Bram Stoker’s Dracula Pinball Machine (Williams 1993)

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.

Lower portion of the Williams MPU board (click for larger), U20 is just left of center. Note battery holder above it.

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.

Fast Draw, Gottlieb pinball machine

Location: Boulder, Colorado.
Symptoms: Pinball machine wouldn’t reset.

A machine not resetting is the most common problem with an electromechanical pinball machine.  In a way, it’s like a self-test, because a lot of things have to be working for the machine to reset.  The reset sequence is like a series of dominoes, one item affects the next item and so on.  If one item fails to reset, then the sequence is either halted, or more likely, gets stuck in a continuous reset cycle.

This Fast Draw had been not working for nearly 12 years.  And typically with an EM machine that hasn’t been used for years, it usually needs some contact cleaning, contact adjustment, and the steppers rebuilt.  This particular machine also had a broken wire associated with the reset relay.