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RCA Volt-Ohm-Milliammeter
Checking Capacitors
The
R X 10,000 range of the W V - 3 8 A permits some indication of
capacitor leakage resistance. A good paper or mica capacitor under
1 id will indicate above the 20 megohm mark ( " 2 K " ) after the initial
surge required to charge the capacitor is completed. If the resistance
indicated is less than 20 meg., the capacitor probably has defective
insulation. T h e time required to charge the capacitor will vary with
the capacity. T h e greater the capacity, the more the pointer will swing
and the longer it will take for it to return to infinity. If the capacitor
is shorted, the pointer will not return to infinity. If the capacitor is
open, there will be no initial pointer deflection. Very small capacitors
will indicate only a slight deflection and a rapid return to infinity.
Electrolytic capacitors, however, normally indicate some leakage, after
charging. T h e amount of leakage is generally related to the capacitance
and voltage rating. F o r example, a low voltage, high capacitance unit
will indicate more leakage (lower resistance) than a high-voltage, low
capacitance unit. W h e n testing an electrolytic capacitor, the red lead
must be connected to the positive and the black lead to the negative
terminal of the capacitor, and the function switch on the W V - 3 8 A
must be i n the " + D C " position.
Checking Rectifiers
The
resistance of copper oxide, selenium, silicon and crystal recti-
fiers can
be measured
in both directions. The resistance
measured
should be quite large in one direction and very small in the opposite
direction. The front-to-back resistance ratio can be checked easily by
setting the Function Switch from " + D C " to " — D C " , thus reversing the
polarity of the test leads.
Use of an Isolation Resistor
In certain sensitive circuits i n radio and T V receivers and other
I.F. or R . F . high-gain amplifiers, it is not possible to connect leads
without disturbing the normal operation of the circuits. Usually, D C
voltage measurements i n such circuits requires the use of a V T V M
with a resistor-isolation probe, such as an R C A VoltOhmyst. However,
when the VoltOhmyst is not available, the W V - 3 8 A can be sub-
stituted, if a resistor is used i n series with the instrument, clipped onto
the end of the red probe. If an exact measurement is not required, almost
any value of resistor, higher than the circuit impedence, will isolate the
instrument so it will not overly disturb the circuit. W h e n an exact
measurement has to be made, a resistor should be used which exactly
multiplies the input resistance of the instrument so that the reading
can be made on one of the higher scales. T h e following table gives
values of resistors to be used:
Range
Resistor
Reading Multiplier
Scale to Read
2.5 V
50 K
2 X
5 V
2.5 V
150 K
4 X
10 V
50 V
1 M e g .
2 X
100 V
250 V
5 M e g .
2 X
500 V
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