fire any solenoid except flippers: repin J4
• Three flashes and 8s or 9s in the hundreds digit
• No solenoids and no game start: corroded power board connector

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