Quantcast Figure 4-11.-Voltage check.
 

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Figure 4-11.-Voltage check.
ohms-per-volt) of the voltmeter used to prepare the
As a supervisor, you should make sure detailed
voltage chart is always given on the charts. Therefore,
t r o u b l e isolation procedures are performed
if a meter of an approximately equal sensitivity is
according to the instruction manual for the
available, use it so the effects of loading will not have
equipment being repaired.
to be considered.
TESTING TUBES
When you are checking voltages, you should
remember that a voltage reading can be obtained across
Electron-tube failure accounts for most of the
a resistance, even if the resistance is open. The
trouble in systems using tubes. It is impractical to try to
resistance of the meter and the multipliers form a circuit
locate faults by general tube checking. Only when the
resistance when the meter prods are placed across the
fault has been traced to a particular stage should tubes
open resistance (fig. 4-11).
be tested. Then, only those tubes associated with the
improperly functioning circuits should be tested.
MEASURING RESISTANCE
MEASURING VOLTAGE
Defective components can usually be located by
measuring the dc resistance between various points in
Since most troubles in equipment and systems result
the circuit and a reference point or points (usually
ground). This is true because a fault will generally
from, or produce, abnormal voltages, voltage
measurements are considered indispensable in locating
produce a change in resistance values. Point-to-point
troubles. Testing techniques that use voltage
resistance charts can be used advantageously when you
measurements have the disadvantage of requiring work
make resistance measurements. The values given,
on an energized circuit. Point-to-point voltage
unless otherwise stated, are measured between the
indicated points and ground.
measurement charts that show the normal operating
voltages in the various stages of the equipment are
Before making resistance measurements, the IC
available to the troubleshooter.
Electrician should make sure the power to the equipment
When voltage measurements are initially taken, it is
under test has been turned off. Since an ohmmeter is
good practice to set the voltmeter on the highest range
essentially a low-range voltmeter and a battery, an
so any excessive voltage in a circuit will not damage the
ohmmeter connected to a circuit that already has
meter. To obtain increased accuracy, the voltmeter may
voltages in it may be seriously harmed. The pointer may
then be set to the range for the proper comparison with
be deflected off-scale, and the meter movement maybe
permanently damaged.
the values given in the voltage charts.
If the internal resistance of the voltmeter and
Filter capacitors must be discharged with a shorting
multiplier is approximately the same in value to the
probe before making resistance measurements. This is
resistance of the circuit under test, it will indicate a lower
extremely important when you are testing power
voltage than the actual voltage present when the meter
supplies that are disconnected from their loads. If a
is removed from the circuit. The sensitivity (in
capacitor discharges through the meter, the surge may
4-13

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