Open At Top Switches

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Case Study Summary
Game Miss America Supreme
Games Applicable To all games
Source phil
Problem Reported timer unit not resetting
Fix confirmed


Mike had a problem with the timer unit not resetting. A peek at the schematic showed that the timer unit resets a couple of ways:

  • A shutter motor cam switch that closes when the shutter opens
  • When the before 4th selector lock trip relay is tripped and the R-button is pushed

There is also an "open at zero" switch on the timer unit which disconnects the reset coil when the timer unit is reset.

Most of the stepper units have switches on the sides of them. These switches are cleverly labelled "open/close at top" or "open/close at zero" to indicate what they do at the top stop/reset position of the unit.

A common switch is open-at-top. When it opens, the step-up coil on the unit is disconnected. That prevents the circuits from trying to step the unit up further (mechanically impossible due to a flat spot in the ratchet teeth). In some "run" circuits, without the open-at-top switch, the unit would be stepping forever if the game wasn't spin cycled to disconnect the power.

Open-at-zero also shows up a lot, and on games with an R-button, it's critical. Without it, a person holding down the R-button would constantly power the timer unit reset coil, and it would eventually burn up.


Miss America Supreme - timer reset circuit

There's two ways to reset the timer unit:

  • Shutter motor cam switch 6 that closes when the shutter opens during game reset
  • When the before 5th selector lock trip relay is tripped and the R-button is long as the game isn't tilted (tilt trip relay tripped and disconnecting wire #21-3 from wire #30. This turns the motors back on.

Since the source of the reset power is coming from the playfield and cabinet, plug connections are also a possibility. However, since separate circuits from both the playfield and cabinet are not working, it's not likely to be an issue in the plugs or either of the circuits. In addition, both the circuit get power from different places!

That leaves the probable culprit the only thing common to both circuits (besides the coil itself) - the "open at zero" switch. If it's always open, then the timer unit can't reset.

Inspection Debug Procedure

I asked mike to describe the switches on the side of the timer unit, and he said they only get moved when the timer unit is at the top step. Something is wrong there, so I asked him to send me a picture.

Mike's timer unit switches. One is open-at-top, two are open-at-zero/reset

The switches aren't right.

  • The top switch is closed (open at top). That's correct. It should only open when the unit is at the top step and a pin bends the pole blade down.
  • The middle and bottom switches (open at zero) are open. That's wrong. Both these switches should be closed unless the unit is reset. When the unit is reset, the pin should be bending the pole blades up to open the switches.

I was trying a new scotch when emailing back to Mike, and apparently it didn't help clarify things, so I took the following set of pictures on a Ballerina unit. The switch stacks are similar - Ballerina has an extra "open at zero" switch.

Timer Unit Reset

top switch closed, others open

The white ratchet/gear steps up in a clockwise direction. At reset, the pin is bending three pole blades left/up. The leftmost/top switch is not being moved at all, and the fish paper insulator is floating in beteen the two metal blades.

Timer Unit Step 1

all switches closed, no pins bending pole blades

Here the ratchet has rotated one tooth clockwise. The pin is no longer holding the bottom/right three switches open.

Timer Unit Penultimate Step (ok, top step -1)

all switches still closed, no pins bending pole blades

One step away from the top step of the unit. On thise game, a second pin is now touching (or about to touch) the pole blade on the top/left switch. The pole is not bending enough to open the switch contacts.

Timer Unit Top Step

"open at top" switch now open. The rest are still closed

Here we see why there is fish paper in between some blades. The top/left pole blade is bent down, and without the fish paper would probably touch the pole blade next to it. In Miss America Supreme, that would short 120V into the 50V circuits....bad news.

Ideally, you don't want to rely on the fish paper. The bottom/right three pole switches are being prevented from moving further right/down by:

  • the pressure from the short blades they are contacting
  • the longest blade in the middle. That blade is not electrical - it is there so the bottom of the left white plastic separator/lifter isn't mashing down directly on the mounting nub of the right plastic separator.

It's likely that the short blades and/or the non-electrical one can be bent down/right to allow the pole blades to move farther, which would create an air gap between the top blade and the one next to it (i.e. if the fish paper wasn't there, the blades would still not touch).

It may be necessary to bend the pole blades down a little if there's not enough tension on the short blades after they are moved, then test to make sure the switches open correctly when the unit is reset.

In this case, the end of the blade(s) could also be kinked a bit to create an air gap. The kink would need to be made past where pin #2 contacts the blade, and/or past the white plastic switch lifter/spacer on the other blade. However, this is not the recommended way to create an air gap.

Voltmeter Debug Procedure

not needed


Adjusting the switches fixed the problem.