3-29
WARNING
The following section discusses voltage measurements on live circuits. BE
SURE YOU ALWAYS FOLLOW PRESCRIBED SAFETY RULES WHEN
MEASURING VOLTAGES.
VOLTAGE TESTS
Voltage tests must be made with the power applied; therefore, the prescribed safety precautions must
be followed to prevent injury to personnel and damage to the equipment. You will find in your
maintenance work that the voltage test is of utmost importance. It is used not only in isolating casualties
to major components but also in the maintenance of subassemblies, units, and circuits. Before checking a
circuit voltage, you should check the voltage of the power source to be sure that the normal voltage is
being applied to the circuit.
The voltmeter is used for voltage tests. In using the voltmeter, make certain that the meter used is
designed for the type of current (ac or dc) to be tested and has a scale with a suitable range. Since
defective parts in a circuit can cause higher than normal voltages to be present at the point of test, the
highest voltmeter range available should be used first. Once you have obtained a reading, determine if a
lower scale can be used that will cause no damage to the meter movement. If so, use the lower scale. This
provides a more accurate reading.
Another consideration in the circuit voltage test is the resistance and current in the circuit. A low
resistance in a high-current circuit could result in considerable voltage drop, whereas the same resistance
in a low-current circuit may be minimal. Abnormal resistance in part of a circuit can be checked with
either an ohmmeter or a voltmeter. Where practical, an ohmmeter should be used because the test is then
carried out with a "dead" circuit.
The majority of the electronic circuits you will encounter in equipment will be low-current circuits,
and most voltage readings will be direct current. Also, many of the schematics will indicate the voltages
at various test points. Therefore, if you suspect that a certain stage is defective, you can check the voltage
by connecting a voltmeter from the test point to ground. If the suspected stage is not defective, the
voltmeter readings should match the voltages given on the schematic.
Some technical manuals also contain voltage charts on which all the voltage measurements are
tabulated. These charts usually indicate the sensitivity of the meter (for example, 20,000 ohms/volt) used
to obtain the voltage readings for the chart. To obtain comparable results, you must use a voltmeter of the
same sensitivity (or greater) as that specified. Make certain that the voltmeter is not "loading" the circuit
while taking a measurement. If the meter resistance is not considerably higher than the circuit resistance,
the reading will be markedly lower than the true circuit voltage because of the voltmeter’s loading effect.
(To calculate meter resistance, multiply the rated ohms-per-volt sensitivity value of the meter by the scale
in use. For example, a 1,000-ohms-per-volt meter set to the 300-volt scale will have a resistance of
300,000 ohms.)
RESISTANCE TESTS
Before checking the resistance of a circuit or of a part, make certain that the power has been turned
off. Also make sure capacitors in the associated circuit are fully discharged. To check continuity, always
use the lowest ohmmeter range. If the highest range is used, the meter may indicate zero, even though
appreciable resistance is present in the circuit. Conversely, to check a high resistance, use the highest
scale since the lower range scale may indicate infinity, even though the resistance is less than a megohm.
In making resistance tests, you must remember that even though the external ohmmeter leads are
