CHASSIS A10.

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