Module 21 − Test Methods and Practices
Pages i ,
5−31, AI−1 to AI−3, Index
Upon completion of this chapter, you will be able to do the following:
1. Explain the
importance of testing individual electronic components.
2. Identify the various methods of testing
3. Identify the various methods of testing semiconductors.
4. Identify the
various methods of testing integrated circuits.
5. Identify the various types of testing batteries and their characteristics.
6. Identify the
various methods of testing RF attenuators and resistive loads.
7. Identify the various methods of
testing fiber-optic devices.
Introduction to COMPONENT TESTING
It is imperative that you be able to troubleshoot an equipment failure to the component level. In the
majority of cases, Navy technicians are expected to troubleshoot and identify faulty components. This chapter,
"Component Testing," will acquaint you with alternative methods of testing various components and their
parameters. a quick glance at the Navy's mission and concept of operation explains why we, in most cases, must be
able to troubleshoot to the faulty component level. a ship must be a self-sustaining unit when deployed. Storage
space is a primary consideration on most ships and a limiting factor for storage of bulky items or electronic
modules as ready spares. Therefore, it is practical to store only individual components common to a great number
of equipment types. This of course, limits the larger replacement modules available to you during troubleshooting.
Q-1. Why are most ships limited in their ability to stock replacement modules for repair of electronic
TESTING ELECTRON TUBES
In equipment that uses vacuum tubes, faulty tubes are responsible for more than 50% of all electronic
equipment failures. As a result, testing of electronic tubes is important to you. You can determine the condition
of a tube by substituting an identical tube known to be good for the questionable one. However, indiscriminate
substitution of tubes is to be avoided for at least the following two reasons: (1) detuning of circuits may result
and (2) a tube may not operate properly in a high-frequency circuit even though it performs well in a
low-frequency circuit. Therefore, your knowledge of tube-testing devices and their limitations, as well as correct
interpretation of the test results obtained, is indispensable for accurate and rapid maintenance.
Because the operating capabilities and design features of a tube are demonstrated by its electrical
characteristics, a tube is tested by measuring those characteristics and comparing them with representative values
established for that type of tube. Tubes that read abnormally high or low with respect to the standard are
suspect. Practical considerations, which take into account the limitations of the tube test in predicting actual
tube performance in a particular circuit, make it unnecessary to use complex and costly test equipment with
laboratory accuracy. For most applications, testing of a single tube characteristic is good enough to determine
tube performance. Some of the more important factors affecting the life expectancy of an electron tube are listed
· The circuit function of the tube
· Deterioration of the cathode coating
· a decrease in emission of impregnated emitters in aging
· Defective seals that permit air to leak into the envelope and oxidize the emitting
· Internal short circuits and open circuits caused by vibration or excessive voltage
the average receiving tube is not overdriven or operated continuously at maximum rating, it can have a life of at
least 2,000 hours before the filament opens. Because of the expansion and contraction of tube elements during the
process of heating and cooling, electrodes may lean or sag, which causes excessive noise or microphonics to
develop. Other electron-tube defects are cathode-to-heater leakage and nonuniform electron emission of the
cathode. These common tube defects contribute to about 50% of all electronic equipment failures. For this reason
you should immediately eliminate any tube known to be faulty. However, avoid blind or random replacement of good
tubes with fresh spares. The most common cause of tube failure is open filaments. Evidence of a tube defect is
often obvious when the filament is open in glass-envelope tubes. You will also notice the brighter-than-normal
cherry-red glow of the plate when the plate current is excessive. Also, when the tube becomes gassy or when arcing
occurs between electrodes, you will probably have visual indication. Metal-encased tubes can be felt for warmth to
determine if the heater is operating. You can tap a tube while it is operating in a circuit to reveal an aural
indication of loose elements within the tube or microphonics, which are produced by loose elements.
tubes are extremely fragile and subject to damage during shipment. When you replace a tube, never make the
assumption that the new tube is good because it's new. You should always test tubes before installing them.
