Comparing Wave Forms

condition is normally to be expected when servicing

burn out the meter movement. Furthermore, contact

with a circuit containing a charged capacitor will

adjustments are made in terms of their effect on the

endanger the life of the person making the test.

shape and amplitude of an observed waveform.

The vertical or horizontal amplitudes of the

COMPARING WAVEFORMS

reference and test patterns may not be proportional. This

will produce apparent differences between the

The measurement and comparison of waveforms

waveforms when actually there is no difference.

are considered to be very important parts of the circuit

Whether or not a minor waveform discrepancy may

analysis used in troubleshooting. In some circuits (for

be disregarded depends upon the type of circuit being

example, pulse circuits), waveform analysis is

traced. A minor discrepancy is not regarded as

indispensable. Waveforms may be observed at test

significant unless the nature of the discrepancy indicates

points, shown in the waveform charts that are part of the

faulty operation of the equipment. In general, time

maintenance literature for the equipment. You should

should not be wasted in searching for faults when

note that the waveforms given in the instruction books

relatively minor differences are detected between the

are often idealized and do not show some of the details

reference waveforms and those obtained by test.

that are normally present when the actual waveform is

displayed on an oscilloscope.

AMPLIFIERS IN ANNOUNCING SYSTEMS

By comparing the observed waveform with the

reference waveform, faults can be localized rapidly. A

In purchasing a public address system, where you

departure from the normal waveform indicates a fault

already know its power requirements, locations and

that is located between the point where the waveform is

types of microphones, and number and types of

last seen to be normal and the point where it is observed

speakers, you may have to decide between a system that

to be abnormal. (For example, if a waveform is observed

uses vacuum-tube amplifiers or one that uses transistor

to be normal at the grid circuit and abnormal at the plate

amplifiers. Your decision should be based on factors

circuit of the same stage, this indicates that the trouble

other than relative costs. Here are some reasons for the

lies in that stage or possibly the input of the following

use of transistor amplifiers instead of vacuum-tube

stage.)

amplifiers:

If there is no trouble present in an equipment or

The life of a transistor is longer than that of a

system, a waveform observed at a point in the equipment

vacuum tube.

should closely resemble the reference waveform given

for that test point. The reference waveforms supplied

Transistors consume less power than vacuum

with maintenance literature are the criteria of proper

tubes.

equipment performance. However, test equipment

Transistor amplifiers use dc power more

characteristics of usage can cause distortion of the

efficiently than vacuum-tube amplifiers.

observed waveform, though the equipment or system is

operating normally. Several of the most common causes

Transistors operate more efficiently at low

of these conditions are summarized in the following

voltages than vacuum tubes.

sections:

Microphonics signals generated by shock or

vibrations are almost completely lacking in transistors.

The leads of the test oscilloscope may not be

placed in the same manner as those preparing the

Power is instantly available at the throw of a

reference waveforms, or the lead lengths may differ

switch.

considerably. This is particularly significant in shielded

Ac hum is minimized.

test leads, where the capacitance per unit length is a

factor.

On the other hand, vacuum-tube amplifiers offer

these advantages over transistor amplifiers:

A type of test oscilloscope having different

values of input impedance, different sweep durations, or

Vacuum-tube replacements take less time than

different frequency response may have been used.

transistor replacements.

The equipment operating (and servicing)

Vacuum-tube amplifiers are less likely to be

controls may not have settings identical to those used

affected by high ambient temperatures or to suffer heat

when the reference waveforms were prepared. This

damage.

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