Star Tech Journal

June 1992
STAR*TECH Joumal
more actual trouble making parts than most check-
ers that give value.
Another caution is that a shorted cap, or a very low
impedance circuit can cause a reading that is erro-
neously good. If you suspect that might be the case,
merely unsolder one of the leads and try the cap out
of the circuit.
OFFSET
Some generators have a feature that can help you
here, too. If your machine has a D.C. Offset control
available, just set it off a half a volt. Then a short
will give you a O volt reading,. while a good cap will
give you that offset. (This is assuming you are
using D.C. coupling on your scope. See Fig. 3) When
in doubt use an ohmmeter, although shorted caps
in monitors are rare.
SAFETY FIRST
Of course, a strong caution is in order here too. This
device is strictly cold chassis! Make sure power is
off each time you make a measurement. Beware of
some of those big power caps too, which can hold
charges for weeks if the fuse blows or the board is
cracked. Youjust got that function generator- don't
smoke it!
_ _ _ 11111111111 . . . . _ _ _ _ _ _ _ lllllllllil _ _ _ ..... {".
Figure 3A (Signal Short)
GROUNDING
Another possibility for false indications is a ground
path. Your test signal has a ground side, as most
scopes and generators do. If you are working on a
chassis that has other test equipment hooked up to
it, then hooking the 'hot' side of your probes to the
grounded section will ground the waveform and
make you think the cap is good. If you work with
chassis that are completely disconnected from
other units this will not be a problem.
BOTTOM LINE
With all these cautions, there is one thing you can
be sure of. A cap that fails to short out your signal
is bad and should be replaced. As you stand looking
at an unfamiliar chassis and wondering whether a
cap job would help, such an inventory will tell you
a lot in a short time.
Next issue: Testing Flyback Transformers in or out
of circuit and Checking the entire Horizontal Cir-
cuit
Figure 38 (Good Cap DC Offset)