6. Serviceable Design
Real pinball machines have numerous design elements that are there
purely for the sake of serviceability - that is, to make the machines
easier to repair, upgrade, and maintain. These features are often the
products of a series of refinements that wouldn't have been obvious
without experience, and a lot them are hidden, internal features that
you wouldn't even realize are there when you're just playing the
It's not surprising that serviceability is such a high priority for
the pinball manufacturers when you consider who their customer is.
The thing to realize is that players are not their customer.
Players are only the "end users". The customer is the
commercial operator who buys the machines for their arcades and
routes. The player mostly wants machines that are fun to play. An
operator certainly cares about that, too, because an arcade game earns
quarters by being fun to play. But what the operator cares even more
about is high reliability and low repair costs. A machine that's down
for repairs isn't earning quarters no matter how much fun it is.
Some of things that they do in real pinball machines to make them
serviceable translate readily into the world of virtual cabs, where we
can copy them to get the same benefits. But new virtual cab builders
are often unaware of how the real machines work, so they don't know
there's a good design they can copy - instead they make things up as
they go. And of course the first time you build anything,
you're likely to come up with quick-and-dirty ways to solve immediate
problems without considering the longer-term costs. So I want to
point out a few of the serviceability features in the real machines
that we can leverage for virtual use, so that you're aware of them,
and so you can keep them in mind as you plan your build.
What makes a machine serviceable
Before we get to implementation details, let's look at what
serviceability really means in the abstract. These are what I
consider the key goals of a serviceable design:
- Accessibility. You should be able to access everything in the
machine easily, without having to destroy anything, without having to
de-construct anything, and ideally without any tools.
- Modularity. You should be able to remove and replace most
individual parts or subsystems without having to destroy anything, and
with as little work as possible. Electrical connections should be
pluggable, for example, rather than being hard-wired or soldered;
parts should be secured with removable/reusable fasteners like screws,
rather than permanently glued or nailed.
With the abstract goals above in mind, here are some concrete
recommendations for how to achieve them, based in part on how the real
machines accomplish the same ends. Things are different in a virtual
cab, naturally, but some of the ideas from the real machines carry
over surprisingly well.
The real machines are set up so that the playfield can be tilted up
from the front and lifted all the way up to lean against the backbox.
This lets you access the entire interior of the main cabinet and the
entire underside of the playfield without taking anything
apart. It's like popping the hood of a car to get to the engine.
It lets you get in and out quickly. Minor jobs remain minor because
it only takes a few seconds to open the machine up.
In my opinion, this is the ideal way to arrange a virtual cab as well.
Accessing the interior of the main cabinet is at least as important in
a virtual cab as a real machine, as that's where we typically install
the PC motherboard and most of the feedback devices. In the virtual
cab, it's the main TV (in place of the playfield) that needs to tilt
up and out of the way like a car hood.
This is why I always advise against any design that involves the main
TV being difficult or impossible to remove, such as installing the TV
in routed grooves along the side walls.
On the real machines, the playfield not only tilts up and out of the
way, but can also be removed entirely. They even make this fairly
easy: the playfield isn't actually permanently attached, but is only
resting on the pivots that let it tilt up, so remove it is just a
matter of lifting it off the pivots.
In a virtual machine, it's equally important to make the TVs
removable. They are of necessity at the very front of every part
of the machine, so you need to get past them to get to anything
else. If it's hard to remove any of the TVs, it's hard to service
what's behind it.
For the playfield TV, the ideal as far as I'm concerned is to
installed it analogously to a real playfield, with the same ability to
tilt it up for smaller jobs and remove it entirely for larger jobs.
The backbox TV(s) should likewise be removable, and like the playfield
TV, this should be possible without a lot of disassembly and certainly
non-destructively (meaning you shouldn't have to rip off the sides or
cut any new holes anywhere).
If you're using a separate DMD TV, that should of course be removable
as well. This one is usually easy to make modular, at least, since
it's so small.
On the real machines, the backbox is attached to the main body with a
hinge that lets it fold forward, so that it lies flat on top of the
main cabinet. This is a key feature to make it practical to transport
the machine, since it's too tall, top-heavy, and fragile to move it
with the backbox upright.
Some pin cab builders figure that they can just remove the backbox if
they ever need to move the machine. Consider the amount of wiring
that you'd have to disconnect to do that, and the risk of breaking
something when redoing the wiring. If a folding backbox is impossible
because of the geometry of your backbox TV, then at the very least, be
sure that the wiring connectors are all modular, so that it's quick
and reliable to reconnect the wiring.
Modular wiring connectors
Real pinball machines have lots of wiring internally, with many
interconnections. (Stern Pinball has estimated that there's about
half a mile of wire in a typical Stern machine from the 2000s. It
might even be a bit higher in the 1990s machines, since they used
lower-tech electronics to implement a similar feature complexity.)
The real machines deal with their many connection points mostly by
using plug-and-socket connectors.
In the 1990s machines, they made heavy use of a few different
connector types made by Molex. You see the term "Molex connector"
used almost generically as though it referred to some specific
physical plug type, but it's actually a particularly unhelpful term
when you're searching for parts. Molex the company makes a
huge array of diverse connector types, so "Molex connector" can refer
to all sorts of things. If you want to be helpful when describing a
particular connector to someone, you need to give them a Molex part
number or at least the Molex product line name.
Pluggable connectors of the sort used in the real machines have two
- You can easily connect and disconnect them at any time
- You can't get a connection wrong when re-connecting something,
because the plug only fits in the one place where it's supposed to go
Most virtual pin cab builders intuitively recognize the important of
modular wiring - of using some kind of removable connectors rather
than soldering everything together permanently. But many new pin cab
builders gravitate towards screw-terminal blocks, and in my opinion,
those don't quite achieve the full goal here. Terminal blocks do
avoid the need for permanently soldered connections, so they address
the first point above right, but they miss the mark on the second
point. Consider what happens if you have a couple of terminal blocks
for wiring a particular part, and you disconnect the wiring: where do
the wires go when it's time to reconnect them?
Proper pluggable connectors of the sort we're talking about are
definitely a bit more work to set up than screw terminals. But
they're great once they're in place, because you can arrange things so
it's practically impossible to plug things in the wrong way. These
connectors can also be time-consuming to select during the design
phase, because there are so many options available. I'm convinced you
could build a nice engineering career on a solid knowledge of the Molex
catalog and of which connector to use when. I've tried to make your
job here a little easier via pointers to some good basic options
There are three clever techniques used in the real machines to make
the connections "idiot-proof", which are worth cribbing when you're
wiring your virtual cab:
- Whenever possible, use a unique connector type. It's impossible to
plug a nine-prong Molex .062" plug into an eight-prong socket. If
there's only one nine-prong Molex .062" plug and one nine-prong socket
in the whole build, that's one connection that you can't plug back
in to the wrong place.
- Wherever you have to use the same connector type more than once, use
a "keyed" connector. That means that you snip off one pin on the plug
side, and block the corresponding socket on the receptacle side. If
you try to plug the wrong 9-pin plug into the wrong 9-pin receptacle,
that wrong plug won't have the right pin clipped, so it won't fit into
the blocked socket.
- Use plug-and-socket connectors that only plug in one way, so that
you can't accidentally plug something in backwards. You get this
benefit automatically for connector types that have asymmetrical
shapes. All of the Molex .062 and .093 connectors are inherently
designed this way, for example.