2-9
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.
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
