Visual inspection is a good maintenance technique. Occasionally, you will find loose wires or faulty
connections, making extensive voltage checks unnecessary.
Transistors can be checked by substitution. Transistors, however, have a characteristic known as
leakage current, which may affect the results obtained when the substitution method is used.
The leakage current may influence the current gain or amplification factor of the transistor.
Therefore, a particular transistor might operate properly in one circuit and not in another. This
characteristic is more critical in certain applications than in others. As the transistor ages, the amount of
leakage current tends to increase. One type of transistor checker used is the semiconductor test set. This
test set can be used either for in-circuit or out-of-circuit tests or for collector leakage current or current
gain. You should use extreme care when substituting transistors. More and more transistors have specific
current and breakdown voltage requirements that may affect how they operate within a given circuit.
Q-12. As a transistor ages, what happens to the leakage current?
Voltage measurements provide a means of checking circuit conditions in a transistorized circuit just
as they do in checking conditions in a tube circuit. The voltages, however, are much lower than in a tube
circuit. The bias voltage between the base and emitter, for instance, is usually 0.05 to 0.20 volts. When
making checks, observe polarity.
Transistors have little tendency to burn or change value because of low voltage in their circuits. They
can, however, be permanently damaged by high-voltage conditions that occur when the collector voltage
is increased. They can also be permanently damaged when the ambient temperature increases and causes
excessive collector current flow. Transistors are easily damaged by high current; therefore, resistance
measurements must not be taken with an ohmmeter that provides a maximum current output in excess of
1 milliampere. If you are not sure that the range of ohmmeter you want to use is below the 1 milliampere
level, connect the ohmmeter to a milliammeter and check it. See figure 2-19 for a method of measuring
the current from an ohmmeter.