This page is about cleaning the score wheels on old Electro-Mechanical (EM) games. You will find the steps I take when cleaning the score wheels listed below. I've also added a couple of pictures to help illustrate the point.
(click any picture to enlarge)
Unlock the back box cover (could be the same key as the one that opens the coin door, but if not, open the coin door and see if there's another key hanging from a hook on the coin door or mounted to the left side of the cabinet) and then remove the metal cover on the back box.
Find the plastic or metal "tang" that holds the score wheel in place and either push it down, lift it up, or slide it sideways to release the score wheel from its rack (depending on what type of game you have). The picture below shows me touching the "tang" that holds the score wheel in place.
After releasing the wheel, slide it out and wipe the surface of the wheel gently with a soft cloth (cut up T-shirt). If the game is old you may actually wipe off some of the painted numbers if you wipe too hard or use something like Windex. I use the soft cloth method and start with my cloth wet with only a little dish soap and luke warm water. I lightly wipe the wheel across the numbers with the wet cloth. I clean the outer surface first and then I clean the inside surface of the wheel.
I check out the contacts and the solenoid (coil) associated with the wheel and ensure that the contacts are cleaned with a business card or filed (if necessary) and are clean. The picture below shows a wheel with the coil that causes it to move and some of the contacts that are associated with it.
I also check the coils to see if they are pulling the plunger in smoothly and the scorewheel rotates freely. If not, you'll have to remove the scorewheel (usually held on with some kind of a clip and a washer) and clean the old grease off before reassembly. I use teflon lube to lightly grease the areas where friction occurs on the bakelite board where the rivets are located after cleaning the mechanism. Note that some scorewheels may have a couple of screws holding on a bakelite board (circuit board) that contains rivets and copper pathways that the "fingers" rotate across as the scorewheel rotates. I plan to create another page showing how to do this, so bear with me until I get this done.
Finally, I put the score wheel back into its rack and ensure that the wheel is locked in place. The type of wheel I show above uses a metal "tang" or wire looking gizmo that must be pushed to one side as it's slid into place. There are other types however and I'll attempt to put their pictures on this page when I get them.
Hope this helps. It's a rather simplistic explanation of what goes on, but assuming it's only a matter of cleaning the surface of the scorewheel, the primary thing is not to use stuff like Windex to clean the scorewheel. Just some light dish soap mixed with some lukewarm water and a soft cloth to gently wipe off the scorewheel while avoiding removing the painted numbers.