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 TM 55-1905-220-14-9
4-48. ALARM SWITCHBOARD - MAINTENANCE INSTRUCITONS (Continued).
(c) The flashing operates on the principle of a relaxation oscillator. Upon closing the switch
contacts, the D.C. voltage slowly charges the capacitor, C1, through the resistor, R1. When the ignition voltage
is reached the lamp will flash, discharging the capacitor, at which time a charge will again build up on the
capacitor causing the lamp to flash again. This cycle will repeat itself as long as the switch is in either the
"Silent Alarm Test" or the "Silent Trouble Test" position.
(d) With the silent alarm test switch in the "Silent Alarm Test" position, the individual circuits are
tested by throwing the associated line circuit test switch to the "Test" position. The operation of the circuit test
switch short-circuits the line and, as a result, the current flow is increased just as it would have been had the
contact maker shorted its supervisory resistor. The alarm target relay now becomes fully operated, displays its
red target and completes the circuit and illuminates the "Alarm Test" lamp, PL2.
(e) The "Alarm Test" lamp, PL2, circuit starts with the source of supply at terminal "L1", going
through the "Alarm Test" lamp, PL2, continuing to the "make contacts" of the silent alarm test switch, S,
through the "make contacts" of the alarm target relay, back to terminal "2F" of the two line alarm unit,
continuing to terminal "2F" on the alarm switchboard, then to terminal "L2", the other side of the source of
supply. The source of supply is 120V 60 cycle single phase A.C. originating from the local machinery space of
the interior communication switchboard.
(4)
Silent Trouble Test.
(a) For this test the silent alarm test switch, S, is operated to the "Silent Trouble Test" position.
Throwing the switch cuts the trouble buzzer, Z, out of the circuit and connects the "Trouble Test" light, PL3, into
the circuit. The "Test Light", PL1, will flash as in the "Silent Alarm Test".
(b) When the silent alarm test switch, S, is in the "Silent Trouble Test" position each supervisory
circuit is tested by throwing the line circuit test switch SLOWLY from "Normal" to "Off" position. Moving this
switch slowly will show the yellow target on the unit panel directly above the "Off" circuit test switch and the
"Trouble Test" lamp, PL3, on the alarm panel will flash momentarily. The yellow target associated with the
circuit will show as long as the switch is left in the "Off" position, but the "Trouble Test" lamp will darken as soon
as the switch is fully operated.
(c) If the yellow targets do not show, the circuit is out of order and this condition may be due to
one of several reasons.
4-1233
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