There's a few things uh...misnamed or misunderstood.
I will try my best to help you.
"I have an S2000 for a 3 reel game. "
An S2000 was a line of slot machines made by a slot company called IGT.
You've placed this Topic in the Bally section of NLG.
Hopefully a moderator will move it to the S2000 area.
"there are times when all 3 reels coils lock on."
These are called stepper reels.
The reels are supposed to "hold" in position after every completed game or spin.
They will turn when another game is played.
"I'm not sure if I need to adjust the reels"
Stepper reel motors cannot be "adjusted".
If the symbols are "off", the reel strips can be removed to verify if the notch is seated into the nibs on the reel baskets correctly.
"the I/O board "
I'm not quite sure what YOU mean by an I/O board but I can tell you that normally I/O cards in an S2000 are located in possibly three places.
Sometimes, there is one mounted on against the MPU enclosure.
It is used I believe with an additional harness when there's a backlight reel driver board
There would be an I/O card mounted on the back door panel, and that would be for communication for main row of buttons on the button deck.
When a machine is equipped with two rows of buttons, there would be an additional I/O card supporting communication for those extra deck buttons.
I hope this helps you?