June 1992
STAR*TECH Journal
, 7":
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CONNECTION
Referring to figure 1, connection is very simple. The
output of the generator is paralleled with the input
of the 'scope, and the two lines come out as test
leads. (Note that the 'scope is obviously an old
model.) On my bench I am fortunate enough to have
a dual trace 'scope, and I just leave this set-up
permanently connected to one channel, using the
other channel for other 'scope functions.
The output of the function generator is normally 50
ohms or a little more and can safely be shorted to
itself. This is the basis of most of the testing de-
scribed here -noting what the device under test
(DUT) does in terms ofloading down the generator.
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Figure 2A (Full Waveform)
SET-UP
Most electrolytic capacitors test well at @ 3kHz or
~a little more. To calibrate your system set the
frequency here and set the output to maximum, or
at the maximum before the output clips. Set the
scope to a setting that just over-scans the screen,
then back off the generator to give a full waveform.
ThsTING
Now, try touching the probes to several different
values of electrolytics. On my setup, a 1 mf d cap will
leave just a trace of ripple (Fig. 2). All higher values
give a direct short indication- if they are good.
Thus, I can go through a chassis, guided only by the
symbols on the solder side, and check every cap on
it in seconds, or at least not many minutes.
Figure 28 (Shorted Signal -Good Cap)
If I get a waveform, I have only to check topside to
find that:
l)the hole was not stuffed in this model,
2) The cap is 1 mfd or less, or
3) I've found a bad cap.
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CAPACITOR NOTES
~ound too easy? There are a few cautions. The first
is that this check is only for internal resistance. If
the cap has merely changed value, this may not
show it. However, electrolytics are not usually
specified to be precision parts. This check finds
Figure 2C ( 7 mfd)
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