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Subel. F
6 questions
Subelement F

Installation, Maintenance & Repair

Section 46

Equipment Faults-3

Silicon crystals are used in RADAR mixer and detector stages. Using an ohmmeter, how might a crystal be checked to determine if it is functional?

  • Its resistance should be the same in both directions.
  • Correct Answer
    Its resistance should be low in one direction and high in the opposite direction.
  • Its resistance cannot be checked with a dc ohmmeter because the crystal acts as a rectifier.
  • It would be more appropriate to use a VTVM and measure the voltage drop across the crystal.

The Silicon Crystal is similar to a diode.

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In a RADAR unit, if the crystal mixer becomes defective, replace the:

  • Crystal only.
  • The crystal and the ATR tube.
  • Correct Answer
    The crystal and the TR tube.
  • The crystal and the klystron.
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An increase in magnetron current that coincides with a decrease in power output is an indication of what?

  • The pulse length decreasing.
  • A high SWR.
  • A high magnetron heater voltage.
  • Correct Answer
    The external magnet weakening.

An increase in magnetron current that coincides with a decrease in power output is an indication of "magnetron magnet weakening."

Explanation: A magnetron is a type of electron tube used in RADAR transmitters to generate microwave signals. It contains permanent magnets that provide the magnetic field necessary for electron beam focusing and interaction with the resonant cavities, leading to microwave generation.

If the magnetron's permanent magnets weaken over time or due to certain conditions, the magnetic field strength within the tube diminishes. This weakening can result in the electron beam not being focused correctly, causing an increase in magnetron current as the system attempts to maintain proper operation.

However, despite the increased current, the microwave power output decreases since the weakened magnetic field affects the electron-beam-cavity interaction, resulting in reduced microwave generation.

Mnemonic: "Magnet Weakening"

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It is reported that the RADAR is not receiving small targets. The most likely causes are:

  • Correct Answer
    Magnetron, IF amplifier, or receiver tuning.
  • PFN, crystals, or processor memory.
  • Crystals, local oscillator tuning, or power supply.
  • Fuse blown, IF amp, or video processor.

Both question and answer have receive

The question says that only weak signal echos from small targets are affected.

Answer A. is the right answer because all the causes listed reduce the level of the signal at the receiver's detector thus specifically keeping small target weak returns from being received.

Answers B., C. and D. are wrong because they affect all targets regardless of signal strength.

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A high magnetron current indicates a/an:

  • Defective AFC crystal.
  • Increase in duty cycle.
  • Correct Answer
    Defective external magnetic field.
  • High standing wave ratio (SWR).

A strong magnetic field is required to keep the electron beam inside a Magnetron focused.

Without the magnetic field current runs away and destroys the Magnetron.

Answer A. is the right one.

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Low or no mixer current could be caused by:

  • Local oscillator frequency misadjustment.
  • TR cell failure.
  • Mixer diode degradation.
  • Correct Answer
    All of the above.
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