Transient protection circuit.

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.