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Subelement A
VHF-DSC Equipment & Operation:
Section 5
Receiver Faults-2:
What condition may cause a significant loss in VHF receiver sensitivity?
  • Correct Answer
    A reading of 9.6 when a voltmeter is connected between the positive side of the radio's DC line and ground.
  • The presence of a -90dbm signal measured at the 2nd IF output with the receiver tuned to the desired input frequency.
  • An internal signal that is very weak in comparison to the desired signal at the output of the second IF stage.
  • Replacing (75 ohm) RG-59/U coaxial transmission line with a (50 ohm) RG-8/U or LMR-400 Ultraflex cable.
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Which of the following test procedures may be used to determine a VHF receiver's minimum discernible signal?
  • Connect an oscilliscope to the antenna input and observe the amplitude of minimum signal voltage plotted on the display's vertical axis as a function of time on the horizontal axis.
  • Correct Answer
    Set a signal generator to the frequency of the VHF receiver, adjust the output signal to near zero, and increase the signal level until an audio voltmeter connected to the AF output point indicates a 3-dB increase.
  • Connect a spectrum analyzer to the antenna input and apply the maximum amount of attentuation to detect minimum signal level plotted on the display's vertical axis as a function of frequency on the horizontal axis.
  • Connect a 156.3 Mhz signal generator to the antenna input, set the VHF receiver to channel 06 and measure dynamic range using an oscilliscope.
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Which of the following procedures may be used in many U.S. ports as a quick field test to determine if a shipboard VHF receiver is operating properly?
  • Tune the VHF receiver to 158.6 MHz and listen for local U.S. Coast Guard announcements or radio traffic from other vessels.
  • Set the VHF radio to a simplex channel, key the transmitter, and monitor the quality of the transmitted signal with the internal receiver.
  • Correct Answer
    Tune the VHF radio to 162.550 MHz, 162.400 MHz or 162.475 MHz and listen for broadcasts from the National Oceanic and Atmospheric Administration.
  • Connect appropriate test equipment to the radio and measure signal levels at the output of the receiver's RF amplifier stage.
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What ohmmeter reading may indicate a faulty component in a VHF receiver?
  • Low resistance across a diode in one direction, high resistance when test leads are reversed.
  • Zero ohms across an in-line fuse in a DC power circuit.
  • Infinite ohms across a ceramic bypass capacitor in a transistor amplifier circuit.
  • Correct Answer
    A transistor indicates low resistance between emitter and base when test leads are reversed across them.
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What condition may indicate a VHF receiver fault?
  • Correct Answer
    The same voltage reading between each lead of a 100K-ohm resistor and ground in a bias circuit.
  • A "click" is heard when an ohmmeter set to R X 1 is touched across loudspeaker terminals.
  • A voltage drop across the leads of a 47K ohm resistor in an intermediate frequency amplifier stage.
  • The presence of an AC signal on the output of an audio frequency amplifier stage.
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What condition may cause noisy operation of a VHF receiver?
  • An output signal that is larger than the input signal in an audio frequency amplifier stage.
  • Correct Answer
    A defective potentiometer in the audio frequency amplifier stage.
  • Low bias voltage to an intermediate frequency amplifier stage.
  • An open winding in a secondary of a coupling transformer in the audio frequency amplifier stage.
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