Re: Found an 'interesting' EM hack repair yesterday

On Nov 18, 2:41 pm, TimMe <TimMeig...@xxxxxxx> wrote:
If you've worked on a lot of Gottlieb EMs, you have probably seen a
lot of stepper add-unit coil stops either broken off or badly
deformed, and in these cases you will also usually find that the end
of the plunger has also mushroomed badly. This is not supposed to
happen. On all Gottlieb steppers, there is an easily overlooked
adjustment related to the add coil stop and plunger. Unfortunately,
this adjustment is almost never done correctly, even though it is
especially important to get it right on steppers that cycle a lot,
like the bonus scanners on these (and other) games and the player unit
of multi-player games. This is the drive arm anti-lash adjustment,
and it helps prevent the early destruction of the add coil stop and
the mushrooming of the plunger. Most people don't even know about
this adjustment or know how to do it correctly, partly because it is a
subtle adjustment and partly because if you don't get it right, the
stepper still seems to work OK. I will explain the anti-lash
adjustment here.

First off, just what is the lashing problem? When the add coil fires
and the plunger snaps into the coil and then instantly stops against
the coil stop, the drive arm has so much inertia that it wants to keep
going, just like what happens to you when you slam on the brakes in a
fast-moving car. This causes the round nylon end of the drive arm
riding in the plunger slot to kick up and hammer against the inside
wall of the plunger slot, sending an extra mechanical shock into the
plunger that must be absorbed at the coil stop. If you know how a
bazooka works to penetrate armor, you know that this particular
hammering effect imparts a surprising amount of force. Over many
cycles, this extra hammering will accelerate the destruction of the
stop and the mushrooming of the back end of the plunger.

Gottlieb solved this problem by adding an anti-lash stop bracket to
the stepper. This is a small angle bracket secured with two small
hold-down screws located between the drive coil and the toothed
ratchet wheel. It has slotted holes so that its position can be
adjusted and then locked in place by tightening the hold-down screws.
The drive arm is stopped by this bracket after the add plunger has
made its full travel into the coil. Note that this bracket is NOT a
drive-arm limit stop! The plunger MUST be stopped by the coil stop
inside the drive coil. This bracket is ONLY to be used to prevent the
drive arm from lashing against the stopped plunger. When improperly
or accidently adjusted as a drive-arm stop, the bracket will break off
after a short time and then (of course) the hammering effect will
occur. Sadly, this bracket is often broken off due to prior improper
adjustment; however if you are working on a stepper that cycles a lot,
it is important to either find or fabricate a replacement for this
bracket and adjust it properly, otherwise the life of the coil stop
and plunger will be significantly reduced.

OK, here is how to adjust the anti-lash stop bracket properly. This
assumes you have already replaced the add coil stop and sleeve,
dressed the end of the plunger, and made proper adjustments so that
the drive arm at-rest position is correct. First, loosen the anti-
lash stop bracket screws and slide the bracket back so that it will
not contact the drive arm when stepping up. Next, loosen the four
screws holding the add coil bracket. While holding the add-coil
bracket, place one finger on the end of the plunger and push it all
the way into the coil so that it rests firmly against the coil stop.
Move the entire add coil bracket and plunger up and down until the
pawl on the drive arm is about 1/3 of the way up the slope of the
tooth on the ratchet wheel. Tighten the four add coil bracket
screws. After the add coil bracket is secured, check your adjustment
by operating the stepper by hand a few times. Then, push the plunger
into the coil until it rests firmly against the coil stop. While
holding the plunger in, confirm that the drive pawl is about 1/3 of
the way up the slope of the tooth on the ratchet wheel. Still holding
the plunger in, try moving the drive arm. You will be able to wiggle
the arm another 1/16th of an inch or so up and down. This is the
lash. Slide the anti-lash stop bracket down so that there is about a
1/32nd inch gap between the stop on the bracket and the edge of the
drive arm. This is a very subtle adjustment and it can take a bit of
practice to get it right, but it's worth it. When you think it's
right, tighten the anti-lash bracket screws.

It's really easy to check the anti-lash adjustment to see if you got
it right. First, hold the plunger firmly against the coil stop. You
should be able to wiggle the drive arm about 1/32 of an inch, between
the plunger slot and the anti-lash stop. If you can't wiggle the
drive arm, the anti-lash stop is down too far and it will break off if
you run the stepper this way. If you CAN wiggle the drive arm, so far
so good. Next, hold the drive arm tightly against the anti-lash stop
bracket and try to wiggle the PLUNGER up and down. It should move
about 1/32nd of an inch. If you can't move the plunger, then the anti-
lash bracket is too far back and you will get premature destruction of
the coil stop and plunger if you run the stepper this way. Re-adjust
the anti-lash bracket position until you can pass both of these
"wiggle" checks.

Yes, it's a bit tedious learning how to make this subtle but critical
stepper adjustment, but it is well worth it. And, knowing how to do
this will help you become a world-class Gottlieb EM tech.

- TimMe (CARGPB3)
Rest in Peace Russ Jensen.

I would add to that this one point:
it's FAR easier to use a feeler gauge to adjust
the lash than to do it by eye. I carry around a
..030" feeler gauge in my tool box, and just
use that. it's MUCH easier to make this
adjustment by sandwiching the feeler
gauge and then tightening the screws.
.

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