try reseating connector J1 on the mpu board, top left.
make sure there isn't any real strain pulling down on that, or any connectors.
whilst in display test try wiggling the wires on J1 to see if the millions digit comes on, you can see the display from behind with the door open.
that failing check the solder joins on the bottom of the display boards for any dry joins/cracks in the solder around the pins.