let us use figure 4-1 and assume that the technician wants to check the
circuit labeled L3. One end of the circuit is at terminal B of P4101.
The technician places one probe of the ohmmeter on terminal B and the
other probe on pin 3 of terminal strip E4002.
He then reads the
ohmmeter. If the circuit is open (incomplete), the meter will indicate
an extremely high resistance (maximum deflection).
If the circuit is
complete, the meter will indicate the resistance of the circuit under
test. If there are no components in the circuit, the reading will be
at or near zero ohm (the resistance of the wire).
If there are any
components in the circuit, the meter indication will include the
resistance of the components in the circuit.
There are four wires
connected at terminal 3 of E4002: one from B of P4101, one from
terminal 1 of relay K4001, one from terminal 3 of blower motor B4002
(the red wire is identified as E4002-3), and the fourth, labeled L3,
from terminal 3 of E4001. Since we have already checked the continuity
between E4002-3 and P4101-B, we now move the probe from P4101-B to
B4002 (the red wire), then to K4001-1 (and when K4001 is closed, to
heater switch S4002), checking the continuity at each point. To check
the switch, place probes across S4002 and set the switch to ON.
Assuming that these circuits are good, we check between E4002-3 and
E4001-3. The final checks are made from E4001-3 to P4204-J, to P4001-
18, and to B4001, in turn.
This entire circuit has been checked for
continuity. The same procedures are used to make continuity checks of
other circuits.
The wiring diagram may be used with other types of
diagrams to help the technician select the proper points for making
other tests and measurements.
b. Cabling Diagrams.
Many communication and electronics sets used by the
military are composed of several units connected by multicolored cables to form an
operating unit. These cables carry signal, power, and control voltages between the
various units that form the complete installation.
The several units may be in
close proximity or may be separated by many feet or yards.
(1) The technician must often install the communication equipment himself.
When making the installation, he follows the cabling diagrams to assure
that each cable is properly connected.
A cabling diagram shows the
cables required to connect the various units of the installation.
Differing in length, diameter, and number of conductors, the
construction of the cable is determined by the purpose for which it is
to be used. For example, the output signal from a radio transmitter is
usually carried to the antenna by a coaxial cable. This minimizes loss
and reduces noise pickup.
On the other hand, power cables carry
operating current to the transmitter. The power cables may contain one
or many pairs of wires, depending on the voltage requirements of the
transmitter.
A typical cabling diagram illustrating interunit
connections is shown in figure 4-2.
(2) In some vehicular installations, cables are difficult to inspect or
test.
The cabling diagram indicates the location of the various test
points that give access to these cables for testing purposes.
309 L4
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