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?

Traveling-Wave Tube

You 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.

Q.6 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.

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?

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 (I_co), 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.

Many common 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.

TRANSISTOR TESTING

When 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.

Resistance Test

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.

Q.9 When you are using an ohmmeter to test a transistor, what range settings should be avoided?