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TM 11-5825-270-23
and 5) to provide remote alarm monitoring provisions.
through C27 to the line transformer T1. The secondary
l. The instant relay K1 deenergizes, P1-13 of the
of T1 provides a balanced and isolated output of the
if. PCB (fig. FO-3) goes low from a normally high (+28 V)
modulation audio for use by a remote monitoring facility.
state.  The high-to-low transition at P1-13 is taken
Connections from T1 secondary are made through pins
through C23 to set the flip-flop formed by U1-A and U1-
2-4 of P1 and terminals 1-2 of TB1 at rear of the unit.
B. Under this condition, the output from U1-B is high
The maximum output from the line amplifier is 250 mW
and Q13 is turned on (TP6 low). This low is fed to the
into 600 ohms.
h. Transistors Q9 and Q10 form an emitter coupled
audio amplifier PCB (through P1-12) which generates
the 3 kHz alarm tone and drives the loudspeaker.
comparator serving as the carrier monitor. When the
m. The resetting of the aural alarm can be
carrier derived dc voltage at Q9 base is greater than the
accomplished in one of two ways:
threshold voltage at the wiper of R22, Q9 goes into
(1) If the operator presses the front panel
saturation. In other words, during normal operation, Q9
ALARM SIL switch S3, pin 15 of the if. PCB (fig. FO-3) is
is on and Q10 is off. In the event the carrier derived de
momentarily grounded.
This causes the flip-flop
voltage at Q9 base falls below the reference voltage at
U1-A/U1-B to be reset with the result that U1-B output
Q10 base, the comparator circuit changes state; Q9
goes low and Q13 is turned off. With Q13 off, the alarm
turns off and Q10 switches on. This action switches on
oscillator in the audio amplifier assembly is disabled and
Q11 because its emitter voltage is now higher than its
the aural alarm is terminated (silenced).  Note that
base voltage. At the instant Q11 switches on, the input
silencing of the aural alarm by ALARM SIL switch is
of nand gate U2-A makes a low-to-high transition and its
completely independant of the actual alarm condition in
output at TP5 assumes a low (0, +0.5 Vdc) state. As
that the ALARM light stays on, NORMAL light off, and
long as TP5 is low, the output from U2-D is high (+12 V
relay K1 remains deenergized.
0.5 V) which results in a low output from U2-C; hence
(2) The only other means of silencing the aural
pin 6 of U3 goes low.
i. Integrated circuit U3 is a programmable timer,
alarm is by removing the alarm condition itself i.e.,
resumption of normal operation. When this occurs, relay
containing an oscillator and a 16-stage binary counter.
K1 is energized and P1-14 (fig. FO-2) goes low. This
The oscillator frequency is determined by C22, R31 and
high-to-low transition is coupled through C24 to reset the
potentiometer R32. The counter is configured to provide
flip-flop U1-A/U1-B. Transistor Q13 now switches off by
a division of 65, 536 which, depending upon R32 setting
the low from U1-B and the alarm oscillator is disabled.
corresponds to a delay between 0.1 second and 60
As normal operation is now restored, NORMAL light
seconds (approximately). The counter output is taken
comes on, ALARM light goes off and relay K1 is
from U3-8 which is normally high but goes low at the
expiry of the count.  Note that the oscillator and the
n. The  signal  at  Q8  emitter  comprises  the
counter are enabled only if the master reset at U3-6 is
modulation audio and a dc voltage proportional to the
low; should U3-6 go high prior to expiry of the count, the
carrier level. In the modulation monitor circuit, formed by
counter is reset and U3-8 retains its normal high state.
j. To facilitate explanation of the carrier alarm
Q5 through Q7 and associated components, only the
modulation audio is of interest and the dc signal is
detection, disregard the output of the modulation monitor
blocked off by C15.
circuit at TP8. Also, assume that the received carrier is
o. When modulation is not present, Q5 is forward
below the threshold setting by R22. This means that U3-
biased through R11, R12 and R13; therefore, transistors
6 is low and both the oscillator and counter in U3 are
Q6 and Q7 are off. At this time the input to U1-C is low
enabled.  At the end of the count, U3-8 goes low,
resulting in a high at the output of U1-C.  When
causing Q12 to turn off. The alarm relay K1 (fig. FO-3),
modulation is present, the negative going peaks of the
which is connected between the collector of Q12 and the
modulating tone cause the voltage at the base of Q5 to
28 V bus, is therefore deenergized.
go lower than the voltage at the base of Q6, thus turning
on Q6 and Q7. Q7 in turn charges C20. Under this
The  deenergized  state  of
condition, a high is applied to U1-C, causing it to
represents an alarm condition.
produce an output. From the aforementioned, it can be
seen that U1-C makes a high-to-low transition each time
k. When relay K1 deenergizes (fig. FO-3) contacts
the modulation is applied and a low-to-high transition
when the modulation is removed.  The out-put from
4-8 close and 4-7 open. Thus, the alarm light DS3 is
U1-C, is differentiated by C21/R25 and a positive-going
switched on and normal light DS2 is turned off. Relay
K1 contacts (1, 2, and 5) are also brought to TB1 (3, 4,

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