Q-2. What is the most common cause of electron tube failure?
Substituting with a tube known to be in good condition is a simple method of testing a questionable tube. However,
in high-frequency circuits tube substitution should be carried out in a logical sequence. Replace tubes one at a
time so that you can observe the effect of differences in interelectrode capacitance in the substituted tubes on
tuned circuits. The tube substitution test method cannot be used to advantage in locating more than one faulty
tube in a single circuit for two reasons: (1) If both an RF amplifier tube and IF amplifier tube are defective in
a receiver, replacing either one will not correct the trouble; and (2) if all the tubes are replaced, there is no
way for you to know what tubes were defective. Under these conditions, using test equipment designed for testing
the quality of a tube saves you valuable time.
Q-3. What is the most accurate method of determining the
condition of an electron tube?
Note ON SYMBOLS USED IN The FOLLowING SECTIONS: IEEE and ANSI standards (see inside front cover) are used to
define various terms, such as anode (plate) current, anode voltage, and anode resistance. This book uses Ea for
anode voltage, Ia for anode current, and Ra for anode resistance. These are the same as E, Ip, and Rp that you
will see elsewhere. This module uses the terms anode and plate interchangeably.
A representative field type of electron tube tester designed to test all common low-power
tubes is shown in figure 2-1. The tube test conditions are as close as possible to actual tube operating
conditions and are programmed on a pre-punched card. The card switch (S101, fig. 2-1) automatically programs the
tube test conditions when it is actuated by a card. a card compartment on the front panel of the tester provides
storage for the most frequently used cards. The cover of the tester (not shown) contains the operating
instructions, the brackets for storing the technical manual, the power cord, the calibration cell for checking the
meter and short tests, the calibration cards, the blank cards, and a steel hand punch.
Figure 2-1. - Electron tube tester.
When a pre-punched card is fully inserted into the card switch
(S101), a microswitch is actuated that energizes a solenoid, causing the card switch contacts to complete the
circuit. The card switch has 187 single-pole, single-throw switches arranged in 17 rows with 11 switches in each
row. The card is used to push the switches closed; thus, the absence of a hole in the card is required to actuate
The meter (M301) contains four scales. The upper scale is graduated from 0 to 100 for direct numerical
readings. The three lower scales, numbered 1, 2, and 3, are read for LEAKAGE, QUALITY, and GAS, respectively. Each
numbered scale includes green and red areas marked GOOD and REPLACE. Inside a shield directly in front of the
meter are five neon lamps (DS301 through DS305), which indicate shorts between tube elements.
The number 2
pushbutton (MP6) is used for transconductance, emission, and other quality tests (described later). The number 3
pushbutton (MP7) is used to test for the presence of gas in the tube envelope. The number 4 pushbutton (MP8) is
used for tests on dual tubes. a neon lamp (DS203) lights when pushbutton number 4 is to be used. Eleven tube test
sockets are located on the panel, plus tube pin straighteners for the 7- and 9-pin miniature tubes.
power ON-ofF spring-return toggle switch (S105) turns the tester on by energizing a line relay. The pilot light
(DS107) lights when this relay closes. Above the power ON-ofF switch are five fuses. Fuses F101, F201, and F202
protect circuits in the tester not protected by other means and have neon lamps to indicate when they have blown.
Fuses F102 and F103 protect both sides of the power line.
group of auxiliary controls covered by a hinged panel is used for special tests and for calibration of the tester.
Two of these controls, labeled Signal CAL (R152 and R155, fig. 2-2), are used with special test cards for
adjusting the regulation and amplitude of the signal voltage. a pushbutton labeled CATH ACT (S302D) is used for
making cathode activity tests. When this button is pressed, DS106 on the front panel (fig. 2-1) lights, and the
filament voltage of the tube under test is reduced by 10%. Results of the test are read as a change in reading on
the numerical meter scale.
Figure 2-2. - Auxiliary compartment.
