STAR*TECH JOURNAL/FEBRUARY 1984
6
VALUE-PACKED
COfflPOnEnTS.
In just a few years NTE replacement
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And to back up that claim of quality,
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QUALITY REPLACEMENT SEMICONDUCTORS
FDR THE ELECTRONIC GAMES INDUSTRY,
INCLUDING THE FOLLOWING
MANUFACTURERS:
• ATARI
• GREMLIN
• BALLY
• MIDWAY
• CINEMATRONICS • STERN
• EXIDY
• WILLIAMS
• GOTTLIEB
Available from your local NTE distributor
NEW-TONE ELECTRONICS, INC.
DEPT. STJ
44 Farrand St . • Bloomfield , NJ 07003
(Formerly TCG)
* ELECTRONIC
TEST PROCEDURES/DIODES
* CINEMATRONICS'
DRAGON'S LAIR DISC PLAYER COMPATIBILITY
By Mark "Bear" Attebery, Vending International Corp., San Juan Capistrano, CA
Recently, I decided that ( after a lot of requests) I
should do a short series on electronic test procedures.
In this future tech article, let's look at the
testing of semiconductors.
The most basic of all semiconductor devices is
the small signal or switching diode. In order to test
a diode, your most useful tool is an ohmmeter.
The best range to operate your ohmmeter at is
the RXlO scale. This will help you not to exceed
the current rating of extremely small signal diodes.
But, in some cases you'll need to use the RX!
range to produce enough forward biasing to cau~e
diode operation (or conduction) on larger "small
signal" diodes.
In many instances, an inexpensive ohmmeter's
supply polarity will be backwards! So don't depend
on the "red is positive/black is negative" approach
unless your meter has been tested ( using another
VOM set to VDC scale) forleadpolarity. The way
you will test a diode is as follows:
Connect the red lead to the anode and the black
lead to the cathode. If your ohmmeter reads
infinite resistance ( doesn't move) then switch the
leads so that the black lead is on the anode and red
is on the cathode. If the meter swings toward 0
ohms, but not all the way, then the diode is good. If
the meter doesn't move ( reads infinite resistance)
then the diode is burned open.
( If your ohmmeter works as above, switch your
leads because your ohmmeter' s power polarity is
reversed.)
If, when you connect your red to anode and
black to cathode, your meter shows conduction
(swings toward the other side [low ohms] of the
scale), then when you switch leads (black to
anode/ red to cathode) you'll look for one of the
following:
I) Infinite resistance (no movement) which
means the diode is good.
(2) Conduction (the meter moves across the
scale) which means the diode is shorted.
OK, now I know many of you have two
questions!
I) Which is anode and which is cathode?
2) Why aren't all ohmmeter power supply
polarities the same?
I'll answer the easiest question first!
The anode and cathode of a schematic symbol
of a diode are:
ANODE
--
HOFFMAN CJ HOFF MAM
CARMEL ·BY -THE-SEA CALIFORNIA 93921 -0896
CATHODE
and the easiest way to remember that is (If you're
easily offended, don't read the next 5 words) A
(anode) stands for Ass end, or the rear, backside,
or behind of the diode. The diodes that most of you
will be looking at will be a glass or plastic case and
they both look like this . ..
ANODE i--E
- - - - · . I CATHODE
This looks like a resistor with one band at one end.
The end with the band closest to it is the
cathode. Now, diodes come in many different
packages but most ( almost all) of the diodes used
in arcade/video games are glass or plastic encased.
OK, now to try to answer question #2 ... _
A possible answer to the "why" of the polanty
question is simply that the designers of the volt/
ohmmeters intend for the resistance scale to be
used simply for measuring resistance and don't
even take the polarity of the power supply into
account!
But a more probable reason was caused by Mr.
Thomas Edison more than a few years ago.
Something that many technical persons don't
realize is that there are two forms of electric
current flow theories! One being the theory of
electron current flow and the other the theory of
conventional current flow.
When Mr. Edison "discovered" electricity, he
had to move in one direction or another before he
could explain electricity well enough to other
people to get fundings for further experiments. So,
what he did was to "father" the theory of conven-
tional current flow.
Now what Mr. Edison said to himself was that
current flowed from positive to negative. He had a
50/ 50 chance of getting it right and he didn't quite
make the right guess. But old ideas die hard ... so
you'll find many books ( especially electrical
engineering) are written in conventional current
flow. But!!! As man progressed, we decided that
since an electron is a negatively charged particle
that moves toward a positively charged particle or
plate, electron current flow was the way to go!
Now this tends to mess things up a bit when a
person who is used to using conventional current
flow tries talking to a person using electron current
flow. They are on different wavelengths.
Due to the fact that there is no standardization
of the theory of current flow, there are sometimes
polarity "problems" or differences that pop up. I
hope that answers question #2.
Now I'm not going to do an "article" for
technicians this month other than to give them
some new information released by Cinematronics
on Dragon's Lair.
Since the new disc players (the LD-VIOOOs)
are being used, (very successfully I might add)
there have been more than a few instances of
operators and distributors either unintentionally or
by a simple (easy to make) mistake, to order the
wrong part when ordering a replacement PCA
( circuit panel assembly), EPROM revision: or
disc player. The cabinet, wiring harness, momtor,
front panel buttons, and joysticks are universal to
all Dragon's Lair games, but the EPROMs, disc
players and PCA board are not EPROMs designed
for use with the PR7820 player are designated as
Continued on next page.