A is at the bottom, yes. Just look at the score motor diagram on the side of the schematics.

The motor should be on a hinge if you take out two of the screws to at least make it a bit easier to access. You can also just find the other ends of the wires going to the switch and test them

It is now my standard during a service job to remove the bottom board, and remove each switch stack for a closer look, especially on a multi-player. There was no way I could be sure everything was correct on a recent Grand Prix without doing this it was so congested. IMG_1931 (resized).JPG

I also set it up like this during final troubleshooting so I could see everything as it activated.

You can often get at the other end of the wire.

Warning: The above is not for beginners.
What to do instead: Slowly and carefully diagnose one problem at a time and then fix only that.
http://www.pinrepair.com/em/index2.htm#clean

Hi redrock
hihi - "C" means "Switch will close when actuated" - "O" means "Switch will open when actuated - "M-B" means "You look at a threebladed Make-and-Brake-Switch". Greetings Rolf

Hmm. I think there's more going on here. They're definitely trying to convey more than cam number and switch stack position but I'm not sure what. Here are some examples from the Stardust schematic:
Williams score motor switch labels (resized).jpg
On the left side I can see:
- "C 6A": the normally open switch Closes
- "O 3B": the normally closed switch Opens
- "M-B 3C": the make/break switch changes

But what about the right side? I don't see how the annotation matches the behavior:
- "C IND. A": the normally closed switch is closed when the motor is at rest (IND. A) but open when the motor is turning
- "5 IMP. A": the normally open switch closes 5 times (IMP. A) for each score motor cycle
- "C 6B": the make/break switch activates as cam 6 passes by
- "O IND.B": the normally open switch is open when the motor is at rest, and closed when the motor is running

One handy bit of trivia is that the numbers represent the order of switches. So when the score motor turns, the 1A, 1B, 1C, etc. switches activate briefly, then the 2 switches activate, then 3 switches activate and so on.

/Mark

Left side is the state of switch. right side is cam number then switch number. A schematic gives you the cam numbers. Index will be the first cam with indent in 12 o'clock position. Impulse will be the last cam.

A quote from pinrepair.com;

" I visually inspect every switch. I look at all the relay switches, score motor switches, and playfield switches. It does not take that long - I can visually inspect all switches in a game in about 5 or 10 minutes. You would be amazed at the problems that can be *seen* - broken switch blades, obviously mis-adjusted switches, wires that have broken solder joints from switch contacts, etc."

To fulfill this inspection (which I agree with) sometimes boards must be removed. So far I have not found an EM that requires just fixing what is broken, not where it will work reliably without something else going wrong. Many times there are multiple issues occurring. You would also be amazed at what cannot be seen bent over/inside a machine.

I also do not make assumptions as to the mechanical aptitude of the OP, if they are willing to undertake a refurbishment of an EM this speaks volumes. The motor switch stacks do appear imposing to a beginner but are no more complicated than rebuilding a stepper.