What the Zero switch does

[OldReno]

Understanding the Zero switch.
The zero switch resets the hopper.  It's kind of like the Terminator, that's what it does -- that's all that it does.
If the zero switch never closes, then you will have shortpay and eventually no-pay problems because the payboard will not reset.
Of course, if your winner paid lite goes off after you pull the handle, then you know hopper has reset, and the zero switch is working just fine.

We really don't need the zero switch to ever be open.  The machine will run perfectly fine if it is permanently closed all of the time.
There are generally only 2 switches in the whole hopper reset circuit, the hopper zero switch, and the reel mechanism 'B' switch.
The B switch is closed when handle is pulled by the action of the variator which lives just above the clock and fan.
Every time the handle is pulled, and the variator moved, the B switch closes.
To check this, set up a pay.  After it pays, push back on the variator bar, and you should hear the hopper reset, and it will pay again.  Try it again.  And again.
This is how we checked for short pays on the machines, without having to coin it, pull the handle and hold the reels each time we wanted to check a specific pay.
You can do it over and over and over and over.

The only reason the zero switch is in circuit, is to keep the hopper reset coil from pulsing every time handle is pulled.
However, by adding the zero switch, Bally was able to keep that from happening, and now the hopper resets ONLY when needed, which is only after a pay, and after the zero switches close.
The only 'bad' thing that will happen if your zero switch never opens is that you may shorten the life of your hopper reset coil (maybe after a couple of years...), and you may eventually get annoyed at hearing the hopper reset whenever you pull the handle. 
If your zero stop bumper gets gummy and worn, you can always bend the zero switch closed so that you can continue to play your machine until you get a replacement bumper in the mail from one of your slot sources.
You really don't need it to play the machine, or to get it to pay....
Just FYI.