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Polarity Test
The 14-15 winding of T2 at both stations couples
the incoming speech to the tapped windings of T2,
To test the polarity of the unit, operate polarity test
which include volume controls S25. Thus, the incoming
switch S214 (fig. 7-30) to the OK WHEN LIT position
signals appear across terminal 7 of T2 and the moving
(not shown). Indicator lamp 1210 should light with full
contact of volume control S25 at both stations. These
intensity if the polarity is correct. Now operate polarity
signal sources are connected in series addition through
test switch S214 to the REVERSED position (not
a closed loop containing both loudspeakers.
shown). The indicator lamp should go out if the polarity
The circuit is from the arm of S25 at station 3,
is not correct. The lamp may glow faintly, but it is not
contact 1-2 of S26, the loudspeaker, contact 4-5 of S26,
important. The polarity test checks the polarity of the
terminals Y4 and Y3, contact 5-6 of K1, terminal Y2
line and signal voltage windings (terminals 10-11 and
over line MC3Y21 to terminal Y1 of station 3A,
8-9, respectively) of the power transformer, T3.
terminal 7 of T2, the arm of S25, contact 1-2 of S26, the
loudspeaker, contact 4-5 of S26, to terminals Y4-Y3,
Call Lamp Test
contact 5-6 of K1, terminal Y2, over line MC3Y12,
terminal Y 1 in station 3, to complete the circuit at
To test the call lamp of the unit, operate call lamp
terminal 7 of T2.
test switch S212 on the test fixture. Call lamp 12 on the
unit under test should be lighted.
The volume at both stations will be the same and
can be controlled by either volume control S25. Both
Amplifier and Reproducer Test
volume controls, however, should be kept at the same
setting.
To test the amplifier and reproducer, depress the talk
If talk relay K1 is operated at either station, the input
switch microphone and talk into the microphone. The
to the audio circuit will be open for both stations.
talker should hear his/her voice clearly through the
reproducer. Rotate volume control knob S25 on the unit
under test while talking into the microphone, and
MAINTENANCE
observe the effect on the output volume. Now place the
microphone close to the reproducer. A microphone
A test fixture is provided with the maintenance parts
feedback should be observed when the volume control
of the equipment to facilitate testing the intercom units.
is in the full-volume position as well as at one step below
The test fixture is housed in a metal case and includes
full volume. This test provides a rough indication of the
the necessary switches, resistors, and controls to
amplifier gain, power output, and the general
perform all essential tests on a unit. It is provided with
performance of the entire unit, except for the signaling
a line cord and plug for connection to the ship's 115-volt,
circuits.
60-Hz power supply, and suitable female connectors for
attaching it to the rear of the unit under test. The front
Station Selector Circuit Test
cover contains 11 double-pole, double-throw (DPDT)
test switches, S201 through S211, a single-pole,
On the test fixture (fig. 7-30), operate talk test
single-throw (SPST) call lamp test switch, S212, an
switch S213 to the TALK position with the microphone
SPST talk test switch, S213, a DPDT polarity test
reasonably close to the reproducer to produce a
switch, S214, and an indicator lamp, I201. A wiring
microphonics howl. Reduce the volume control to the
diagram of a test fixture is illustrated in figure 7-30.
minimum position at which the howl can still be
To use the test fixture, remove the intercom unit to
obtained by moving the microphone as close to the
reproducer as required. This position will produce the
be tested from its case. Attach the test fixture to the rear
minimum objectionable howl during the subsequent
of the unit by plugging it into the unit. Connect the line
cord and plug to the ship's 115-volt, 60-Hz power. On
station selector circuit tests.
the test fixture, operate talk switch S213 to the OFF
When test switch S201 is in the TEST position, it
position and the 11 test switches, S201 through S211, to
places a short circuit across terminal 1 and 1C to
the STANDBY position. On the unit under test, depress
interrupt the microphonics howl.
release push button S1, turn volume control S25 and
On the unit under test, depress station selector push
dimmer control S27 to the extreme clockwise positions,
button S2 (adjacent to release push button S1). On the
and connect a microphone or handset into microphone
test fixture, operate test switch S202 to the STANDBY
jack J6.
7-30

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