Hold-in circuits
By Zac Oliver
Schematics are the simplest way to
understand the operation of a
machine . However, describing three -
dimensional circuits and mechanisms
on a two-dimensional piece of paper,
requires from the reader a constant
visualization of the machinery in
order to understand whats happen -
ing .
For example, most pinball manu-
facturers feed the control circuitry
through a switch on the Reset relay .
When the machine starts the reset
cycle , power to playfield targets and
associated circuits is cut off, to
prevent a situation where a second
ball is in play while the game is being
reset . With power applied to the
playfield targets and the ball bounc-
ing around , points would be scored
at the same time the score units are
being reset.
Therefore, it is necessary to cut the
power to the playfield targets during
game reset.
Most manufacturers use a normal-
ly closed switch which often fails . The
resulting situation is where one can
put credits , start the game but the
playfield or part of it won 't work .
The knowledge of power distribu-
tion is the first step in understanding
schematics. The language and sym-
bology is another. Few differences in
symbology and language occur
between manufacturers . However ,
similar mechanism symbols will
differ .
There 's also a logic process in the
way the circuitry and mechanisms
work , that is a common point among
designs .
The holding circuit is a good
example of the similarity mentioned .
Given any relay controlling a timed
function , when switch A closes (see
Figure One) the coil energizes pulling
in the armature and closing switch B
which is located on the relay itself.
Switch B is in series with a switch
actuated by a motor cam .
During the total revolution of the
cam , switch C will open , interrupting
the current path for the coil. B now
opens, preventing the coil from being
energized when C closes again . This
simple circuit might take quite a
rlifferent shape if you add other
72
switches , however the basic principle
remains .
Figure Two shows another typical
holding circuit where the holding
time is determined by the movement
of a mechanism situated by a coil.
When the control relay is energiz-
ed , it will activate the bumper coil.
The mechanism will move and
contact the end of stroke switch
(E.O .S .) which will open , releasing
the control relay , which in turn
releases the bumper coil. This
interaction assures proper electro-
mechanical operation . On a bumper
circuit , the longer the control relay
RE.L. /\'/
stays energized, the stronger the
stroke.
As one can see , it 's easier to
understand these circuits when one
can combine the action of three-
dimensional objects moving inani -
mate symbols on a piece of paper .
Next time you're around schematics ,
look around the circuits , where the
manufacturers supply information.
If you have the chance , follow the
main lines that supply power to the
control circuitry, the w~v they are
laid out in the machine . Get used to
where these wires go . It will save you
time on troubleshooting , especially
on shorted circuits .
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PLAY METER , January, 1978