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GCUBIC
.
.
COMMUNICATIONS
SIW',AItI
DIVIBION
flo~ing,
so no collector current,
so no drop across the emitter resistor).
And so on.
It isn't hard to figure out just what
is wrong 'with the device,
although this isn't really
necessary--if tne voltages are no{ close to what
you calculated, the device is probably bad.
INTEGRATED CIRCUIT TROUBLESHOOTING
Very little can be written concerning the finding of internal faults in
integrated circuits.
Often,
the exact
internal circuitry is both complex
and unknown to the service technician, for manufacturer's data sheets gener-
ally show only an equivalent circuit at best, and at worst, only the pinout.
The data sheets often do,
however, give a truth table or timing (wave-
form) diagram for
digital circuits,
or some other description, not of what
the internal circuitry is, but rather what it does.
This, then,
is the ap-
proach we must use in troubleshooting them alSOf
In short,
we troubleshoot IC's by the process of
elimination; Are the
voltages at each of the pins
about what they should be?
Is the proper sig-
nal going into it?
Then, if the proper signal isn't coming out of it, it is
bad and
must be replaced.
Never mind
what is wrong
inside--you can't get
inside to fix it anyway, and time spent figuring it out is wasted.
Just re-
place
it.
A good voltmeter,
a high impedance VTVM or better yet one of the newer
digital
voltmeters that will
read to tenths of a volt or less,
and a good
oscilliscope are then necessary pieces of test equipment, and no shop should
be without them.
If you don't have them,
you'll have to spend your time in
speculation, and for the most part,
the results
will depend
on guesswork.
Either you'll use more
time than necessary (and have to charge the customer
for it) or you'll
wind up missing the cause of
intermittant problems, re-
placing perfectly
good parts,
or the like.
In either case,
the increased
cost to the customer (or decreased profit to the shop) is something that you
can hardly justifY.
MEASURING SIGNAL TO NOISE RATIO
The procedure
for measuring the signal to noise ratio of a receiver is
really quite simple, and it is a check that should be made on every receiver
before it leaves your shop--finding
a weak receiver
and fixing the problem
while it's in the shop is one of the best ways to keep a customer happy, and
it will pay dividends in repeat business.
Conneet an unmodulated signal generator that has a 500 output impedance
to the antenna terminals,
and tune it for about
1 kHz beat note audible in
the speaker.
Adjust the generator's output for about 0.25 uV at the antenna
terminals, so that the AGC circuit isn't
act~vated.
(Note that some gener-
ators are calibrated in terms of their open circuit output voltage, not what
is
present when
connected to a
load--check yours out,
so you can get the
right setting.)
Conneet an AC
voltmeter across
the speaker terminals,
and adjust the
volume control
for a convenient
reading on the
meter.
Note that reading.
Now remove the signal generator connection
and short out
the antenna term-
inals without
changing
anything else on
the receiver.
Note that reading.
The first is the reading for the
signal-plus-noise; the latteris the
reading for noise only.
The
siN
ratio is 20 times the common log of the ra-
tio between
the first
and second values.
Some AC meters have log (or dB)
scales and can be used directly.
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Chapters
Table of Contents
Troubleshooting
