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 TM 55-1905-220-14-9
4-48. ALARM SWITCHBOARD - MAINTENANCE INSTRUCITONS (Continued).
(b) The circuit of the common trouble buzzer, Z, may be traced from terminal "L1" to one side of
the buzzer, through the buzzer to the closed contact of silent trouble test switch, S, terminal "T" of the alarm
switchboard, terminal "T" of the two line alarm units, the closed contact of the line unit switch of each two line
alarm unit, the closed contact of each supervisory target relay, terminal "2F" of each two line alarm unit,
terminal "2F" on the alarm switchboard to terminal "L2" which is the other side of the source of supply.
(c) The circuits described in paragraph b and c are the normal operative circuits. The rest of the
circuits are for testing purposes and are described under paragraph d entitled "Routine Tests".
d. Routine Tests.
(1) The supervisory alarm system is designed to require very little attention. Almost any trouble that
may affect the system will give both an audible and a visual signal in the form of either an alarm or trouble
alarm. However, the system is so arranged that periodic tests of all circuits may be made easily and quickly
from the alarm switchboard. This procedure tests only the panel and the line unit modules. External wiring
must be checked at the panel for insulation resistance and (most important) continuity.
(2)
Silent Alarm Test.
(a) A three-position rotary silent test switch, S, is centrally mounted on the alarm panel at the top
of the alarm switchboard. Normally, this switch is maintained in the central or "Normal" position. For this test,
it is operated to the "Silent Alarm Test" position. This action disconnects the extension signal relay, K,
controlling the alarm bell, B, and connects the "Alarm Test" light, PL2, into the circuit in place of the bell.
Whenever the test switch is thrown to either silent test position, the "Test Light", PL1, on the panel will flash.
(b) In order to test the capacity of each circuit to function as an alarm: first, place the silent test
switch, S, in the "Silent Alarm Test" position, then place the circuit test switch on the two line unit in "Test"
position. The "Alarm Test" lamp will then glow if the circuit is in proper condition. To turn off the "Alarm Test"
lamp, PL2, return the station to normal.
(c) If the "Alarm Test" lamp, PL2, fails to glow, the circuit is out of order. This condition may be
due to one of several reasons. First, the armature of the alarm target relay may be binding. To check, remove
the two line unit on which the affected unit is mounted, and inspect the alarm target relay. If the relay is in good
working order, the armature will pull in against the magnet core and the
4-1231
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