Quoted from Pecos:
This is what I do. There are a lot of ways to restore a pinball machine. My way may or may not be best for you.
Remove all mechs from the mech board. Be extremely careful to pull the mechs up and over the labels! Put the screws, nuts and other hardware somewhere safe where it won't get lost. (Do as I say and not as I do!) I need to get one of those magnetized metal bowls! You will need to remove every relay from the mech board anyway and this method is not much more work.
It might be easiest for you to remove the labels on the board. I find it is too easy to damage the labels.
For non-painted boards, sand down the board with large grit sandpaper. Finish with a fine grit sandpaper. Sand CAREFULLY around the labels. You can clear-coat the mech board, but I found that it darkens the wood too much and I prefer the original look. For painted boards, clean with a good cleaner. Repaint if necessary. Do NOT get the mech board wet! The wood will swell and the labels will get soaked. You can, carefully clean the labels with Isopropyl alcohol and cotton swabs, but be aware that this might remove some of the ink on the labels.
Set the mech board aside in a safe place.
Screws and Washers
If you have a tumbler, you can use it to polish up the screws.
I don't have a tumbler, so I put Mother's Mag and Aluminum Polish on an old T-shirt in one spot and some Mother's Carnauba wax in another. I manually rub the head of the screw or washer in the Mother's Mag and Aluminum Polish until it is shiny. I do the same with the wax. I then do the same on a clean part of the cloth to finish.
As you take apart the assemblies, the score unit for example, clean the screws as you go and keep them together! They are easy to lose!
Shiny screws leave a professional look to your work.
Take a lot of pictures of stepper unit from every angle!
Take all stepper units apart, clean all loose parts in the sink with Bon Ami and Gain dishwashing soap, using a toothbrush.
Sand any surface rust down with 500 grit sandpaper and blue glass cleaner.
Optional: Polish copper gears and parts with Mother's Mag and Aluminum Polish. Just buy it. Great stuff!
Remove any remaining dried grease and oil with Isopropyl alcohol, especially on the gear teeth.
Remove the cotton from a cotton swab and using Isopropyl, clean out the shaft holes in all nylon pieces.
Using a cotton pad and Iso, thoroughly clean the shafts on the stepper unit base. Continue until cotton pad is clean.
You can polish up the stepper metal base with Mother's Mag and Aluminum Polish, but I usually just clean it off with iso.
I used to put some SuperLube on the parts with metal to metal contact, but not so much anymore. Put some on the teeth of the gear though - only the teeth that interact with other stepper unit parts.
Put the stepper unit back together again!
Check for loose or missing wires. I had an Aztec that had a lot of bare wires on a stepper unit. This can wreak havoc when the game is turned on. Remove any bare wires, use shrink tubing to cover the bare spots and solder back on. Do this one wire at a time!
Clean all contacts and switches with cotton swabs and Iso.
Using a Dremel with a #443 wire brush, polish the contacts until they are bright and shiny. I used to use burnishing tools and 500 grit sandpaper but this method is far superior.
Clean all contacts, again, with cotton swabs and Iso
Look for missing springs, loose wires, missing wires, anything that is 'wrong.' Lightly pull and move each wire. Sometimes the wires appear to be soldered on properly but may actually be loose. This is rare but cause intermittent problems that are hard to track down. The wires tend to get bent down and flattened over time. I like to restore them to 'loops' when testing the wires - just that extra little touch that looks nice when done.
Optional. Clean the metal relay base and the edges of the spacers with a cotton swab and Iso. It's amazing how much dirt and grease will come off!
Tighten all switch screws with the proper-sized screwdriver.
Using a switch adjustment tool or small regular screwdriver, adjust all switches so the switches are open when they should be and closed when they should be. The gap should be about 1/16 of an inch. Manually move the armature to mimic when the relay is energized. Normally open switches should now be closed and normally closed switches should now be open. The key word here is deflection. Does the switch move just a tiny bit when the switches are opened and closed? If not, you may not be getting good contact and, over time, the switch will come out of adjustment more quickly.
Check each switch by manually moving the relay armature to confirm the proper gaps, closed switches that open and open switches that close. Then, go back and double-check your work. Later, when the mech board is put back into the cabinet, repeat the process for each relay and switch. Switches can come out of alignment when bumped during work on other parts on the mech board.
Clean all male plugs with iso and Dremel #443 brush
I move the male Jones plugs up and down in the female Jones Plug to, at least, try to remove some of the dirt and to make better contact. Sorry about how that sounds! I still haven't found a good way to clean the female contacts. Anyone?
Look for loose or missing wires on the Jones Plug! These problems are very hard to track down, so make sure now that all is good.
You can clean up the rest of the Jones Plug with Iso and cotton swabs if you like. I always clean the area where the screws are since these areas collect dust.
Cotton swabs - I like the paper swabs. The cotton comes off too easily on the plastic swabs. I like Q-Tips. They cost more but hold up better in use.
Cotton pads - I was getting the expensive pads at Walmart and found that the cheap 100 for a buck pads at the local 99¢ store were good enough.
This is a thorough restoration and may not be appropriate of you are doing your first electro-mechanical restoration. Only you can decide that.
More to come.