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Apply a signal from test equipment by starting
see to place the overall layout of the chassis in your mind
with a low output signal setting, then proceeding to the
and to quickly focus your attention on the area of
required signal level. Be sure the signal applied is below
the rating given for the circuit under test.
Many commercial manufacturers have developed
Make sure all parts in a circuit are secure before
methods for quick identification. One of the most
starting to test the circuit or turning on the equipment
common is to impose a grid over a drawing of the board
power. Do not cause an inductive kickback or transient
and a corresponding table that lists the part location.
current by moving loose connections, disconnecting
Another technique is to number points of interest on the
parts, inserting or removing transistors or similar
schematic, then provide a guide to locate the points on
components, or changing modular units while the
the board.
equipment power is on or under test.
Circuit tracing of the printed-circuit board maybe
Remove all capacitance charges from parts and
simpler than that of conventional wiring, due to
test equipment before attaching them to a modular
increased uniformity. If the circuit board is translucent,
a 60-watt light bulb placed under the side being traced
will simplify circuit tracing. However, be careful not to
overheat delicate circuit components. In this way, you
can locate test points without viewing both sides of the
If a specified part cannot be obtained a suitable
substitute part may be used temporarily, provided the
substitute is replaced with the specified part as soon as
Resistance or continuity measurements can be made
it can be obtained. Make sure the failed part is the source
from the component side of the board. In some cases, a
of trouble and not the symptom of another failure.
magnifying glass will help in locating very small breaks
in the wiring. Voltage measurements can be made on
either side of the board. However, a needlepoint probe
is needed to penetrate the protective coating on the
The use of two or more resistors or capacitors in
wiring. You also can locate hairline cracks by making
either series or parallel combinations to replace a
continuity checks.
burned-out component is a common practice.
As a supervisor, you should make sure the people in
When the proper size resistor or capacitor is not
your shop or maintenance crews observe the following
available, check your spare parts drawer to see if you
can use what you have on hand in a series or parallel
Observe power supply polarities when measuring
the resistance of the circuits containing transistors or
Tubes and Transistors
other semiconductors. Such parts are polarity and
voltage sensitive. Reversing the plate voltage polarity of
Substitution for defective tubes or transistors is
a triode vacuum tube will keep the stage from operating,
usually an easy task. By consulting the proper
but generally will not injure the tube. Reversing the
substitution manual, and using the specifications for the
voltage applied to a transistor or other semiconductor
tube or transistor you wish to replace, you can normally
will ruin it very quickly.
locate an acceptable substitute. Be certain to match the
minimum specifications for the tube or transistor that
CAUTION: Ohmmeter voltage for checking
you are trying to replace.
transistors and their circuits should be 3 volts or less.
Make certain power line leakage current is not
Relays and Switches
excessive before applying ac power-operated test
equipment or soldering irons. Use an isolation
When replacing a relay, switch, or similar control
transformer with all test equipment and soldering irons
device, be sure to consult the proper instruction book or
operated on ac power, unless the equipment contains a
blueprint to check the voltage and current ratings before
transformer in its power supply or shows no leakage
you start looking for a substitute. When replacing a
current. With all test equipment (whether transformer
relay, also consider the resistance of the coil and the type
operated or not), connect a ground lead from the ground
and arrangement of the contacts. It may also be possible
to make the replacement part yourself or do something
of the circuit to be tested to the test equipment ground.

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