2-6
As soon as the card switch is actuated, the tube under test is automatically subjected to an
interelement short test and a heater-to-cathode leakage test. A blinking or steady glow of any of the short
test lamps is an indication of an interelement short. If the short test lamps remain dark, no interelement
shorts exist within the tube. If a short exists between two or more elements, the short test lamp or lamps
connected between these elements remain dark, and the remaining lamps light. The abbreviations for the
tube elements are located on the front panel just below the short test shield so that the neon lamps are
between them. This enables the operator to tell which elements are shorted. Heater-to-cathode shorts are
indicated as leakage currents on the #1 meter scale. If the meter reads above the green area, the tube
should be replaced. A direct heater-to-cathode short causes the meter to read full scale.
To make the QUALITY test, push the number 2 button (fig. 2-1) and read the number 2 scale on
meter M301 to determine if the tube is good. (This test may be one of various types, such as
transconductance, emission, plate current, or voltage drop, depending upon the type of tube under test.)
To test the tube for GAS, press the number 3 button and read the number 3 meter scale. The number
2 button also goes down when number 3 is pressed. If a dual tube having two identical sections is being
tested, the neon lamp (DS203) will light, indicating that both sections of the tube may be tested with one
card. To do this, check the tube for shorts, leakage, quality, and gas as described previously; then hold
down button number 4 and repeat these tests to test the second section of the tube. Dual tubes with
sections that are not identical require two cards for testing. A second card is also provided to make special
tests on certain tubes.
AUXILIARY TEST.—As mentioned previously, two special tests (cathode activity and sensitive
grid shorts) may be made by use of controls located in the auxiliary compartment (fig. 2-2). The cathode
activity test (CATH ACT) is used to indicate the amount of useful life remaining in the tube. By reducing
the filament voltage by 10 percent and allowing the cathode to cool off slightly, the ability of the cathode
as an emitter of electrons can be estimated. This test is made in conjunction with the normal quality test.
To make the CATH ACT test, allow the tube under test to warm up, press button number 2 (fig. 2-1),
and note the reading of scale number 2 on meter M301. Note also the numerical scale reading on M301.
Next, lock down the CATH ACT button (fig. 2-2), wait for about 1.5 minutes, then press button number 2
(fig. 2-1) again and note the numerical and number 2 scale readings on meter M301. The tube should be
replaced if the numerical reading on M301 differs from the first reading by more than 10 percent or if the
reading is in the red area on the number 2 scale.
It is sometimes desirable to check certain tubes for shorts at a sensitivity greater than normal. To
make the SENSITIVE GRID SHORTS test, push S302C (fig. 2-2) and note if any short test lamps (fig.
2-1) light.
HIGH-POWER HF AMPLIFIER TUBE TESTS
You normally test high-power amplifier tubes, which operate in the low-to-high frequency range, in
the transmitter in which they are to be used. When you operate the tube in a transmitter, its condition can
be determined by using built-in meters to measure the grid current, plate current, and power output and
comparing those values with those obtained when using tubes known to be good.
Q-4.
Normally, how are high-power rf tubes tested?
Klystron Tube Tests
You can check low-power klystron tubes for gas, frequency of the output signal, and output power
by placing them in the equipment where they are to be used. You measure the beam current, output
