When we talk about switches, we are talking about leaf switches. A leaf switch has two or more blades with contacts on them, and the blades bend to connect/disconnect the contacts.

I guess we might as well tackle one of the major sources of confusion - normally open vs. normally closed switches. The concept is simple enough: when a switch is normally closed, it's contacts are touching and current can flow through it. When a switch is normally open, there is an air gap between the contacts and the switch is like an infinite resistance - no current can flow.

The problem is....when is "normally"?

"Normally" normally means when the switch is in it's normal state, which normally is right after reset and the game is sitting there doing nothing. Although normally we consider reset to be the normal state, it's not necessarily the normal state in circuits which normally are active only at other times, like right after the game is over normally.

I would have thought that was clear enough, but some sober people aren't happy with it.

Another way of looking at is that "normally" means when the switch is not doing it's intended function. For example, a carry-over switch on the ball lifter keeps the motor powered until the lifter is back to it's home position. This switch is normally open (if it wasn't, the ball lift motor would stay powered all the time).

Fortunately, most switches are pretty obvious once you get used to things a bit, and "normally" is just after game over, or just after game reset.

Switches
Schematic Symbols Reality
normally open NO switch
This is a normally open single pole, single throw switch (SPST). The pole is the moving blade. Single throw means there is one other blade it can reach when the pole is bent. On the schematic, notice how the pole pivots. If you rotate the line segment representing the pole counter-clockwise (CCW), it will have to go almost all the way around to touch the other wire. That's the sign of a normally open switch.

The reason for talking about CCW rotation is that switches are sometimes drawn horizontally, and the pivot end can be either side of the symbol.

Somewhere near the switch will be a label. In this example, the switch is the B switch on the control unit cam stack #3. Which switch is B? Where is stack 3? The manual will show you....and so will we in a little while.

normally closed NC switch
This is a normally closed single pole, single throw switch (SPST). Note that although the switch is closed, the schematic symbol still has a gap between it's end and the other wire. I suppose if the pole was touching the other wire, it may not look much like a switch. In any case, when CCW rotation of the pole segment on the schem results in a connection quickly, the switch is normally closed.

This time the label just says "Search index sw". At least we know it's one of the switches on the search index unit. Which one? Back to the manual (or use the wire colors).

SPDT
SPDT
SPDT
SPDT switch
An SPDT switch is more complex. It still has a single moving pole, but there's blades on both sides of it. When the pole is bent one way, it touches one of the blades. Bend the pole the other way, and it connects to the other blade.

Normally the pole does not connect (make) to both blades at the same time (while it's moving from one blade to the other, it should disconnect/break first, then connect/make).

A special case switch is a MBB (make before break). This is an SPDT style switch that has the pole contacting both switches at the same time when the pole is changing position. MBB switches are very rare. Even in normal SPDT switches, it's often harmless for it to act like an MBB, but sometimes it can cause problems.

Shown are three of the orientations the symbol may appear in on the schematic. The CCW rule applies - the wire that the pole touches first when rotating the pole counter-clockwise is the normally closed part of the switch.

Again, each switch is labelled somewhere near the symbol. The label placement depends on where there's room to put it. Sometimes the label identifies specific switches in a relay or cam, sometimes it just tells you what relay or cam the switch is on and you need the manual to get the exact location.

Common Problems

  • Dirt/oil buildup - Dry black carbon buildup on the switch contacts is not a problem, especially in the 50V circuits. An oily black coating is usually bad, and the contacts should be cleaned.
  • Shrunken spacers - The switch stacks have bakelite spacers between the blades to space things correctly. Over time, the bakelite shrinks. This causes the blades to reposition themselves. First, snug down the two screws on the switch stack. Tighten the screw closest to the blade contacts first. Then if necessary, adjust the blade(s).
  • Misadjusted blades - Changes in temperature, age, and accumulated stress can affect the switch blades. You never adjust the pole/moving blade (well, almost never), always adjust the stationary ones. You bend the stationary blade at the stack end to reposition it. When properly adjusted, the pole contact will hit the stationary blade contact and move the stationary blade a little. This will cause the contacts to rub across each other, which has a self-cleaning affect. What are the exceptions for bending the moving blade?
    1. to fix a mangled blade.
    2. if the moving blade is providing tension - like lifting the pin at the bottom of the ball lift chute - you may need to bend the moving blade so it applies more force.
    You may then need to bend the stationary blade for proper switch operation.
  • Pitted contacts - Sometimes a contact will develop a crater in it due to arcing. You can usually file down the contact to remove the pit, and adjust the blade. This is the only time I'd recommend filing. You should not file just to clean a contact. Small pits don't matter, so if in doubt, don't file. Note that filing is different from burnishing. A burnishing tool is mildly abrasive and removes surface crud without scratching the contact. A file takes metal off the contact as well.

Troubleshooting

You can sometimes use an ohmeter to measure the resistance of a closed switch (game power off!). It should be very close to zero. However, you must make sure that you are measuring ONLY the switch. If there are other circuits connected to the switch at the time, and they go through low resistance devices like coils, you will get a false reading.

Using a voltmeter is a better option. A voltmeter can be used to probe both sides of a closed switch. You should get the same voltage on both sides. If you're thinking that means you stick both the voltmeter probes on both sides of the switch, you're in trouble. Getting this right is so fundamental, we're going to flog this topic to death in a little while. First, though, let's take a quick peek at how switches are operated.

A switch can look closed and still not pass enough current. Dirt or pitting can raise the resistance of the contacts and reduce current flow enough to kill the circuit.