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LOW-PASS FILTER (U1B)
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TM-11-5820-815-14 Service and Circuit Diagrams RADIO SET AN/GRC-171 Manual
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COMPRESSION


TO 31R2-2GRC171-2
TM 11-5820-815-14
NAVELEX 0967-LP-544-5010
variable resistor A10A1R2 are connected to give fine
removed and strap E13 to E10 is added. This removes
control for high percentages of modulation and course
keyer driver transistor Q4 from the remote ptt keyer
control for low percentages of modulation.  Resistor
circuit and adds a voltage divider composed of resistor
A10A1R6 in parallel with A10A1R7 provides a nearly
R109 (P1-C) or R110 (P1-D) in series with zener diode
constant load to the audio signal from the audio module.
VR7 and resistors R40 and R41.  For this strapping
The variable audio output from the modulation control is
option, ptt keyer transistor Q5 turns on whenever the
applied to the clipper and low-pass filter circuits (A4P1-
keying voltage applied to pin C or pin D of P1 exceeds
14/A10J13-14) of audio module A4.
about 4.6 volts.
4-155. CLIPPER AND LOW-PASS FILTER (U7A). To
4-159.  Optional 26-Volt Remote Keying.  For 26-volt
prevent overmodulation of the transmitter, the clipper
keying, straps E4 to E5. E6 to E7, and E8 to E9 are
(CR3 and CR4 of figure FO-21) removes excessive
removed and strap E13 to E11 is added. This removes
voltage peaks that may be present on the transmit audio
keyer driver transistor Q4 from the remote ptt keyer
signal.  The output of the clipper is applied through
circuit and adds a voltage divider composed of resistor
voltage divider R31-R32 to the low-pass filter (U7A). The
R109 or R110 in series with zener diode VR5 and
voltage divider attenuates the audio signal by a factor of
resistors R40 and R41.  For this strapping option, ptt
about 0.24 before applying the signal to the low-pass
keyer transistor Q5 turns on whenever the keying voltage
filter.
applied to pin C or pin D of P1 exceeds about 22 volts.
4-156. The low-pass filter (U7A of figure FO-21) shapes
4-160.  Optional 48-Volt Remote Keying.  For 48-volt
the high-frequency response of the wide bandwidth data
keying, straps E4 to E5, E6 to E7, and E8 to E9 are
to be transmitted. The filter is a 2-pole, active filter with
removed and strap E13 to E 12 is added. This removes
unity gain and cutoff frequency of 25 kHz. The two poles
keyer driver transistor Q4 from the remote ptt keyer
are determined by resistor-capacitor networks R33-C64
driver and adds a voltage divider composed of resistor
and R35-C65.  In the passband, the filter response
R109 or R110 in series with zener diode VR6 and
varies less than +1 or -3 dB when referenced to the
resistors R40 and R41.  For this strapping option, ptt
output at 1000 Hz. Because of the high cutoff frequency,
keyer transistor Q5 turns on whenever the keying voltage
high-pass filter U7A does not affect the frequency
applied to pin C or pin D of P1 exceeds about 35 volts.
response of the voice audio signal to be transmitted.
4-161. The remote ptt keyer output (P1-A) is connected
4-157. REMOTE PTT KEYER. The remote ptt keyer
to the REMOTE/LOCAL switch (A10A1S3 of figure FO-
(Q4 and Q5 of figure FO-21) converts external keying
16) located on the front panel of chassis A10. When in
information applied to the center tap of the main audio
the remote position, the remote ptt keying signal is
input (P1-C or P1-D) into remote ptt keying (P1-A).
applied to the ptt input (P1-K) of the keyer control circuit.
When operating in the remote mode, the remote ptt
output is applied to the keying control circuit input (P1-K).
4-162. KEYING CONTROL CIRCUIT. The keying con-
Figure FO-16 shows the interconnection between the
trol circuit (Q6, Q7, Q8, Q9, and Q10 of figure FO-21)
remote ptt keyer and the keying control circuit. Strapping
converts ptt keying into two key-line circuits (key 1 and
options on audio module A4 provide for selection of the
key 2) and one ready lamp circuit.  Key 1 key line
following types of external keying: normal ground keying
provides fast attack-slow release keying to the 30-MHz
or optional 6-, 26-, or 48-volt keying. Strapping as shown
frequency shift circuit of frequency synthesizer module
on figure FO-21 provides for ground keying. When a
A2, the receiver mute circuit of receiver rf module A3,
ground is applied (keyed) to pin C or D of P1, keyer
and transmit/receive switch in rf filter module A7. Key 2
driver transistor Q4 turns on to forward bias ptt keyer
key line provides slow attack- fast release keying to the
transistor Q5 turning it on.  When on, Q5 applies a
transmit/receive switch of frequency synthesizer module
ground to the ptt input (P1-K) of the keying control circuit
A2 and to the ALC circuit of power amplifier module A8.
to key the radio set (remote operation).  When the
In addition, when a fault is detected in the phase locked
ground is removed (unkeyed) from pin C or D of P1, +22
loop (p11 fault), or in the rf filter (rf filter fault), or in the
V dc applied through resistors R111, R36, and R37
d/a servo amplifier circuit (rf filter fault), the keying
reverse bias diodes CR1 and CR2. This prevents key-
control circuit inhibits grounding of the key 2 key line and
line noise from turning on keyer driver transistor Q4
turns off the ready lamp.
which is reverse biased by +5 V dc applied through
resistors R38 and R39. When Q4 turns off, ptt keyer
4-163. When the ptt fault (P1-L) and rf filter fault (P1-10)
transistor Q5 turns off to unkey the radio set.
inputs are both logic 1 (no fault), fault switch Q8 is on
and key 2 inhibit transistor Q9 is off. When on, fault
4-158.  Optional 6-Volt Remote Keying.  For 6-volt
switch Q8 provides a ground to the ready lamp circuit to
keying, straps E4 to E5, E6 to E7, and E8 to E9 are
turn
on
the
radio
control
4-30


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