Red & Teds Road Show, Williams Pinball Machine (1994)

Location: Broomfield, CO

Symptoms: Delivers too many balls to the shooter lane.

In this era of pinball machine, the ball trough is monitored by infra-red emitters (LEDs) and detectors (photo transistors).  The detectors are on one side of the trough, the emitters on the other side of the trough.  The game senses the ball when it breaks the beam of light between the two.

The owner had suspected faulty opto boards and had replaced them to no avail.

The test diagnostic (“switch edges”) showed that the Trough Jam opto was not working.  It said there was a trough jam all of the time, even when no balls were present.  This caused the firmware to think the ball was jammed and to try shooting it again into the shooter lane.

Switch Matrix (click for larger and clearer)

Even though the owner had replaced the opto boards, I wanted to start at the beginning and make sure it was working. I checked the signal at the collector of the photo transistor while the owner blocked the light in the trough.  When the light was allowed to hit the detector, the collector measured 12 volts (or close to it).   When the light was blocked, it measured near 0 volts.  This is correct.

Photo-transistor schematic; the red dot indicating the measurement point.

At this point, I thought it would be a good idea to check the other switches in the same row and column as the “Trough Jam” to see if there was a wiring or MPU problem in the switch matrix.  Several other switches in the row did not work.

We spent some time tracing the row wire through the various bundles, not an easy task when 40-50 wires are tie-wrapped together.  The row and column wires are daisy-chained from one switch to the next, zig-zagging across the playfield. We found it broken at the White Standup switch.

I re-soldered the wires to the switch and everything worked.

While the switch edges diagnostic was running, we discovered and unrelated intermittent switch.  We found it easily at one of the eddy current sensors because the LED on the sensor board would blink whenever the connection was lost.  One of the wires was pulling out of the connector.  I tie-wrapped the top of the connector to act as a strain relief and hopefully it will hold.

Police Force, Williams Pinball Machine

Location: Littleton, Colorado

Symptoms:  Weak flipper, lower playfield general illumination not working.

I started with the flipper.  I checked the mechanical aspects of the flipper by manually turning it. Then I checked the end-play by lifting it up and down, and tried to rock it back and forth to check whether the flipper bushing should be replaced.  Mechanically, everything seemed fine.

Next I turned to the electrical aspects of the flipper.  Flippers of this type have two coils wrapped around the solenoid coil bobbin.  One coil provides a strong magnetic field for starting the movement of plunger (slug) into the solenoid.  The other coil provides a weaker magnetic field for holding the plunger once it has been drawn inside the bobbin.  The force required to hold the plunger is less than what it takes to draw it in.

When the flipper button is pressed, both coils are initially energized.  Once the flipper is at it’s ending position, it opens a switch that turns off the more powerful coil and leaves the weaker coil energized.  This allows the flippers to be strong, but prevents them from burning up if someone were to hold the flipper button.  If power were applied constantly to the more powerful coil, it would overheat and start burning.

The switch that opens when the flipper reaches its end position is called the End of Stroke switch (EOS).  I check the EOS switch with the power off and the flipper in its normal resting position with my ohm-meter, and it was “open”, when it should have registered a short.  I cleaned the contacts with some 440 grit sandpaper, and adjusted the contacts to make sure they were closing when the flipper was at rest.

Drawing of flipper unit from the Police Force manual, showing the End of Stroke switch.

I rechecked the switch contacts with the ohm-meter and they were working properly.  We powered-up the machine to verify the flipper was working fine.  Basically, since the switch wasn’t closing, the flipper was only running with the weaker coil.

Next I took a look at why all of the General Illumination (GI) lights in the bottom half of the playfield were not working.  The GI lights are the ones that stay illuminated all of the time and light up the plastics and the playfield areas so that you can see the ball.

First I checked to make sure no voltage was present at one of the light sockets.  Although it was a remote chance, I wanted to be sure that it wasn’t just a case of the lights being burned out (which could have happened if there was a short with a higher voltage somewhere else in the machine).  There was no voltage (be sure to measure on AC scale).  I checked where the power was coming from the Backbox Interconnect board, and there was 6.3 volts there.  So somewhere between the backbox and the playfield, the voltage was getting lost.

I tipped the playfield all of the way up and found the GI Relay on a small circuit board at the very bottom edge.  The connectors that connect the wires to the board were burnt and falling apart.  The connector on the circuit board was burnt as well.

Connector failure is a common problem with GI lighting in pinball machines. Oxidation builds up on the connector pin, or the crimp, and causes the resistance of the connection to go up.  This causes the connector to heat up until it fails. Because each machine and manufacturer uses different types and sizes of connectors, it’s difficult to to keep connectors on hand.  In this case, I didn’t have replacements with me.

The owner didn’t want to wait to get the proper connectors and asked that I solder the wires directly to the circuit board.  I did so, and the lights all worked fine after that.  Normally I am a purist about having the connectors fixed properly, but in this case the connectors only served to make the board replacement easier, which is unlikely to ever need replacing.

GI lamp relay board.

We checked and replaced several burned out lights.  The game was working well and in good condition.


Judge Dredd follow up

Location: Denver, Colorado.

A few months later, I was called back to the Judge Dredd machine mentioned in this previous post.

The thermistor in the power line module had burned out.  I replaced it and tested the machine.  I noticed that the shorted switch-row problem had returned.  The nice thing was, it was a solid failure this time and I was able to track it down.

In my previous post, I mentioned the short would go away if I unplugged J212.  This was the case again, and it deceived me into thinking the short was somewhere along this stretch of wires that heads towards the coin door.   In the switch table in the service manual, Switch 24 is listed as Always Closed.

(The switches and lamps in a pinball machine of this vintage are multiplexed in a grid.  This reduces the complexity of circuitry and wiring.  Rather than having 64 separate wires to 64 switches, by multiplexing, 64 switches can be monitored with just 16 wires.)

Somewhere in the machine, there is a diode connecting Column 2 and Row 4.  It turns out it’s on Coin Door Interface Board, which is were the wires from J212 end up (D2).

What is deceiving is that when a column is shorted to ground, any switch that is closed will show that row to be shorted.  Since switch 24 bridges column 2 and row 4, every switch in row 4 was shown as closed, leading me to believe the problem was in row 4.  But when I closed other switches in column 2, the entire corresponding row would show as closed.  And as a verification, I open the diode at D2 and the remaining switches in row 4 began functioning normally.

Once I determined the problem was actually in column 2, and the symptom indicated it was shorted to ground, I unplugged J205 and J212 to isolate the CPU board from the rest of the machine.  I measured the outputs of each column driver and found the output of U20, pin 17 was stuck at ground.

I replaced the ULN2803 with a new part and everything in the pinball machine tested normal.