Fun With Pinball

Animated Score Reel Reset circuits from EM Pinball Machines

In the 1960s and 1970s electromechanical pinball machines used Score Reels to show each player's score. At the start of each game the Score Reels all need to be reset to zeros to clear scores from the previous game. Although there are variations in the Score Reel reset circuits used they are all very similar to the example shown here. Note that this schematic shows only the reset circuitry. Circuits to add points by firing the Score Reel coils are not shown here.

A Typical Gottlieb Score Reel Reset circuit

This is a simplified example of how Score Reels are reset in a Gottlieb game. On the right side are three symbols for the 10 position Step Units representing three Score Reels. Below them are digits representing the view of the Score Reels through windows in the backglass.  In the bottom right is a push button representing the Replay or Start button on the game.

The wiring diagram in the middle of the schematic has three main sections or circuits for the Reset Done relay coil, the three Score Reel coils and the Score Motor.
Typical Score Reel Reset circuitTypical Score Reel Reset circuit
The Score Motor circuit is activated by a normally open switch on an Interlock relay. An Interlock relay has two coils for tripping and resetting the relay. In this case the relay is tripped when the Start button is pressed.  It will remain tripped even after the Start button is released until the Reset Done coil fires. The switch in the Score Motor circuit therefore closes when the Start button is pushed and opens when the Reset Done coil fires. The Motor 1C switch in the Score Motor circuit keeps the Score Motor running once started until the end of the Score Motor cycle so the motor stops only in its home or index position. A Score Motor cycle in this case is a 120 degree or 1/3 of a turn of the Score Motor.

The circuits for the three Score Reel coils are all very similar.  They all share a normally open switch at position 1A on the Score Motor. This switch closes five times each time the Score Motor runs through one cycle as shown in the timing chart on the left side. This switch will send five pulses towards the Score Reel coils in every Score Motor cycle.

The Score Reel coil circuits also share a normally closed switch driven by the Reset Done relay coil. This switch allows pulses generated by the Score Motor 1A switch to continue towards the Score Reel coils until the reset sequence has completed.

The circuit for each Score Reel coil has its own normally closed POS=0 or Zero Position switch.  This switch is mounted to the Score Reel itself and opens only when the Score Reel is showing a zero. When any other number is showing the switch is closed. These switches determine how many of the pulses generated by the Score Motor 1A switch are allowed to get to the Score Reel coil.  Once a Score Reel has reached zero the switch opens and prevents any more pulses from reaching the Score Reel coil.

At the top of the wiring diagram is the Reset Done relay coil which is driven by four normally open switches. The first three are Zero Position switches mounted on each of the three Score Reels. Each one closes when its Score Reel shows a zero. The fourth switch is driven by the Score Motor at position 2B. It closes at the very end of each Score Motor cycle to evaluate whether the Score Reels have reached zero yet.

The Score Reel Reset Circuit animation

This is the animation to reset the three Score Reels to zero.  Milestones in the animation are detailed below.

Score Reel Reset circuit animation

Before this animation starts the Score Reels show a score of 976 from the previous game as shown by the double arrows in each Step Unit symbol and by the Score Reel windows below them:

Animation before the startAnimation before the start

The animation starts when the Start button is pressed which in turn starts the Score Motor turning.  When the Score Motor has turned 20 degrees the Motor 1A switch closes for the first time which energizes all three Score Reel coils since none of them have reached zero:

Animation at 20 degreesAnimation at 20 degrees

When the Score Motor reaches 27 degrees the Motor 1A switch opens and the three Score Reel coils relax.  Only then can the score reels advance one step.  The coils stretched a spring in the Score Reels when they activated, but it's the spring that actually advances the Score Reel once the coil has relaxed.  The Score Reels advance from 976 to 087:

Animation at 27 degreesAnimation at 27 degrees

After turning 41 degrees the Score Motor again closes the Motor 1A switch.  This time however the Zero Position switch on the 100s Score Reel is open because the Score Reel has reset so only the 1 and 10 point Score Reel coils energize in preparation for stepping from 087 to 098:

Animation at 41 degreesAnimation at 41 degrees

The process repeats until all three Score Reels reset to zero.  Notice how the Score Motor and switch 1A don't know how many pulses are required to reset the Score Reels. Each Score Reel has to determine for itself how many pulses to receive by opening its own zero position switch at the right time. In this case the 100 point reel needed just one pulse, the 10 point reel needed three pulses and the 1 point reel needed four pulses as shown in the timing chart on the left.

When the motor has turned 113 degrees the Motor 2B switch finally closes to test whether all of the Score Reels have reset:

Animation at 113 degreesAnimation at 113 degrees

In this case since all score reels have reached zero all of the switches in the Reset Done circuit are closed and the Reset Done relay coil energizes which opens the Interlock relay switch in the Score Motor circuit. The Motor 1C switch will keep the Score Motor running to the end of the cycle.  If one of the Score Reels had not reached zero the Reset Done relay coil would not have energized and the Score Motor would have continued through a second cycle and sent another 5 pulses towards the Score Reel coils.

A Typical Williams Score Reel Reset Circuit

Once you're familiar with the Gottlieb example above compare it to a common reset circuit used by Williams below. Note that the Williams Score Motor creates different pulses compared to the Gottlieb motor and rotates 180 degrees or half a turn instead of 120 degrees or 1/3 turn.

Williams Score Reel Reset circuit animation

The significant difference between these two approaches is how they detect that the Score Reels have all reset to zero.  In the earlier Gottlieb example normally open Zero Position switches are wired in series to detect that reset is done when:

(the 100 point reel is zero) AND (the 10 point reel is zero) AND (the 1 point reel is zero)

The later Williams example instead uses normally closed Zero Position switches that are wired in parallel to detect that reset needs to continue when:

(the 100 point reel is NOT zero) OR (the 10 point reel is NOT zero) OR (the 1 point reel is NOT zero)

These are subtly different approaches that achieve the same result.

Common Problems and Troubleshooting

A common problem with EM pinball machines is that they don't complete the reset sequence when a new game is started and instead the Score Motor runs indefinitely.  There are two Score Reel related scenarios that could cause this behavior:

  • One or more of the Score Reels doesn't reset to zero. In this situation the cause is obvious because getting all the Score Reels to zero is required to complete the reset sequence. If that's not happening the Score Motor will likely keep running, sending pulses to the Score Reels and trying to get them back to zero.
  • The Score Reels all reset to zero but the Score Motor still keeps running. If you look at the schematics above you'll see that there are two sets or types of Zero Position switches.  Each Score Reel has one switch of each type.  On each Score Reel one switch blocks any more pulses from reaching the Score Reel coil once it has reached zero.  The other switch tells the reset circuitry that the Score Reel has reached zero and has completed the reset sequence for that reel.  Even if all Score Reels correctly reset to zero, one of the Zero Position switches in the reset circuit may not have properly activated so the game doesn't recognize that the reset sequence can complete.  Both switches on each Score Reel need to be operating correctly for the game to completely reset.

Animated Schematic Diagrams

Follow these links to other animated schematic diagrams: