How They Work - Magic Screen Movement

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 screen section 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
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. More importantly, the wipers on the magic screen unit contact plate are centered on the appropriate rivets. If the slats are halfway between numbers, you've got the index cam in 180 degrees off where it needs to be relative to a c-shaped bracket above it that connects the index cam to the screen and chain drive gears. If that happens, easiest thing is to unscrew the plate enough to disengage the bracket and spin the cam 180 degrees.

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 makes sure power is on the move right motor coil and move right relay before powering the index coil to allow the screen to move right.

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. If the index coil plunger is pushed in, the screen will move to the left. You need to hold down the right button relay plate and then push in the plunger to move the screen to the right.

Before plodding on, we need to be clear about one thing. Pushing the right button moves the screen slats to the right. On an OK game, that would mean sliding the orange section into view. The left button obviously goes the other way. Any mention of the words left/right in the below mess is talking about the slats sliding in the direction the words indicate.

Screen Movement Schematic

Grab a drink, buckle up, and prepare for a bit of bafflement. If you've read other writeups of mine without that warning, you should realize that by "grab a drink" I mean grab a bottle. You may want to open another browser window so you can keep an eye on the schematic while scrolling down this long page.

screen movement schematic
Bikini screen movement schematic.

Homing the Screen at Reset

On games without OK (or oddballs like Twist), the screen can only move left from the home position. Reset on these games is just a matter of moving the screen to the right until it's back to the home position. The blue circuit takes care of that. Wire #21-3 is directly connected to wire #30 when the tilt trip relay is untripped, and wire #30 is connected to the transformer. The panel switch is activated by the playfield shutter. When the shutter opens to drop the balls, the panel switch closes. Wipers on the magic screen unit will pass the current along the blue path until the screen reaches home, at which time the wipers disconnect and the motor power is turned off.

The blue circuit powers the move right coil on the motor and the right button relay, but we still need the index coil to power and release the screen. The purple circuit takes care of that. Note the panel switch is one of the oddbal schematic symbols. While it looks similar to an SPDT switch, it's not. When the shutter panel opens, wire #21-3 connects to both #61-4 and #50-5 at the same time. The purple and blue circuits are tied together.

If the screen has been moved to the right of the home position at reset time, we need to move it left to home it. In this case, the magic screen unit will not connect wire #50-5 to #45-5, so the current will not pass down the blue circuit to the move right coil and button relay. Only the purple circuit will pass current to the index coil, and the default action of powering the move left motor via the red circuit will cause the screen to move left.

Moving the Screen With Rail Buttons

You can look at the schematic like a map and you want to be able to walk from one side of the transformer through the circuits and back to the other side without getting stopped by an open switch. In this case, wire #21-3 is connected to one side of the transformer, and when the playfield shutter is closed and the game not tilted, we can meander along to the left end of the orange circuit with no problems. Our goal for this section of the writeup is to get to the other end of the orange circuit.

For the moment, ignore the light green branches and stick to orange. We hit the select before 4th ball lockout trip relay. That relay trips when the 4th ball is shot and the yellow circuit is connected to enable the R-button to release the search wipers. Assuming the 4th ball hasn't been shot yet, moving along runs into before 5th and after 5th trip relays. They disconnect the circuit to disable screen movement when the 5th ball and first extra ball are shot. Since we haven't even shot the 4th ball yet, these switches should be closed.

Finally we hit search wiper cam #20A. This switch is on the stack that is lifted by a pin sticking out of the search wiper lock cam. It's job is to make sure you can't move the screen while the game is searching for winners. Assuming you aren't trying to do that, you should now be standing on wire #20-2 at the right end of the orange circuit segment.

The light green branch paths we previously ignored make moving the screen before/after 5th work. The before 4th trip relay always trips when the 4th ball is shot. If we have after 5th enabled, the light green path through the red pollover trip relay is closed and we can walk around the before 4th and before 5th switches that way. The yellow rollover trip just gets you around the before 4th switch, but not the before 5th one.

You're standing at wire #20-2, and by default the left button switch is directing you down the dark green path when the left button switch is not pressed. Down you go and get stopped immediately because the right button switch is open. So good, nothing is happening because neither the left nor right button switch is pressed. Backtrack to the end of the orange circuit.

Moving the Screen Left

When the left button is pushed, orange circuit wire #20-2 connects to cyan circuit #31-16. Move along that and pass through the closed right button relay switch (it opens when the screen wants to go right to prevent powering both motor coils at the same time. That would cause the motor to stall and eventually burn up.)

You enter a rat maze of connections between the magic screen unit disc (the one hanging beneath the screen) and the magic screen feature unit (stepper unit that steps up to enable ABCDEFG(H)). Assuming the current position of the screen and the magic screen feature unit says it's ok for the screen to move left, you pop out on wire #41-5 and head straight down to merge onto the purple circuit to power the index coil. As soon as the index coil is powered, the index arm disengages from the index cam, the index switch closes, and the move left motor coil is powered via the red circuit.

The screen moves left. If you get to a position the screen is not allowed to move further left, the rat maze disconnects, the index coil loses power, and the index arm drops onto the index cam edge. At that time, the index switch must still be closed to keep the motor powered. When the index cam rotates enough for the arm to fall into the notch, the motor is then turned off.

Moving the Screen Right

If you left the orange circuit, went down the dark green path and patiently waited until the twit pushed the right button, you keep going down and merge into the blue circuit. The magic screen unit disc lets you through if/when you are allowed to move right - when the screen is left of home or you have OK enabled and haven't moved all the way right yet.

You then notice you are twins, so one of you continues down the blue path to power the right button relay and right motor, and the other one continues along the dark green where you go though the magic screen unit disc again (and wonder if this couldn't have been simplified a bit so it wasn't necessary to go through twice).

If the right button is still closed, you pass though a switch on it and a closed shutter panel switch and power the index coil to release the index cam. Since the right motor is running, the screen moves right

There is a tiny amount of time where it's a race as to whether the right button relay closed and prevented the move left coil from powering. Doesn't really matter what happens in that fraction of time, it sorts itself out.