Chapter 7 — COMMUNICATIONS EQUIPMENT AND OPERATIONAL PROCEDURES 



The calibration procedure consists essentially 

 of the following steps: 



1. Tune the receiver to a point where the 

 frequency indicator dial shows an exact multi- 

 ple of 100 kHz. 



2. Turn the ZERO ADJ knob clockwise to 

 disengage the tuning controls from the fre- 

 quency indicator. 



3. With the function switch in the CAL posi- 

 tion, turn the KILOCYCLE CHANGE control to 

 give the maximum response to the calibration 

 signal. 



4. Turn the ZERO .ADJ knob counterclock- 

 wise to reengage the tuning control to the fre- 

 quency indicator. 



TUNING CONTROLS. — Two front panel knobs 

 provide the tuning control of the R-390A. They 

 are the MEGACYCLE CHANGE knob and the 

 KILOCYCLE CHANGE knob. The MEGACYCLE 

 CHANGE knob selects any 1-mHz bandwidth of 

 the tuning range. Turning this knob changes the 

 reading of the first two digits of the frequency 

 indicator. The KILOCYCLE CHANGE knob tunes 

 the receiver to any desired frequency within 

 the megahertz band selected by the MEGACYCLE 

 CHANGE control. The last three digits of the 

 frequency indicator dial provide the kilohertz 

 reading. The tuning controls actually adjust 

 the tuning circuits in the RF stages and in the 

 local oscillator in order to select the desired 

 station frequency and to provide simultaneously 

 the desired IF signal to the IF portion of the 

 receiver. The DIAL LOCK knob is associated 

 with the tuning controls. This knob locks the 

 KILOCYCLE CHANGE control so that the fre- 

 quency setting will not be changed accidentally. 



BANDWIDTH CONTROL. — Some transmis- 

 sions use narrower bandwidths in the RF spectrum 

 than others. Receivers are therefore provided 

 with a control that allows the operator to adjust 

 the pass band of the receiver so only the de- 

 sired bandwidth is received. On the R-390A 

 receiver, this control is achieved by the BAND- 

 WIDTH KC switch. It adjusts the tuned circuits 

 of the IF portion of the receiver, thereby con- 

 trolling receiver selectivity. Proper adjustment 

 of this control helps to eliminate noise and 

 interfering signals. If the bandwidth is set too 

 narrow, part of the incoming signal will be 

 lost. 



BEAT FREQUENCY OSCILLATOR. — Some 

 radio transmissions, such as Morse telegraphy 

 and FSK teletype, contain no audio frequency 

 information when they are received. The R-390A 

 is equipped with a beat frequency oscillator 

 (BFO) to produce an audible output if required. 

 The BFO is activated by the BFO On-Off switch 

 and the pitch of the audio output can be ad- 

 justed by the BFO Pitch knob. 



GAIN CONTROL. — The R-390A has three 

 front panel gain controls. The RF GAIN control 

 permits manual adjustment of the gain of the 

 RF and IF sections of the receiver. The LOCAL 

 GAIN and LINE GAIN knobs control the gain 

 c* the AF circuits. The LOCAL GAIN controls 

 adjust the level of the output to the phone 

 jack. The LINE GAIN controls the level of the 

 audio output used to operate terminal equip- 

 ment. 



ANTENNA TRIMMER. — The front panel con- 

 trol labeled ANT TRIM adjusts the input circuit 

 in such a manner that optimum coupling from 

 the antenna to the receiver can be achieved at 

 each frequency. 



AUDIO RESPONSE. — The AUDIO RESPONSE 

 control, which adjusts the bandwidth of the 

 audio circuits, has two settings: SHARP and 

 WIDE. The setting of this control depends on 

 the type of signal received. 



LIMITER. — When the control labeled LIM- 

 IT ER is activated, the operator can control the 

 amplitude of the audio output circuits to pre- 

 determined limits. The setting of the limiter 

 control depends on the type of signals received. 

 A low setting of the control, for example, would 

 be desirable to prevent loud crashes of static 

 in the output when monitoring voice signals. 

 If the received signal is FSK-modulated, it may 

 be desirable to remove all amplitude varia- 

 tions by using a high setting on the LIMITER 

 control. For many types of reception, however, 

 the LIMITER should not be activated. 



BREAK-IN. — The ON-OFF switch labeled 

 BREAK-IN is used when a receiver and trans- 

 mitter are operated together as a radio set. 

 In the ON position, circuits are activated for 

 removing the antenna from the receiver and 

 for grounding the antenna and receiver audio 

 circuits whenever the transmitter is energized. 



135 