Pushbutton S302E and potentiometers R401 and R405 (fig. 2-2) are used for balancing the
transconductance (Gm) bridge circuit under actual tube operating current. Pressing S302E removes the grid signal
and allows a zero balance to be made with one potentiometer or the other, depending upon whether the tube under
test is passing high or low plate current. Lamp DS108 on the front panel lights when S302E is pressed. Pushbutton
S302C is used for checking grid-to-cathode shorts at a sensitivity much higher than the normal tests. Results of
this test are indicated by the short test lamps on the front panel.
Certain special tests require the use
of a continuously adjustable auxiliary power supply. By pressing pushbutton S302B, you may use meter M301 to read
the voltage of the auxiliary power supply on meter M301. This voltage may be adjusted by the use of the
potentiometer R142. The rest of the potentiometer controls are calibration controls and are adjusted by the use of
special calibration cards and a calibration test cell.
All circuits in the tester, except the filament
supply, are electronically regulated to compensate for line voltage fluctuations. The filament supply voltage is
adjusted by pressing pushbutton S302A and rotating the filament standardization adjustment switch S106 until meter
M301 reads midscale.
The circuits to be used in testing are selected by a pre-punched card. These cards are made of tough vinyl plastic
material. The tube numbers are printed in color on the tabs of the cards and also at the edge of the card for
convenience in filing. a special card is provided to use as a marker when a card is removed for use. Blank cards
are provided so that additional test cards may be punched for new tubes that are developed or to replace cards
that have become unserviceable.
Before operating the tester for the
first time, and periodically thereafter, you should calibrate it using the calibration test cards as described in
the equipment technical manual.
NORMAL TESTS. - The tester is equipped with a
three-conductor power cord, one wire of which is chassis ground. It should be plugged into a grounded 105- to
125-volt, 50- to 400-hertz outlet.
Before operating the tester, open the auxiliary compartment (fig. 2-2)
and ensure that the FILAMENT STD ADJ and the Gm BAL knobs are in the NOM position. The GRID SIG and CATH ACT
buttons (S302E and S302D) should be up and lamps DS108 and DS106 on the front panel should be out.
the tester and allow it to warm up for 5 to 10 minutes, then press the CARD REJECT KNOB (fig. 2-1) down until it
locks. If a non-test card is installed in the card switch, remove it. This card is used to keep the switch pins in
place during shipment and should be inserted before transporting the tester.
Plug the tube to be tested
into its proper socket. (Use the pin straighteners before plugging in 7- and 9-pin miniature tubes.) Select the
proper card or cards for the tube to be tested. Insert the card selected into the slot in the card switch until
the CARD REJECT KNOB pops up. The card will operate the tester only if it is fully inserted and the printing is up
and toward the operator. Do not put paper or objects other than program cards into the card switch, because they
will jam the switch contacts. If the overload shuts off the tester when the card is inserted in the switch, check
to see that the proper card is being used for the tube under test and that the tube under test has a direct
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.
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
frequency, and output power with the transmitter's built-in test equipment. You can check the output
of klystrons used as receiver local oscillators by measuring the current in the crystal mixer unit.
Klystron tubes that remain inoperative for more than 6 months may become gassy. This condition occurs in klystrons
installed in stored or spare equipment as well as in klystrons stored as stock supplies. Operation of a gassy
klystron at its rated voltages will ionize the gas molecules and may cause excessive beam current to flow. This
excessive beam current may shorten the life of the klystron or produce immediate failure. You can detect gas in a
klystron tube by setting the applied reflector voltage to zero and slowly increasing the beam voltage while
observing a meter that indicates the beam current - excessive beam current for a specific value of voltage
indicates that the tube is gassy.
a gassy klystron tube can usually be restored to serviceable condition if you temporarily operate it at reduced
beam voltage. Eight hours or more of reduced voltage operation may be required for klystrons that have been
inoperative for periods in excess of 6 months.
