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packaged functional assembly of wired electronic
can compensate quickly for the efficiency of the horn
components for use with other such assemblies. A
being greater than the cone's (fig. 4-18, view C).
modular assembly is constructed with standardized
Suppose you want to get as much power out of a horn
as the cone is delivering, and the cone is tied to the 5-watt
units or modules. An equipment that consists of
replaceable assemblies (any type) is said to be of
tap of its transformer. Tying the horn to the 1-watt tap
unitized construction. Modular construction is a type of
of its transformer would take into account the 5:1
unitized construction that consists of modular
efficiency ratio. Now you can make the horn twice as
loud as the cone by stepping up its tapped position to the
assemblies.
2-watt level.
For example, think of a carton of cigarettes. If each
There is a limit to how many speakers you can tie
pack were a module, the carton would be an equipment
of modular construction. Notice that the packs can be
across the line in this system, a limit that holds for the
arranged differently without changing the outside
transformerless matched system as well. The total load
dimensions of the carton. Although the cigarette packs
on the system by all the speaker transformers and the
are all the same size, the assemblies in many pieces of
speakers should not exceed the total power capacity of
equipment are not.
the amplifier. In the transformer system, you find the
limit by adding the rating capacities of the transformers.
By their original concept, many modular assemblies
In the matched-impedance system, you must measure
were not to be maintained in the field. The intention was
the voltage across voltages to power values, then add
to replace a faulty assembly and ship it to a repair
them up.
facility. As assemblies became more complex, the point
On a constant-voltage line, what do you do with a
was reached where the supply system required for the
transformer that is marked in impedance with no
replacement was too costly. Many of the original
mention of power? Using the formula,
modules were potted to discourage maintenance
personnel from tampering with the insides.
When the Navy reassessed this concept and called
for shipboard maintenance of as much equipment as
possible, most manufacturers began to make
convert the primary impedance of the transformer to
components accessible. However, it is difficult to dispel
power. As an example, suppose the transformer has a
the conviction that modular assemblies are impossible
primary marked 167 ohms and a secondary marked 8
to repair. This conviction may stem from a lack of
ohms. How much power does this transformer take from
experience in working with the printed circuits and the
the 70.7-volt line for its 8-ohm speaker load?
other components in modular assemblies. It is true that
Solution:
special tools and techniques are required. It is also true
that satisfactory repairs can be made to any printed
circuit by using a little care and common sense. With a
little experience, repairs can be made as easily as in
conventional assemblies, and often more easily because
of improved accessibility.
MODULAR UNITS
REPAIR OF DEFECTIVE COMPONENTS
The demand for small, maintainable circuitry in
military equipment has led to many different
One of the time-consuming steps of troubleshooting
construction techniques. One of the most popular is
is the identification of specific components. In
modular construction. Since modular assemblies
conventionally wired equipment, components are not
incorporate several subminiaturized features not found
always easy to locate. The circuitry in the chassis can
in conventional equipment, some specialized
become confusing since related components are often
knowledge and tools are required for efficient repair and
positioned in different areas of the chassis.
maintenance.
In equipment that includes printed-circuit boards,
MAINTENANCE CONCEPT
identification of circuitry and components may be
relatively simple. This type of circuit construction
A few definitions are helpful in understanding the
allows uniform placement of components. Just a quick,
terms involved. A module is, in the electronic sense, a
once-over glance of the circuitry is often all you need to
4-19

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