Substitution of Parts

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

concern.

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,

assembly.

a 60-watt light bulb placed under the side being traced

will simplify circuit tracing. However, be careful not to

SUBSTITUTION OF PARTS

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

board.

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

RESISTORS AND CAPACITORS

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

precautions:

can use what you have on hand in a series or parallel

combination.

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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