Qacember 1986
STAR*TEC~ J9urnal
Various and Sundry Problems
Concerning Electronics
By Sam Cross
Godwin Distributing Co.
North Little Rock, Arkansas
SI'AR"'TOCH JOURNAL has asked Sam Cross to
reply to an assortnent of service problems
most often encountered in everyday
maintenance of coin-operated electronic
entertainnent equipnent. what follows is
the first in an ongoing series of
discussions identifying the nost cornmon
trouble areas as seen by the service
corrmunity of our Industry.
\U1l7lGE MEASOREJmUS
It all depends on how you look at it,
whether you think the power supply is
sirrply one part of the game, or an
integral section concerned with providing
a well-regulated voltage and current to
the logic board.
If you think of it in the former manner,
you p:>ssibly set the voltage of a
switching p:>wer supply at the supply's
output terminals; if you are of the latter
frame of thought, however, you neasure the
voltage of the supply on the board the
supply is p:>wering -knowing that the board
is the only place to neasure correct power
supply voltage.
even better place to measure and set
the p:>wer supply voltage is on the chip
which is the furthest away from the logic
board connector -on the logic board.
An
?-t>st all of the switching supplies I have
encountered have a voltage adjust on the
supply accessible from the outside by a
p:>eket screwdriver, (some of the supplies
have p:>tentioneters which are flimsily
nounted -so don't use much pressure down
on the p:>t, only enough to get a grip in
the grooves and turn it).
'lllese supplies should only be adjusted
with the power on. This requires some
agility; holding the voltneter probes in
one hand, while a screwdriver is in the
other hand -with the added task of looking
at the meter, but it is the correct way to
adjust the supply.
(12)
ICFOiml.Em
.A
Don't know where the p:>wer supply
•
terminals on a chip are located?
.
Here is a quick lesson. The ground side of
a chip on alnost all 'ITL (74xx type
chips), and irost all Eproms (the meirory
chips), is the last leg on the first side.
The p:>wer (+SVOC) input is on the last leg
of the last side.
Let's look at this through some simple
drawings. Chips don't install in_either
direction, so how do you know which leg is
pin one?
Well, the manufacturers in their
competitive nature have made things hard
on us. One thing is certain though, don't
go by the identification writing on the
chip!
Most all manufacturers use the notch to
indicate the "top" of the chip, some use
the dot at pin one, and still fewer use
the dot on the other side (Fairchild).
FIRSTtEltB
F1RsrS1DI
1
~OTCH
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"
't\~T"
Just remember that you count legs on a
chip in the sane way you would write the
letter "U"; in the counterclockwise
direction.
last leg First side= ground.
Last leg Last side = power.
CAUTION! Some 'ITL ships, few in fact, have
their p:>wer supply legs on pins other than
the standard above (such exarrples are:
7441, 7473, 7476, 7477, 7478, 7481, 7482,
7483, 7493), so an Eprom chip is your best
bet as a measuring point.
If you don't have an Eprom chip near where
you want to measure, use the legs of a
noise filter capacitor on the board. There
are usually a lot of these on the board,
commonly found near the edges of the board
-straddling the ground and power busses. -
NEXT ISSUE: Installing Line Filters and
MOVs when performing garne conversions.
Also tie-downs for logic systems utilizing
"piggy-back" mini-boards.