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Antenna VSWR and PA/Antenna Power Temperature Monitor
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TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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CHASSIS A10.


TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
temperatures, the negative voltage developed across
A8RT1 and applied to the noninverting input of U7B is
and a transmitter keyer circuit. The squelch operated
more negative than the voltage developed across
relay circuit provides a set of spdt relay contacts at the
resistor R79 and applied to the inverting input of U7B.
receiver-transmitter input/output connector J22 for use
This causes the output of U7B to be negative. As the
external to the AN/GRC-171.  The squelch relay is
temperature  increases,  the  resistance  of  A8RT1
activated whenever the receiver is not muted by squelch
decreases causing the voltage across it to decrease
or when squelch is turned off. Keying the transmitter
also. When the heat-sink temperature at the rf amplifier
inhibits the squelch relay from being activated.
transistors exceeds about 100 degrees Celsius, the
voltage across A8RT1 becomes less negative than the
4-294. The keyer circuit provides interface between the
voltage applied to the inverting input of U7B.  This
transmitter key line and external audio loop, current loop,
causes the output of U7B to go positive (approximately
or voltage loop type push-to-talk or keying circuits. For
10 V dc). To prevent the detector from tripping more
audio loop keying, the transmitter can be keyed by a
than once at threshold, resistive divider R75-R76 applies
300- to 3000-Hz tone at a level of -10 to 0 dB mW. An
a  positive  voltage  through  resistor  R77  to  the
optional jumper on keyer module A9 provides for voice
noninverting input of U7B. This provides hysteresis so
operated keying (VOX) through the audio loop.  For
that the temperature has to decrease by a certain
current loop keying, the transmitter can be keyed by a
amount before the voltage across A8RT1 becomes more
20- to 60-milliampere dc current loop. For voltage loop
negative than the voltage applied to the inverting input of
keying, the transmitter can be keyed by external push-to-
U7B.
talk or keying circuits that employ 6-, or optional 26-, 28-,
or 100-V dc keying voltages. Strapping options on keyer
4-289.  When an antenna vswr fault, or pa/antenna
module A9 allow for selection of one of the voltage
power fault, or a heat-sink temperature of over 100
levels.
degrees Celsius is detected, the voltage at the cathodes
of diodes CR2 and CR3 goes to a positive level. This
4-295. SQUELCH OPERATED RELAY CIRCUIT. Refer
causes the output of the pa power turndown switch, U2A,
to figures 4-7 and FO-32.  Squelch comparator A9U1
to be low (ground) to reduce the power output from the rf
compares the voltage applied to the squelch on/ off line
power amplifier module.  Otherwise, for no fault, the
to a bias voltage (about -0.26 V dc) developed across
voltage at the cathodes of CR2 and CR3 is about 0 V dc.
resistor A9R2 of voltage divider A9R1-A9R2.  The
This causes the output of U2A to be high (open) for
voltage applied to the squelch on/off input is dependent
normal power output.
upon the squelch mode of the receiver which can be one
of the following: squelch off, squelch on and the rf input
4-290. Refer to paragraph 4-272 for a description of the
signal level to the receiver below the squelch threshold,
envelope feedback control switch, U2F-U2D.
or squelch on and the rf input signal above the squelch
threshold.  With squelch off (ground applied to the
4-291.  The one-shot, U3B, stretches a momentary
squelch on/off input), the bias applied to the inverting
antenna vswr fault or pa/antenna power fault into
input of A9U1 causes the output of A9U1 to be positive
approximately a 300-millisecond pulse.
When a
(about +3.2 V dc). The positive voltage turns squelch
momentary fault is detected, the output of U2A goes to
relay driver transistor A9Q1 on to energize squelch relay
ground. This allows capacitor C5 to discharge to ground
A9K1.
through U2A and develop a negative voltage across
resistor R25. When applied to the inverting input of U3B,
4-296. With the squelch turned on and an rf receive
the negative voltage causes the output of U3B to go high
signal level above the squelch threshold as set by
(about 10 V dc) and develop a positive voltage (about 2.3
squelch potentiometer A10A1R4, the output of squelch
V dc) across resistor R29 of voltage divider R28-R29.
control A4U5B is a positive dc voltage of approximately
Diode CR1 applies the positive voltage to the input of
10 V dc. (Refer to paragraph 4-175 for a discussion of
U2A to hold the output of U2A at ground. At the same
the squelch control circuit.) This positive voltage is
time, capacitor C4 begins to charge toward the voltage
applied through A4R78 and A9R3 to the noninverting
across R29.  The positive voltage developed across
input to A9U1 causing the output of A9U1 to be positive
resistor R23 as C4 charges holds the output of U3B high
to energize relay A9K1. Diode A9CR1 limits the positive
as long as the voltage exceeds an approximate +0.54-V
voltage applied to the input of A9U1 to about 0.7 V dc.
dc bias voltage developed across resistor R25 of voltage
divider R26-R25. The RC time constant of C4 and R23
4-297.  Under the above conditions with relay A9K1
determines the length of time the output of U3B is held
energized, keying the transmitter applies a ground (key 1
high
input) through diode A9CR4 to the base of A9Q1. The
ground turns A9Q1 off to deenergize A1K1 whenever the
4-292. KEYER MODULE A9.
transmitter is keyed. Diode A9CR5 biases the emitter of
A9Q1 one diode drop above ground to ensure A9Q1 will
4-293. GENERAL. Refer to figure FO-32. The keyer
turn off when a ground is applied to key 1 input.
module is composed of a squelch operated relay circuit
4-50


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