The beam current is also an indication of the power output
of the klystron. As klystrons age they normally draw less beam current; when this current decreases to a minimum
value for a specific beam voltage, the tube must be replaced. You can usually determine the power output of
transmitter klystrons by measuring the transmitter power output during equipment performance checks.
Q-5. What should you do if a klystron becomes gassy?
can usually test a traveling-wave tube (TWT) in the equipment in which it is used. When the TWT is installed, you
can usually measure the collector current and voltage and check the power output for various inputs. Any deviation
greater than 10% from normal specifications may be considered to be an indication of a defective tube. Most
amplifiers are supplied with built-in panel meters and selector switches so that the cathode, anode, helix, focus,
and collector currents may be measured. Thus, continuous monitoring of amplifier operation and tube evaluation is
possible. Adjustments usually are provided for you to set the helix, grid bias, and collector voltages for optimum
operation. If variation of these controls will not produce normal currents and if all voltages are normal, you
should consider the tube to be defective and replace it with a new tube or one known to be in good operating
condition. To avoid needless replacement of tubes, however, you should make an additional check by measuring the
input power and output power and determining the tube gain. If, with normal operating conditions, the gain level
drops below the minimum indicated value in the equipment technical manual, the tube is defective.
When used as an amplifier, what is the best indication that a TWT is operating properly?
In the absence of
special field-test sets, you may construct a laboratory test mock-up similar to that shown in figure 2-3. Because
of the variations in power and gain between tubes and the large frequency ranges offered, we can illustrate only a
general type of equipment. The equipment you select must have the proper range, impedance, and attenuation to make
the test for a specific type of TWT. To make gain measurements, you turn the switch shown in figure 2-3 to
position 1 and set the precision attenuator to provide a convenient level of detector output. Then turn the switch
to position 2 and insert attenuation until the detector output level is identical to that obtained without the TWT
in the circuit. The gain of the traveling-wave tube is equal to the amount of added attenuation.
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.
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.
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
Q-7. What is the major advantage of a transistor over a tube?
Since certain fundamental characteristics indicate the condition of semiconductors, test equipment is
available that allows you to test these characteristics with the semiconductors in or out of their circuits.
Crystal-rectifier testers normally allow you to test only the forward-to-reverse current ratio of the crystal.
Transistor testers, however, allow you to measure several characteristics, such as the collector leakage current
(Ic), collector to base current gain (b), and the four-terminal network parameters. The most useful test
characteristic is determined by the type of circuit in which the transistor will be used. Thus, the
alternating-current beta measurement is preferred for ac amplifier or oscillator applications; and for
switching-circuit applications, a direct-current beta measurement may prove more useful.
transistors are extremely heat sensitive. Excess heat will cause the semiconductor to either fail or give
intermittent operation. You have probably experienced intermittent equipment problems and know them to be both
time consuming and frustrating. You know, for example, that if a problem is in fact caused by heat, simply opening
the equipment during The Course of troubleshooting may cause the problem to disappear. You can generally isolate
the problem to the faulty printed-circuit board (PCB) by observing the fault indications. However, to further
isolate the problem to a faulty component, sometimes you must apply a minimal amount of heat to the suspect PCB by
carefully using a low wattage, heat shrink gun; an incandescent drop light; or a similar heating device. Be
careful not to overheat the PCB. Once the fault indication reappears, you can isolate the faulty component by
spraying those components suspected as being bad with a nonconductive circuit coolant, such as Freon. If the
alternate heating and cooling of a component causes it to operate intermittently, you should replace it.
Q-8. Name two major disadvantages of transistors.
trouble occurs in solid-state equipment, you should first check power supplies and perform voltage measurements,
waveform checks, signal substitution, or signal tracing. If you isolate a faulty stage by one of these test
methods, then voltage, resistance, and current measurements can be made to locate defective parts. When you make
these measurements, the voltmeter impedance must be high enough that it exerts no appreciable effect upon the
voltage being measured. Also, current from the ohmmeter you use must not damage the transistors. If the
transistors are not soldered into the equipment, you should remove the transistors from the sockets during a
resistance test. Transistors should be
removed from or reinserted into the sockets only after power has been
removed from the stage; otherwise damage by surge currents may result.
