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 TM 55-1905-220-14-9
4-48. ALARM SWITCHBOARD - MAINTENANCE INSTRUCTIONS (Continued).
(3)
Trouble Alarm Operation.
(a) If the trouble buzzer, Z, operates, examination of the unit panels will show one or more yellow
targets showing. This is an indication that the circuit on which the target is displayed is open and out of order
(b) The circuit in trouble should be disconnected by throwing the switch to "Off" for the same
reason that it would be disconnected for an alarm condition. When the circuit has been repaired, it can be
switched back into operation by simply returning the associated line switch to the normal position.
(4) Operation in Case of Grounds. Positive and negative ground detector lights are provided on the
alarm panel at the top section of the alarm switchboard. Under normal conditions both ground lamps are
darkened. If either the positive or negative ground detector lamp glows, this is an indication of a ground in the +
or - side of the power supply. For removal of these grounds, see paragraph e "Grounds". ANY
REPLACEMENT LAMPS MUST BE AGED. To age lamps, operate 24 hours at 180 volts A.C. and then 12
hours at 100 volts A.C.
c.
Description of Operating Circuits.
(1) The electrical operation of the supervisory alarm system may be understood by a study of the
wiring diagrams(Foldout 1). These diagrams show the complete wiring of the alarm switchboard and the wiring
for a two line alarm unit. Other line alarm units are wired in exactly the same manner. The number of contact
makers may be varied as required.
(2)
Supervisory circuit.
(a) Referring to the wiring diagrams (Foldout 1) the supervisory circuit may be traced from the
positive (red) terminal of the full wave selenium rectifier, through the winding of the alarm target relay, through
the line unit test switch to the "FL" side of the line circuit. The circuit enters one side of the contact maker,
through the supervisory resistor to the other side of the contact maker to the "FFL" side of the line. The circuit
continues through the line unit test switch, through the supervisory target relay coil to the negative terminal of
the rectifier.
(b) With the current flowing in this circuit as outlined, the supervisory relay is energized
sufficiently to operate its armature, the alarm relay is also energized but not sufficiently to operate its armature,
and the supervisory resistor is in the circuit holding the current down to a point where the alarm target relay will
not operate. This current is approximately .012 amperes.
4-1229
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