2-90. You can identify transistors by a JAN designation printed directly on the case of
the transistor. The marking scheme (shown below) and explained earlier for diodes is also
used for transistor ID. The first number indicates the number of junctions. The letter "N"
following the first number tells us that the component is a semiconductor. The 2- or 3-digit
number following the N is the manufacturer's ID number. If the last number is followed by
a letter, it indicates a later, improved version of the device. For example, a semiconductor
designated as type 2N 130A signifies a three-element transistor of semiconductor material
that is an improved version of type 130.
2-91. You may also find other markings on transistors that do not relate to the JAN
marking system. These markings are manufacturers IDs and may not conform to a
standardized system. If in doubt, always replace a transistor with one having identical
markings. To ensure that an identical replacement or a correct substitute is used, consult an
equipment or transistor manual for specifications on the transistor.
2-92. Transistors, unlike electron tubes, are very rugged and are expected to be relatively
trouble free. Encapsulation and conformal coating techniques now in use promise
extremely long life expectancies. In theory, a transistor should last indefinitely. However,
if transistors are subjected to current overloads, the junctions will be damaged or even
destroyed. The application of excessively high operating voltages can also damage or
destroy the junctions through arc-over or excessive reverse currents. One of the greatest
dangers to the transistor is heat, which will cause excessive current flow and eventual
destruction of the transistor.
2-93. To determine if a transistor is good or bad, you can check it with an ohmmeter or a
transistor tester. In many cases you can substitute a transistor known to be good for one
that is questionable to determine the condition of a suspected transistor. This method of
testing is highly accurate and sometimes the quickest. However, use this method only after
you are sure that there are no circuit defects that might damage the replacement transistor.
If more than one defective transistor is found in the equipment where the trouble has been
localized, this testing method becomes cumbersome, as several transistors may have to be
replaced before the trouble is corrected. To determine which stages failed and which
transistors are not defective, all the removed transistors must be tested. You can perform
this test by using a standard Army ohmmeter, a transistor tester, or by observing whether
the equipment operates correctly as each of the removed transistors is reinserted into the
equipment. A word of caution, avoid randomly substituting transistors in critical circuits.
When transistors are soldered into equipment, substitution is not practicable; it is generally
desirable to test these transistors in their circuits.
2-94. Transistors, although generally more rugged mechanically than electron tubes, are
23 June 2005