Transistor circuits, other than
pulse and power amplifier stages, are usually biased so that the emitter current is from 0.5 milliampere to 3
milliamperes and the collector voltage is from 3 to 15 volts. You can measure the emitter current by opening the
emitter connector and inserting a milliammeter in series. When you make this measurement, you should expect some
change in bias because of the meter resistance. You can often determine the collector current by measuring the
voltage drop across a resistor in the collector circuit and calculating the current. If the transistor itself is
suspected, it can be tested by one or more of the methods described below.
You can use an ohmmeter to test transistors by measuring the
emitter-collector, base-emitter, and base-collector forward and reverse resistances. a back-to-forward resistance
ratio on the order of 100 to 1 or greater should be obtained for the collector-to-base and emitter-to-base
measurements. The forward and reverse resistances between the emitter and collector should be nearly equal. You
should make all three measurements for each transistor you test, because experience has shown that transistors can
develop shorts between the collector and emitter and still have good forward and reverse resistances for the other
two measurements. Because of shunting resistances in transistor circuits, you will normally have
to disconnect at least two transistor leads from the associated circuit for this test. Exercise
caution during this test to make certain that current during the forward resistance tests does not exceed the
rating of the transistor - ohmmeter ranges requiring a current of more than 1 milliampere should not be used for
testing transistors. Many ohmmeters are designed such that on the R x 1 range, 100 milliamperes or more can flow
through the electronic part under test. For this reason, you should use a digital multimeter. Be sure you select a
digital multimeter that produces enough voltage to properly bias the transistor junctions.
you are using an ohmmeter to test a transistor, what range settings should be avoided?
Laboratory transistor test sets are used in experimental work to test all characteristics of transistors. For
maintenance and repair, however, it is not necessary to check all of the transistor parameters. a check of two or
three performance characteristics is usually sufficient to determine whether a transistor needs to be replaced.
Two of the most important parameters used for transistor testing are the transistor current gain (beta) and the
collector leakage or reverse current (Ic).
The semiconductor test set (fig. 2-4) is a rugged, field type
of tester designed to test transistors and semiconductor diodes. The set measures the beta of a transistor,
resistance appearing at the electrodes, reverse current of a transistor or semiconductor diode, shorted or open
conditions of a diode, forward transconductance of a field-effect transistor, and condition of its own batteries.
Figure 2-4. - Semiconductor test set.
In order to assure that accurate and useful information is gained from the transistor tester, the
following preliminary checks of the tester should be made prior to testing any transistors.
POLARITY switch (fig. 2-4) in the ofF position, the meter pointer should indicate exactly zero. (When required,
rotate the meter adjust screw on the front of the meter to fulfill this requirement.) When measurements are not
actually being made, the POLARITY switch must always be left in the ofF position to prevent battery drain.
and Direct Current
||Alternating Current and Transformers
||Circuit Protection, Control, and Measurement
||Electrical Conductors, Wiring Techniques,
and Schematic Reading
||Generators and Motors
||Electronic Emission, Tubes, and Power Supplies
||Solid-State Devices and Power Supplies
||Wave-Generation and Wave-Shaping Circuits
||Wave Propagation, Transmission Lines, and
||Introduction to Number Systems and Logic Circuits
||- Introduction to Microelectronics
||Principles of Synchros, Servos, and Gyros
||Introduction to Test Equipment
||Radio-Frequency Communications Principles
||The Technician's Handbook, Master Glossary
||Test Methods and Practices
||Introduction to Digital Computers
||Introduction to Fiber Optics
|Note: Navy Electricity and Electronics Training
Series (NEETS) content is U.S. Navy property in the public domain.