[4] What are stepper units? How do they work?
Stepper units are another huge foundation behind the inner workings of an EM pinball machine, which control things like the bonus, ball in play/player up (if applicable), match/0-9, and much more.
In pre late-50's games, before score reels became a thing, the "lightbox scoring" was also controlled by one very large stepper unit or more.
A stepper unit looks complicated but the idea behind it is actually pretty simple. You have a circle set of contacts (which can either be "receivers" or "powered", depending on what role they play), and one or more conductive metal arms/fingers to again either send or receive voltage to/from a contact.
Each time a unit steps, all of the arms will advance a position onto the next set of contacts. Stepper units come in different designs across each manufacturer and also different types.
There are non-resetting units that continuously rotate, never stepping backwards or "fully" resetting. Then you have the ones that do reset - but there are two types of THESE! Some do a full reset, meaning they pass back over all contacts to a specific starting position in one go (one pulse of the coil), while others reset by going backwards one point at a time, much like how they step forward - they step back the same way - one contact at a time.
This kind of unit is needed for something like the bonus unit, which needs to tally each 1,000 point contact one at a time as it drains down. Did you know: The bonus unit actually doesn't really "know" how much bonus it needs to add? While the unit is resetting (say, at the end of a ball), a circuit livens up that sends solenoid voltage to an arm, so each time the unit and arm steps over each contact (remember, one at a time backwards) while it resets, it adds another 1,000 or whatever amount back to the bonus until it is fully reset.
This is another great example of EM technology, they need to reset the unit for the next ball/player, but also utilized the reset as a secondary usage for tallying up the bonus as well, both at the same time. Rather than each contact being specific, each is rather just an 1,000 point contact. So if you have a 9,000 point bonus racked up, your unit has to reset 10 positions to 0, so 1,000 * 9 = 9,000. Each of those positions share the same exact circuit that leads to the 1,000 point relay, rather than needing a circuit for each specific amount like a 50 point relay. It simply just shoots 1,000 points up for each step backwards until it knows the unit is reset. Not much sentience there.
That being said, if your bonus unit gets stuck on a contact, it will continue to add 1,000 points regardless of what your bonus is until the unit is unjammed. The machine never knows what your bonus is, it just knows "not done resetting bonus" and "done resetting bonus". 1's and 0's...
Another interesting circuit is the 3-ball play circuit on an EM. Back in the day, operators had the choice to set an EM between 3 balls and 5. (and 8 of all numbers, if you count Gottlieb add-a-ball games from a range of around 1968-early 1970, thanks Gottlieb I guess... I doubt that was rarely EVER used)
If your EM is set to 5 balls (the standard, I must add!) the ball in play unit will continue to step with each ball coming up through the trough until the last ball, when it steps itself to the game over position (or just fires a game over relay if applicable and stays on ball 5 position until reset for the next game). However, on 3 ball play there is a problem.
You have the ball 4 and ball 5 position/contacts standing between you and game over. Oh no! Well, never fear. (This may be specific to Gottlieb 70's single-players, I never tried 3 ball on Bally or Williams - pre-70's Gottlieb single-players and Gottlieb multiplayers just used a game over relay) Much like the switch that kicks on when you drain on ball 5 (controlled through the score motor as well, just before the outhole kicker is set to fire), when you drain on ball 3 on a 3 ball set game, it triggers a relay to advance the unit 3 times into the game over position.
Of course since operators didn't want people to know they were missing out on two balls, first the lighting circuit to the unit is disabled (so you don't see the ball 4 and ball 5 lights come on quickly as it passes over them), and then it automatically pulses the ball count unit three quick times, ball 4 for about a second, ball 5 for about a second, and then finally the game over position. (again, if applicable)
I was very fascinated by this feature even though all my EMs in my gameroom are faithfully set to 5 ball play (as I believe it should be for this era - SS is 3 as well), if you listen to a 3 ball set game as the game ends, listening for the loud distinctive "CLICK CLICK CLICK" of the ball in play unit swiftly/quickly passing over these unused contacts, as pulsed by the score motor in usual fashion.
So, is your stepper unit not working, gummy, or STUCK? Well, the first thing I always like to do is get rid of the grime on those contacts, even that can be enough to slow up a unit. Some people get specific and use alcohol to clean contacts, or a Brite-Boy, etc., but if you don't mind scuffing up the unit a little some very fine sandpaper used in the direction of travel (meaning follow how to the stepper arms move) will do the trick.
I don't like this as much because it does scuff up the units and I would like to look into a better alternative but if you are not concerned it gets the job done well and damn well. Most of the time it is only the Gottlieb units (my favorite) that have large gaps between the contacts, so like on Williams and Chicago Coin units you don't have much to scuff up besides metal contacts anyways, so go for it. Bally is in-between on how their contacts are laid out. Sand the contacts (some look more like large flat square pads, like Chicago Coin games) lightly until the contacts or pads are back to a shiny gold or silver color and all "travel" marks (little trails left by the arms) are gone or mostly gone.
Make sure you don't overdo it or sand too hard, it's not easy to do but remember you do not have unlimited metal there and you also don't want to make one contact uneven or it will provide a very intermittent connection.
Next up is checking out the back of the unit, many units have a screw or pin in them that allow them to be folded down for service without having to remove the entire unit. Make sure all of your springs appear to be intact. These things can become stretched out over time and no longer step up the unit, so if this appears to be the case I would recommend ordering new ones from somewhere like Marco Specialities. You can also shorten an old spring to give it a tighter pull more reminiscent to the original, but I would just recommend buying new ones. There is a giant spring in the center of the unit that controls reset functions and give the unit a "push back" if it is a full-resetting unit. This spring is a giant pain in the ass, so try not to mess with it unless you really need to. Note, your springs may be fine but the unit may be gummed up. Before replacing them or messing with them, it is often a good idea to disassemble the back of the unit (and remove the springs anyways) and clean it up with some cleaner like Simple Green. There was oil applied to these units at the factory or by clumsy operators that dries up over time and freezes up or bogs down the unit.
Cleaning it will remove this oil and allow the unit to operate smoother. I never re-oil my units as they are mostly designed to "run dry". Some people choose to lubricate the contact disc itself with the appropriate lubricant (not oil, and HELL NO to WD-40! Flammable and never dries!) but I and others also choose to just let this be and run it dry. You can make your own decision there without ill effects, as long as it is done correctly with the right specific kind of lubricant. (teflon lubricant)
Last but not least, make sure your stepper unit's shaft is not too tight up against the disc (pulling it too close), or it won't spin even with the best of your efforts! There are a few allen wrench screws on this shaft, and while they need to be tight themselves, you need to make sure the rotating disc is not offset hard into the unit, or the physics won't allow it to spin or spin well. (and will also wear down the contacts quicker) You want a solid connection that touches the contacts fully, but also not having the disc grinding into the unit basically.
Another common misconception is an issue where it is not the stepper's problem at all. You can step up (and down/reset) a unit by hand, but you do need to "act" like a coil and do it incredibly swiftly to accurately emulate the results. Quickly in, quickly out, a nice hard emulated "pulse". This is a good way to test if your stepper is acting up or if it's an issue in another part of the game.
A common culprit is a dirty or misadjusted score motor switch not pulsing a stepper for long enough. People always say "Oh, it's NEVER the score motor!" but that is probably the BIGGEST lie I have ever heard in the history of this hobby! While it is probably one of the later things you should check (before you know what you're doing for sure), it is definitely not impossible and 30 EMs later