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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.
4-14

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