2-8
Figure 2-3.—Traveling-wave tube test arrangement.
When you use the twt as an oscillator, failure of the tube to break into oscillations when all other
conditions are normal usually indicates a defective tube. In the case of a tube used as a receiving
amplifier, an increase of noise with a normal or reduced output can indicate that the tube is failing but is
still usable. All the general rules applying to klystron tubes mentioned previously are also applicable to
the twt.
Magnetron Tube Tests
You test a magnetron tube while it is in the transmitter equipment in which it is to be used. When
you install the magnetron in the transmitter, the condition of the tube can be determined by the normal
plate-current measurement and the power, frequency spectrum, and standing-wave-ratio tests of the
output signal. An unusual value for any of these measurements may indicate a defective tube.
Crossed-Field Amplifier
You usually test a crossed-field amplifier (cfa) tube while it is in the equipment in which it is used.
Like the klystron, if you do not operate the cfa for more than a few months, the tube may become gassy.
If a cfa tube is suspected of being gassy, we recommend that you consult the technical manual for the
particular piece of equipment in which the crossed-field amplifier is used.
TESTING SEMICONDUCTORS
Unlike vacuum tubes, transistors are very rugged in that they can tolerate vibration and a rather large
degree of shock. Under normal operating conditions, they will provide dependable operation for a long
period of time. However, transistors are subject to failure when they are subjected to relatively minor
overloads. Crystal detectors are also subject to failure or deterioration when subjected to electrical
overloads and will deteriorate from a long period of normal use. To determine the condition of
semiconductors, you can use various test methods. In many cases you may substitute a transistor of
known good quality for a questionable one to determine the condition of a suspected transistor. This
method is highly accurate and sometimes efficient. However, you should avoid indiscriminate
substitution of semiconductors in critical circuits. When transistors are soldered into equipment,
substitution becomes impractical - generally, you should test these transistors while they are in their
circuits.
Q-7.
What is the major advantage of a transistor over a tube?
