2-32
discharge from the human body when the device is handled. You may avoid such damage before starting
repairs by discharging the static electricity from your body to the chassis containing the transistor. You
can do this by simply touching the chassis. Thus, the electricity will be transferred from your body to the
chassis before you handle the transistor.
To prevent transistor damage and avoid electrical shock, you should observe the following
precautions when you are working with transistorized equipment:
1. Test equipment and soldering irons should be checked to make certain there is no leakage current
from the power source. If leakage current is detected, isolation transformers should be used.
2. Always connect a ground between test equipment and circuit before attempting to inject or
monitor a signal.
3. Ensure test voltages do not exceed maximum allowable voltage for circuit components and
transistors. Also, never connect test equipment outputs directly to a transistor circuit.
4. Ohmmeter ranges that require a current of more than one milliampere in the test circuit should not
be used for testing transistors.
5. Battery eliminators should not be used to furnish power for transistor equipment because they
have poor voltage regulation and, possibly, high-ripple voltage.
6. The heat applied to a transistor, when soldered connections are required, should be kept to a
minimum by using a low-wattage soldering iron and heat shunts, such as long-nose pliers, on the
transistor leads.
7. When it becomes necessary to replace transistors, never pry transistors to loosen them from
printed circuit boards.
8. All circuits should be checked for defects before replacing a transistor.
9. The power must be removed from the equipment before replacing a transistor.
10. Using conventional test probes on equipment with closely spaced parts often causes accidental
shorts between adjacent terminals. These shorts rarely cause damage to an electron tube but may
ruin a transistor. To prevent these shorts, the probes can be covered with insulation, except for a
very short length of the tips.
LEAD IDENTIFICATION
Transistor lead identification plays an important part in transistor maintenance; because, before a
transistor can be tested or replaced, its leads or terminals must be identified. Since there is no standard
method of identifying transistor leads, it is quite possible to mistake one lead for another. Therefore, when
you are replacing a transistor, you should pay close attention to how the transistor is mounted, particularly
to those transistors that are soldered in, so that you do not make a mistake when you are installing the new
transistor. When you are testing or replacing a transistor, if you have any doubts about which lead is
which, consult the equipment manual or a transistor manual that shows the specifications for the transistor
being used.
There are, however, some typical lead identification schemes that will be very helpful in transistor
troubleshooting. These schemes are shown in figure 2-17. In the case of the oval-shaped transistor shown
in view A, the collector lead is identified by a wide space between it and the base lead. The lead farthest
from the collector, in line, is the emitter lead. When the leads are evenly spaced and in line, as shown in
