Figure 4-11.-Voltage check.

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

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