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Table D-2. List of Test Frequencies
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SB-740-5820-91-006 Radio Set AN/ARC-51BX Manual
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Figure 1. Equipment and test units for making power output and sidetonr measurements.


If the voltage does not rise adjust A3R31 on the third
oscillator for an output of 1000 Hz and a signal level
IF amplifier to achieve the above results. The ad-
of 2.73 volts (17 DBM) at test point "A" as read on
justment of the main receiver AVC is now complete.
the VTVM. Connect the VTVM to test point "B" the
voltage shall read 0.39 volts (ODBM). Key the trans-
(d) Adjust the signal generator for 1,000 micro-
volt 243.00 MHz modulated 30 percent at 1,000 Hz,
mitter by operating the CW switch and adjust the
and with the switches of the control box set at GD
modulation gain control R20, located on the modulat-
XMIT and TR + G position, adjust A4R20 on the audio
or and audio subassembly lA4, to the point at which
modulator for 100 milliwatts across 150 ohms. The
the modulation pattern just reaches the clipping
audio level of the guard receiver is now set.
level as viewed on the oscilloscope OS-8C/U.
(e) Adjust the signal generator for 1.8 to 2.2
(b) Refer to figure 1 and connect the test
microwatt output modulated 30 percent at 1,000
equipment as shown. Set the function switch on the
Hz and re-peak the signal generator to 243 MHz.
control unit to T/R+G position and allow the equip-
Slowly turn A7R51 on the guard receiver to open
ment to warm up for at least 5 minutes. Adjust the
the squelch. The squelch level of the guard receiver
audio oscillator for an output of 1000 Hz and a
is now set.
signal level of 0.8 volts. With the control head fre-
(f) Adjust A3R46 on the guard receiver so that
quency set at 304.7 MHz key the transmitter by
operating the CW switch. The modulation meter
the audio output as read on the audio wattmeter TS-
shall read between 80 percent to 95 percent. If the
585 D/U does not vary more than 3 db when the
modulation meter does not read these percentages
signal generator is varied from 10 to 100,000 micro-
volt. Adjustment of guard receiver AVC is now
adjust A4Pl on the modulator and audio module for
complete.
approximately 85 percent modulation.
(10) Sensitivity tests. With the equipment con-
(8) Transient protection circuit. To check the
nected as shown in figure 3, adjust the "SENS"
transient protection circuit when testing the RT-
742B/ARC-51BX, connect a lK to 1.5K ohms, l-watt
and/or "VOL" knobs of the control box to the extreme
clockwise positions. Modulate the signal generator
resistor from ground to the top of the power diode
CR2, main frame, under the power amplifier and
30 percent at 1,000 Hz. Adjust the signal level of the
listen for the sound of the blower motor lugging. If
signal generator to 4.2 microvolt output to the main
receiver or 5.0 microvolt to the guard receiver. The
it lugs, the transient protection circuit is good. If it
resulting receiver audio output shall be at least 14
does not, reject the radio set as Q1 is shorted.
milliwatts and the signal-plus-noise to noise ratio
(9) Receiver tests. Unless otherwise specified
shall be at least 10 db. If these results are not ob-
herein, the receiver tests shall include those to be
tained, check the receiver sensitivity resistor A3R34
made at approximately each of the extreme and mid-
making sure it is adjusted maximum clockwise and
point frequencies available and also the guard chan-
repeat the above tests. Reject the equipment if the
nel frequency. All tests should be made using manual
required test results are not obtained.
control C-4677/ARC-51 X or preset control C-6287/
(11) Squelch tests.
ARC-51BX, set to the T/R + G position and given at
(a) Main receiver tests. With the equipment
least a 5 minute warm up period. With the equip-
connected as shown in figure 3, adjust the signal
ment connected as shown in figure 3 the following
generator for minimum output. Check that the
alignment procedure will be used when bench test-
squelch opens and closes with operation of the SQ
ing the RT-742B/ARC-51BX.
DISABLE switch on the control box. With the SQ
(a) Adjust the signal generator for a 1,000
DISABLE switch in the OFF position, increase the
microvolt input modulated 30 percent at 1,000 Hz.
output of the signal generator to the point which
With a frequency of 304.7 MHz adjust A4R21 on the
just opens the squelch. The signal level shall be
audio modulator for 100 milliwatts output across
between 1 and 3 microvolt. If these results cannot
150 ohms impedance. The audio level for the main
be obtained, adjust the signal generator output to
receiver is now set.
the level which produces a 6 db signal-plus-noise
(b) Adjust the signal generator for 1.5 to 2
to noise ratio and adjust the receiver squelch resistor
microvolts output modulated 30 percent at 1,000
A3R50 for sqelch threshold. Reject the equipment
Hz and re-peak the generator to 304.7 MHz. Slowly
if this adjustment cannot be made.
turn A3R50 on 3rd IF amplifier until squelch opens
(b) Guard receiver tests. With the equipment
(threshold). Squelch level of the main receiver s
connected as shown in figure 3, adjust the signal
now set.
generator frequency to the receiver guard channel
(c) Connect the VTVM to measure DC volts at
frequency (243.00 MHz). With the main receiver
AlJ2 of the RF preamplifier. Increase the output
squelch operative and the signal generator adjusted
of the signal generator until the voltage begins to
for minimum output, the receiver audio output
rise. This shall occur between 50 and 150 microvolt.
8


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