Location: Broomfield, Colorado.
Symptom: Error message indicating several switch rows shorted to ground.
The owner had checked the playfield and the coin door wiring, looking for any obvious shorts. None were found. I suspected there was a problem on the MPU board, where all of the columns and rows of the switch matrix connect.
I disconnected all of the switch connectors from the MPU. With the pinball machine in the Switch Edges test routine, I took a couple of jumper leads and a diode and connected a Row 1 with Column 1. Instead of seeing a single switch closure, the entire Row 1 lit up as being closed. I repeated with a few of the other rows and got the same result. This pointed to the column driver chip U20 (ULN2804).
To be sure, I checked the column outputs with my oscilloscope. Instead of seeing a signal pulsing from 12 volts to ground, I saw a signal pulsing from about 2 volts to ground. I checked that there was 12 volts on the pullup resistors to make sure the PCB wasn’t damaged from leaky RAM batteries. The 12 volts on the pullup resistor was fine. I then checked the input to U20 to make sure the upstream chip was functioning correctly.
I replaced U20 with a socket because this is a common problem with these Williams MPU boards. I then placed a new ULN2804 into the socket. The board was retested in the machine and everything was fine.