Magic Screen Feature Unit Burnt Coil

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Case Study Summary
Game Golden Gate
Games Applicable To magic screen
Source phil
Problem Reported magic screen feature unit step-up coil burning up
Fix confirmed


The magic screen feature unit steps up occasionally during the spin cycles to enable the A-G movement range of the screen.

The step-up coil on the unit was getting constant power. When this happens on the plunger-type coils, get out your marshmallows. The coil will become red-hot in a couple minutes, and will self destruct shortly thereafter.


Simplified Golden Gate magic screen feature unit step-up circuit

Since the problem is a stuck-on coil, find the coil on the schem and see how it's supposed to be powered. In this case, it's a rat-maze of circuits, any/all/none of which may be the problem.

The blue path is used to step the unit up once right after reset. Most stepper units related to scores and features don't sit at their reset positions. They get reset by playfield shutter switches when the shutter opens to drop the balls, and get immediately stepped up once by a control unit cam switch.

The green path is the OK/Red Letter win circuit. When you collect an OK win, the magic screen unit needs to step up to the correct level defined on the award card.

The red path is one branch of the step-up circuit that ignores what other features are enabled. Highlighted is a circuit through the reflex unit that will get disconnected after the reflex unit steps up far enough (i.e. you've won too much), but the unhighlighted path to the left of the reflex unit into the spotting disc is always active, so there is always a chance that the unit will step up.

The remaining unhighlighted paths are route from the spotting disc through feature units and/or trip relay switches. These paths are disconnected when the feature units step up far enough or the trip relay trips (e.g. you get the yellow or red supersection enabled).

Inspection Debug Procedure

Assuming you discovered a burnt coil and replaced it (and after powering up the game you fried the replacement coil again and put in a third one), you either have a good coil installed or are using a voltmeter in it's place.

It's now a process of elimination to figure out where the constant power is coming from. The OK game feature is pretty convenient here, because Bally placed a yellow stop relay switch (1) in a handy place. It's job is to disconnect all the normal step-up circuit when an OK game win is being collected, but you can use it too. Look in the manual to see which switch on that relay is the one in this circuit - or use the wire colors to figure it out - and stick a business card in between the switch contacts to open the switch. If the power disappears, you know it's coming from the normal step-up circuits and not the OK stuff...and now you have a simple way to remove the power so it doesn't cook another coil while you test further.

Next I'd jump up in the circuits a bit to (2). A lot of circuits are routing through the magic screen feature unit itself, and those circuits disconnect as the unit steps up. While you could get out the manual and figure out what the unit is doing, the easiest thing is to disconnct the circuits by sticking more business cards under all the wiper fingers on the unit. In this case, the power stayed on, so it's coming from the red path or something lower down.

Moving onto (3), the open-at-top switch on the unit. This switch disconnects the circuit when the unit reaches the top step, and is needed because otherwise an endless stream of pulses could reach the unit and operate the plunger forever if the spotting wipers stopped in the right place. The simple test is to manually step the unit up all the way and see if the power disappears. It did, so now we know the problem is along the red path and above the open-at-top switch.

The circuit branch through the blue score booster relay (4) can be eliminated by opening that switch, though the features lock relay switch right above it should already be open unless in green button mode. However, we're looking for a short, so it doesn't hurt to open the switch and see. Didn't help.

The power is coming either from the spotting disc, the reflex disc, or a wiring short along the way. After double-checking that the spotting slip ring wipers are in the right grooves on the wiper hub, you could stick paper under the spotting wiper fingers...or deal with a smaller case first - the reflex unit (5). Sticking paper under those wipers made the power disappear. Either wire #54-18 going into the reflex disc is always powered, or there's something wrong with the reflex unit.

Voltmeter Debug Procedure

With paper under the reflex wipers, use the voltmeter to measure between wire #70 (fat orange wire on all the 50V coils, including the reflex unit ones) and wire #54-18 attached to the reflex unit. 50V was not seen, so the problem is on the reflex unit itself.


The reflex unit diagram below shows the wipers in what should be their fully reset (counterclockwise rotation) position.

On this game, the wipers were rotated further, and the large span of 6 fingers was touching the rivet under solder lug 10. This connected wire #54-18 to wire #75-5. #75-5 has power most/all of the time, depending on the position of the mixer #1 rotor and the reflex unit adjust plug. The result is a continuously powered coil, and if left that way for a while, a burnt up coil.

The fun part is that bally does not provide reflex unit diagrams in the manual, so how do you know where the wipers should stop (assuming you have a game that I don't have a diagram for posted on the web site)? Well, as far as we know, the six finger span never travels further counterclockwise than the adjacent row of consecutive rivets on the outside ring. On some games if may reset enough that only the rightmost finger is on the leftmost rivet. On Golden Gate, this should be impossible as it would cause the leftmost fingers to bridge other circuits.

On many games, there's small arcs of a few rivets connected by a couple wipers. The maximal reset position would be on the end rivets of the arcs.

The root cause was a bent stop pin inside the gearbox. It could also have been a mispositioned shaft adapter (the wipers connect to the reflex unit shaft using an adapter with set screws on it. If the set screws are loose, the shaft will slip inside the adapter and reposition the wipers relative to internal stops.

Reflex unit W-770-5b used in Golden Gate