Screen Wont Move

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Case Study Summary
Game Bikini
Games Applicable To magic screen
Source randy
Problem Reported the screen won't move in either direction
Fix not reported


The whole point of having a magic screen machine is moving the screen. When the left/right buttons don't do anything, business picks up at the local depression hotlines.

Of course, there's some prerequisites for screen movement:

  • at least A must be enabled. Most if not all machines enable at least AB
  • you can't have shot the ball that disables movement. Minimally, that's the 4th ball, but if you have the feature enabled, it could be the 5th ball or the 1st extra ball.

The picture below shows the components involved with screen movement.

screen movement components

The index cam is a metal disc with notches. When the index arm falls in a notch, the screen slats are centered over the number on the display panel, and more importantly, the wipers on the magic screen unit contact plate are centered on the appropriate rivets.

The motor has two stators/coils. Which direction the motor spins is determined by which coil is powered.

The index switches have a critical function. One powers the motor and keeps it powered until the index arm falls into a notch on the index cam. The other switch removes the power from the index coil so it is not held in if a player holds down one of the left/right buttons. None of the plunger-style coils can be powered for a long time without turning into toasters.

The index coil is what kicks off the entire process of screen movement. Pushing the left/right buttons will power the index coil. That sucks in the plunger, releasing the index cam and operating the index switches.


bikini screen movement schematic. Circuit is mostly the same on all screen games, though simpler on non-ok machines.

Since in-line scoring works, the search index unit is positioned ok, the search relay switches are functional, and the slip ring feeds connecting the search wiper hub to the game are in the right grooves.

Generally, either all the sections work, or none do. When only one/some sections don't pay, usually the problem is just dirty contacts on the search disc.

When none of the sections work, it's usually an issue with the "changeover" switches. These switches are operated by a cam attached to the search wiper locking ratchet. When the search wipers spin around far enough, the cam flops the changeover switches to disconnect the inline circuits and connect the section circuits.

On Lido and Golden Gate, that cam is #15. When these switches flop over to connect the pink-highlighed circuit, that should give you a connection through the rightmost circuit branch when a switch in one of the search relays 1-4 is closed, and should power the search index coil to stop the wipers.


If you are trying to figure out how the circuit works, and you're puzzled by the sequence unit reset plunger switch that is shown as normally open, would it help if you knew that switch is normally closed? This is one of the schematic errors. Some screen game schems got it right, some are wrong. The switch is always NC (it opens when the plunger sucks in to reset the sequence unit)

Inspection Debug Procedure

You pretty much check CU switch 15A and 15B and the search disc wipers and rivets, the sequence unit "close at zero" switch (on the side of the sequence unit operated by a pin sticking out the ratchet/gear), and verify the sequence unit is resetting so the close-at-zero switch is, umm, closed.

There's also a quick-n-dirty check. When the search wipers are locked, cam 15 switches are usually still in the section-scoring state. You can just manually close one of the search relays 1-4 and the search index unit will power (assuming the before 4th trip relay is tripped). Just be prepared that the sequence unit will also start stepping up if the motors are running, so don't jump backwards and land on the dog.

Voltmeter Debug Procedure

The quick-n-dirty test works because power is in this circuit when the search wipers are locked. Pretty handy for voltmeter work, but probing depends on if you want to hold/jumper a search relay switch closed.

If you want to skip messing with the search relays, then put one meter probe on wire #18 (start of the pink bit at the top), and poke around in the pink circuit below the search relays. You should see 50V everywhere.

If you want to hold/jumper a search relay, then put one meter probe on fat orange wire #70 on any convenient 50V coil and probe anywhere in the pink circuit to see where the 50V got lost.


Jeffrey writes:

Of course I checked switches 15A and 15B and they were good, holding voltage properly in either mode, in-line or color section. I found my problem on "Big Yellow" and "Little Green" to be the two outside wipers and rivet rings on the search wipers/disc. I had to clean these really well and re-solder the wire connections.

But "Big Red" still wouldn't test all the way through. If it did start to pay only a portion of the credits would be awarded then the whole circuit would drop out.

The cause was a slight "wobble" on the control shaft effecting the position of cam 15. 'BUT' only when cam 15 was in the position where the rotating search disc wiper was testing "Big Red". The voltage never fluctuated on any other color sections. A simple adjustment on Sw 15A and the problem was cleared.

The first thing I tell any person trying to make their game score properly, "clean and adjust switches 15A and 15B". Still, I get caught with a problem on 15A even after I was sure it was good!

Besides the dirty wipers/rivets, , switch 15A simply wasn't making a good was barely closing. Switches should always "overtravel" a little after the contacts touch. This causes the contact faces to wipe across each other, scrubbing them clean and insuring a good mechanical connection.