FCC Commercial Element 8 pool, section 46

Subelement F

Installation, Maintenance & Repair

Section 46

Equipment Faults-3

846F1

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?

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

846F1

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846F2

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

A.
Crystal only.
B.
The crystal and the ATR tube.
Correct Answer
C.
The crystal and the TR tube.
D.
The crystal and the klystron.

846F2

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846F3

An increase in magnetron current that coincides with a decrease in power output is an indication of what?

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

846F3

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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846F4

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

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

846F4

Both question and answer have receive KD9TIP

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846F5

A high magnetron current indicates a/an:

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

846F5

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846F6

Low or no mixer current could be caused by:

A.
Local oscillator frequency misadjustment.
B.
TR cell failure.
C.
Mixer diode degradation.
Correct Answer
D.
All of the above.

846F6

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