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Subelement A
VHF-DSC Equipment & Operation:
Section 8
Transmitter Faults-2:
What condition may be indicated if a VHF radio cannot contact nearby stations on any channel, an inline wattmeter indicates 25 watts output (HIGH power mode), and no modulation is heard when the transmitted signal is monitored with an external receiver?
  • A fault in the transmitter's Push-To-Talk circuit is causing the loss of audio.
  • Correct Answer
    There is an open connection in the microphone cable or connector.
  • The microphone's Automatic Gain Control (AGC) has malfunctioned.
  • Degenerative feedback applied to the AF amplifier exceeds specifications.
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What condition may be indicated if a VHF radio cannot contact nearby stations, an inline wattmeter indicates less than 1 watt output (HIGH power mode), and nominal voltage (e.g., 12.8 vdc) is applied to the VHF radio power input line?
  • The RF power applied to the input of the Power Amplifier (PA) stage is excessive.
  • There is an open connection between the radio's PA stage and the antenna connector.
  • Correct Answer
    The bias voltage applied to the Power Amplifier (PA) stage may be insufficient.
  • A defective capacitor is blocking DC voltage from appearing at the antenna connector.
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A spectrum analyzer (SPECAN) can be a useful test instrument to troubleshoot a marine VHF radio. Which of the following is the best procedure to evaluate the input/output stages of a marine VHF transmitter?
  • Connect the radio to the SPECAN through an in-line attenuator, apply maximum attentuation and set the radio to HIGH power. Use an inductive probe to evaluate signals at the input/output of AF transmitter stages.
  • Connect the radio to the SPECAN through an in-line attenuator, apply maximum attentuation and switch the radio to HIGH power. Use an inductive probe to evaluate signals at the input/output of RF transmitter stages.
  • Connect the radio to the SPECAN through an in-line attenuator, apply minimum attentuation and switch the radio to LOW power. Use an inductive probe to evaluate signals at the input/output of AF transmitter stages.
  • Correct Answer
    Connect the radio to the SPECAN through an in-line attenuator, apply maximum attentuation and switch the radio to LOW power. Use an inductive probe to evaluate signals at the input/output of RF transmitter stages.
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Which in-line RF power reading indicates the proper operation of a VHF marine transmitter?
  • Correct Answer
    25 watts forward/1.5 watts reflected.
  • 25 watts forward/25 watts reflected.
  • 27.5 watts forward/1.5 watts reflected.
  • 2.5 watts forward/1.5 watts reflected.
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What condition may cause distortion in the modulator stage of a marine VHF transmitter?
  • The reactance modulator may be out of alignment.
  • The excessive use of pre-emphasis on audio signals above 25 kHz.
  • A defective varactor diode in the transmitter's voltage controlled oscillator.
  • Correct Answer
    Transmitter modulation in excess of 150%.
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What terms are used to describe a device used to test the RF power output of a marine VHF transmitter without emitting a signal on the air?
  • Correct Answer
    Dummy load, artificial antenna.
  • Phantom Antenna, dummy load.
  • Isotropic suppressor, artificial antenna.
  • Artificial attenuator, dummy load.
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