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 TM 55-1905-219-14-9
4-47. ALARM SWITCHBOARD-MAINTENANCE INSTRUCTIONS (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.
4-1205
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