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KEYER MODULE A9.
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TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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TEST POINTS - TM-11-5820-815-140097


TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
4-298. With the squelch turned on and the rf receiver
developed across R14 and R15 is applied to keyer
signal level below the squelch threshold, the output of
comparator U2 through the resistive divider formed by
A4U5B is a negative dc voltage of approximately -10 V
resistors R16 and R17 and the approximate 4890-ohm
dc. This negative voltage is applied through resistors
equivalent resistance of the reference voltage resistor
A4R78 and A9R3 to the non-inverting input of A9U1
network. The resistive divider attenuates the voltage by
causing the output of A9U1 to be negative (about -0.6 V
about -26 dB to set the keying threshold at about 1.57
dc). The negative voltage turns A9Q1 off to de-energize
volts.  This represents a current flow of about 14
A9K1.
milliamperes through resistors R14 and R15.
`
4-299.  KEYER  CIRCUIT.  Refer  to figure FO-32.
4-302. For audio loop keying, the audio tone is applied
Three resistive divider networks attenuate the three loop
to the input of U2 through resistive divider R20-R21-R22-
input signals and apply the attenuated signals to keyer
R23 which attenuates the input signal by about -'5.9 dB.
comparator U2.  Under normal use, only one loop is
Positive audio peaks which exceed the reference voltage
employed at any one time.  Keyer comparator U2
(about 109 mV rms at the audio loop input) are amplified
compares the attenuated input loop signal to a reference
by U2 to produce a square- wave output. Diode CR3
voltage (about 0.078 V dc) applied across the differential
rectifies and capacitor C3 filters the square wave to
input of U2.  The reference voltage is developed by
produce a dc voltage that turns keyer transistor Q2 on.
voltage divider action of 12 V dc applied through
For VOX operation, strapping of E7 to E8 adds capacitor
resistors R18 through R24 and is negative with respect
C4 in parallel with capacitor C3.  Capacitor C4 and
to the plus-to-minus input of U2. Attenuated input loop
resistors R25 and R26 determine the keyer attack and
signals greater than the reference voltage cause the
release times. When the output of keyer comparator U2
output of U2 to go positive (about +3.2 V dc).  This
goes positive, the charge time of C4 through diode CR3
provides base current to keyer transistor Q2 causing it to
and the output impedance of U2 set the attack time at
turn on.  When the AN/GRC-171 is operating in the
less than 10 milliseconds. When the output of U2 goes
remote mode, turned-on keyer transistor Q2 applies
negative, CR3 is back biased.  This causes C4 to
aground through REMOTE/LOCAL switch AlOAlS3 to the
discharge through R25 and R26 and the base resistance
ptt key line (figure FO-16) to key the transmitter.
of Q2 to set the release time at about 1.25 seconds.
Attenuated input loop signals less than the reference
voltage cause the output of U2 to be negative (about -0.6
4-303. Zener diodes VR2, VR3, VR4, and VR5 limit the
V dc). This causes keyer transistor Q2 to turn off and
input voltage to keyer comparator U2 to about 4 volts.
remove the ground from the ptt key line to
This provides circuit protection against voltage spikes
unkey the transmitter.
that may be conducted into the radio set on the voltage
loop, current loop, or audio loop input lines.
4-300.  For voltage loop keying, input loop voltage is
applied to the input of keyer comparator U2 through the
4-304. CHASSIS A10.
resistive divider formed by resistors R7, R8, R12, and
R13 and the equivalent resistance of the reference
4-305. Refer to the schematic diagram of figure FO-33.
voltage resistor network (resistors R18, R19, R20, and
Chassis A10 contains connectors and wiring that
R21  in  parallel  with  resistors  R22  and  R23,
interconnect the nine modules of the receiver-transmitter
approximately 4890 ohms). The attenuation ratio of the
and provide connections through the emi filter (A10A3) to
resistive divider determines the input level (threshold) at
the rear panel power connector (J7), control connector
which keying occurs.  For 6-voltkeying ( no strap),
(J6), and input/output connector (J22). Also contained
resistors R12 andR13 are test selected so that the
on chassis A10 are power supply components and filter
resistive divider attenuates the input loop voltage by
networks that form part of the power distribution circuit of
about -35 dB to set the keying threshold between 4 and 5
the receiver-transmitter.  The front panel (A01A1) of
V dc (4.4 V dc nominal). For 26-volt keying, strapping E5
chassis A10 contains controls and indicators that are
to E6 adds R11 to the resistive divider. This changes the
used to setup and locally operate and maintain the radio
attenuation of the resistive divider to about -47 dB to set
set. With the exception of the emi filter and the front
the keying threshold at about 18 V dc.  For 48-volt
panel meter and test points, all circuits of chassis A10
keying, strapping E3 to E4 (strap E5 to E6 removed)
have been previously discussed and will not be repeated
adds R10 to the resistive divider.  This changes the
here.
attenuation of the resistive divider to about -52 dB to set
the keying threshold at about 30 V dc.  For 100-volt
4-306. EMI FILTER. The emi filter (A10A3 of figure FO-
keying, strapping E1 to E2 (strap E3 to E4 removed)
33) provides filtering for all wires that enter and leave the
adds R9 to the resistive divider.  This changes the
receiver-transmitter. This prevents internal interference
attenuation of the resistive divider to about -56 dB to set
from being conducted into the
the keying threshold at about 49 V dc.
4-301. For current loop keying, input loop current is
applied through resistors R14 and R15.  The voltage
4-51


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