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 TM 55-1905-220-14-9
4-48. ALARM SWITCHBOARD - MAINTENANCE INSTRUCITONS (Continued).
contacts above the coil will close when the line circuit test switch is placed in the alarm test position. If the
armature does not close, it may be due to dirt or foreign matter on the pivots of the armature and target
assembly, between the target drum and the drum mount, or on the drag link between the armature and the
target drum. It may also be due to a dirty "make" contact or poor adjustment of the "make" contact of the line
circuit test switch when in the alarm test position.
(d) Second, if cleaning and freeing the armature and target assembly does not cause the
armature to close when the line circuit test switch is thrown to the test position, and the circuit is in all other
respects normal, the coil of the alarm target relay is defective and should be replaced with a new one.
(e) Third, if the alarm target relay armature is in good order, failure of the "Alarm Test" lamp, PL2,
to glow may be due to poor con- tact in the make springs of the alarm target relay. These springs must be
adjusted to make good contact.
(f) Fourth, if the alarm target relay and the line circuit and silent test switches are in good order,
the trouble may be due to loose connections in any of the following places: the connections on the line circuit
test switch or either connection of the alarm test lamp.
NOTE
When the silent alarm test switch, S, is placed in the "Silent Alarm Test"
position, the extension signal relay, K, controlling the alarm bell, B, is
disconnected from the circuit, and the "Alarm Test" light, PL2, is operated
directly from the contact of the alarm target relay.
(3)
Description of Silent Alarm Test Circuits.
(a) When the silent alarm test switch, S, is placed in the "Silent Alarm Test" position, the switch
opens the circuit to the extension signal relay, K, and holds it open so that the relay and alarm bell, B, will not
operate while the alarm switchboard is being tested.
(b) The operation of the silent alarm test switch, S, also closes the circuit for the "Test Light",
PL1, from the positive (red) terminal of the rectifier, CR1, to a 2 mfd capacitor, C1. The other side of the
capacitor is connected to a .75 meg resistor, R1, and through the make contacts of the silent alarm test switch
to the negative rectifier terminal. The test light which is an NE79 neon lamp is connected across the 2 mfd
capacitor, C1. The lamp will flash with the silent alarm test switch in either silent test position, serving as a
warning that one audible signal or the other is out of service.
4-1232
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