Location: Littleton, Colorado.
Symptoms: Two of the displays had digits that were blank (off). General illumination lights on the backbox were not working.
I ran the display self-test to determine the full extent of the display failures. One digit on the Player 1 score and one digit on the Player 4 score were not working. They were different digits, so it wasn’t a problem with connections from the CPU board.
I started with the Player 4 display. I disconnected and removed the display from the backbox panel and checked the digit driver transistors with a mulitmeter on the “diode” scale. The transistors checked OK. This was a little surprising to me. So I connected an oscilloscope to the digit drive signal. It was OK where it was coming into the display board. There was no signal on the collector of level shifter transistor (Q6 in the diagram). Surprisingly, the 100K resistor was bad and read “open”.
I had previously read about the power rating of these 100K resistors being too small as originally designed, but I wasn’t expecting to see a failure with no visual indication of the resistor being over-heated. I didn’t have any spare 100K resistors with me, but fortunately Radio Shack is still good for some very common parts.
I disconnected and removed the Player 1 display and immediately checked the resistor for the failed digit and it too was bad. With the resistors from the nearby Radio Shack, I was able to desolder and replace the resistors on-site. I re-assembled the displays into the backbox and re-tested. Everything worked fine.
As for the general illumination lights not working on the backbox, this is a common problem and is often related to a connector. This was no exception. I traced the non-working branch of lights back to the connector at the power supply board. A portion of the connector on the PCB had been previously replaced, with a version with slightly shorter pins. The lights started working with just touching the connector. I checked the solder connections on the back side of the PCB and cleaned the connector contacts.
As I usually do when I work on a pinball machine, I ran a light test and replaced several bulbs that were burned out on the playfield.