nature often occurs during transistor servicing by applying the incorrect polarity voltage to
the collector circuit or excessive voltage to the input circuit. Careless soldering techniques
that overheat the transistor have also been known to cause considerable damage.
2-95. One of the most frequent causes of damage to a transistor is the electrostatic
discharge from the human body when the device is handled. To avoid such damage before
starting repairs, discharge the static electricity from your body to the chassis containing the
transistor. You can do this by simply touching the chassis. Therefore, the electricity will be
transferred from your body to the chassis before you handle the transistor.
2-96. There are a number of ways to prevent transistor damage and avoid electrical
shock. Observe the following precautions when you are working with transistorized
Check test equipment and soldering irons to ensure that there is no leakage current
from the power source. If leakage current is detected, isolation transformers should
Always connect a ground between test equipment and circuit before attempting to
inject or monitor a signal.
Ensure test voltages do not exceed maximum allowable voltage for circuit
components and transistors.
NEVER connect test equipment outputs directly to a transistor circuit.
Ohmmeter ranges, which require a current of more than one milliampere in the test
circuit, should not be used for testing transistors.
DO NOT use battery eliminators to furnish power for transistor equipment because
they have poor voltage regulation and, possibly, high-ripple voltage.
When soldered connections are required, keep the heat applied to the transistor
leads to a minimum by using a low-wattage soldering iron and heat shunts (such as
NEVER pry transistors from printed circuit boards when it becomes necessary to
Check all circuits for defects before replacing a transistor.
Remove power from the equipment before replacing a transistor.
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.
TRANSISTOR LEAD IDENTIFICATION
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. Pay attention 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
23 June 2005