Chassis Wiring

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