Stepper units in pinball machines are counters which maintain state like ball in play, bonus, or player up counters. Some steppers count up one and down one like a bonus stepper. Some count up one and and reset to zero like ball in play or player up steppers. Others simply count up in a loop like a match stepper which loops over and over.
TAKE PHOTOS OF THE UNIT at each step in case you forget where something goes.
Here is a Williams player up stepper unit. It's located in the back box and selects which player (1-4 in this case ) is currently up. The unit is responsible for routing scoring signals to the correct set of score reels. The problem with this reel was that it wouldn't select the correct player. The springs were over tightened as shown in the photo. You can verify operation with the power off by manually pushing the solenoid plungers in and watching the stepping action. It should count up smoothly and reset properly and fully to the home position. One solenoid counts up, the other one resets the stepper.
Note the spring adjustment:
After quickly putting the spring back the unit now really bogged down and was difficult to move. This indicates things are not as they should be.
There's 2 screws on the back box which will release the unit so you can position it to use a screwdriver on. There's not much slack in the wiring harness.
Remove the center screw on the circuit side will allow you to remove the contact fingers. Hold the white plastic gear on the opposite side of the assembly to prevent it from popping out. Note the alignment of this gear and the spring tension.
To remove the levers for cleaning you have to remove the coils. They're attached (sometimes) with lock tite (or similar) on the screws. You can put a vise-grip on the opposite side of the screw to loosen it. Remove the 2 screws and the coils. There will be a round spring on one end of the coil - don't lose this. Use a magnetic screwdriver to prevent the screws from getting lost. If you drop one DON'T JUST LEAVE IT - It may fall into the circuitry and fry your pinball.
Here's a spot that gets gummed up. We'll remove this lever for cleaning. Remove the two springs attached to this lever completely and note the position of each. Remove the clip holding the lever in place and remove the lever leaving the white gear and everything else in place.
While I don't use WD-40 for lubrication, it's excellent for breaking up old grease from these parts. Shoot a little into the joint and work the lever until the joint moves freely. Thoroughly clean off all the WD-40 you've sprayed on the lever.
Spray a rag with WD-40 and wipe down the armature rod which the lever was on. Remove all WD-40 residue when done. Repair the process for the other lever.
Remove the nylon gear. This will just pull up through the stepper chassis. The spring will loosen unless you secure it. Note the yellow gunk on the armature. Again, clean this with WD-40 and remove all residue. Also clean the inside of the armature guide in the stepper housing. Re-assemble everything and verify alignments and spring tension. While you're hear it's a good idea to clean the EOS contact switch blades with a file or fleckstone.
The stepper now moves smoothly without the contact fingers attached. Good. Next, re-solder the wire connections to the pcb. These are prone to loosen through expansion/contraction over the years and could create a hard to find intermittent problem. Resoldering all the connections will insure proper performance. While you're at it clean the disk. Again, wipe down with WD-40 and clean of all residue. Note the burn marks. These are cause by the mis-alignment of the contact fingers. These may require sanding to smooth out the pcb otherwise the burrs will cause the fingers to wear and and cause them to misalign themselves.
Replace the contact fingers. At this point the stepper again was sluggish. Note the bent fingers. At some point someone attempted to adjust the fingers. These need to lightly contact the pcb but not so lightly as to not make good electrical contact. In this case they had been over-bent in the direction towards the pcb which created a tight spring which caused the unit to bind. This is why the spring on the levers had been over-tightened. The fingers should also be straight and contact the pcb flush. It's nearly impossible, once bent, to get these back to factory but you can get them 'close enough for rock&roll' otherwise you'd need to locate a perfect set of contact fingers which is not likely.
The alignment is performed by loosening the 2 screws that attach the pcb to the stepper chassis and rotating the pcb slightly to have the fingers sit center on the pcb contact pads. Loosen the screws, position the stepper somewhere between the reset position and max position, and adjust the pcb. In this case I reset the stepper with the reset coil plunger and stepped it 2 times. You may have to step the unit to access the screws. When done, reset and step the unit to max a few time to verify it operations smoothly. Remount to the backbox and play away.
Additionally, some people wipe the pcb with a LIGHT lubricant such as a 3-in-one oil or white lithium grease. Put a drop on your finger and wipe the pcb. Whether you use none or some it's inevitable the unit will fail at some point in the future. It's a mechanical device with lots of opportunity for wear. The field notes on pinballs recommend clean and lube every thirty days although everyone just waits for something to break and then fixes it. The lube will help delay the inevitable wearing of the fingers and pcb contacts. Remember, a problem existed that caused someone to try to fix it. It's very rare someone opens a working machine and breaks it. Lube was widely used by professional field mechanics for years. You may have to de-gunk the machine in the future but that's better than having to replace a contact blade or a stepper finger assembly.
Hope this helps. It should take less than an hour to do this and should last for years of home play.