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REPAIRING CHASSIS WIRING
(called functional block diagrams) and the overall block
diagrams (called the function description diagrams).
A major consideration in repairing chassis wiring is
Instead of only one MDC for each major function,
the amount of time you have to complete the repair and
newer manuals will have additional MDCs for the
get the equipment back in operation. If the equipment is
functional block and schematic diagrams. The MDCs
to be "down" for a long time, be sure the repair is
also will be provided with an acetate or Mylar overlay
completed correctly. As a temporary repair, it is
permissible to run a straight wire instead of following
so troubleshooters can use grease pencils to mark their
the original cable run. This temporary lead should be
progress. The size of the new SIMM will be 11 by 27
soldered in place, after you disconnect the faulty lead.
inches, instead of 15 by 35 inches. This new size gives
This practice should only be done when there is
it a folded dimension of 9 by 11 inches, the same size as
insufficient time to make the proper repairs.
conventional manuals.
When a few wires are to be replaced, lace them to
the outside of the existing cable run. When many wires
CHASSIS WIRING
are to be replaced, remove the damaged wiring and
re-run the new wires along the route of the old cable run.
Chassis wiring is loosely defined as the wire or
After connecting the wires, you should lace or clamp
wires installed in an equipment cabinet that interconnect
them in their original positions.
the assemblies of the cabinet. Chassis wiring varies in
You should avoid unnecessary slack in the cable
application and size of run. It may range from a harness
runs and always try to keep your repairs neat and
or run of two wires, used in sound-powered amplifiers
workmanlike. A properly replaced wire or cable run also
and telephone cases, to large cable runs, such as those
will make it easier to trace later.
in MC cabinets, dial telephone switchboards, and gyro
binnacles. IC Electricians must be able to test, repair,
REPLACING CHASSIS WIRING
and replace chassis wiring, regardless of its run size and
application.
Sometimes your personnel may have to replace
chassis wiring in components of IC systems, such as
TESTING CHASSIS WIRING
21MC units. Chassis wiring is usually the last part to fail
in electronic equipment, but it can wear out from minor
In testing chassis wiring, it is not possible to
friction and abrasion in the course of making other
substitute for good common sense. Your first step in
repairs, or it can be damaged by a malfunction that
testing chassis wiring of a faulty piece of equipment
causes overheating or fire.
should always be a detailed visual inspection of the
Individual leads in chassis wiring must be replaced
wiring harness, from terminal to terminal. A complete
wire for wire. This is a tedious job, but one that requires
visual check will save you many hours of work looking
only skill with a soldering iron. A wiring harness
for trouble that you could have detected in the first place
containing damaged wires is best repaired by replacing
by inspecting the equipment for visible damage. If
it with a prefabricated harness. Prefabricating a
certain wires are obviously at fault, you should replace
replacement harness is usually easier than repairing
them before going any further in the test.
individual wires in the old harness. An installed harness
is hard to work on, and soldering on it may damage other
Your sense of smell can help in pinpointing burned
wiring insulation or electronic components. If the unit
or damaged wire that you may not see when making the
to be repaired is still in working order, the replacement
visual inspection. Most burned electrical insulation will
harness can be made, and then installed whenever it is
give off a noticeable, unpleasant odor that is readily
convenient to have the unit off the line.
detectable. The location of a burned wire, however, does
Tee first step in making a wiring harness is to make
not necessarily reveal the cause of the trouble.
a wiring form or jig, as shown in figure 4-9. Build the
Signal tracing is the most reliable method of
form on a sheet of 1/4-inch plywood, using small (6d)
locating shorts, opens, or grounds in chassis wiring. The
finishing nails. Using a plastic ruler, measure the
best method of making continuity tests for shorts and
original harness and chassis in the unit to be repaired.
grounds in chassis wiring calls for the use of a
Your measurements should let you outline the chassis
multimeter and manufacturers' instruction books.
and the location of the harness run and mark the
4-10

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