CMRT Troubleshooting & Repair Techniques 5 — Questions and Answers
Question 1: A radar technician is troubleshooting a system where the display shows no video but the timebase sweep is visible. What does this indicate?
- Magnetron has failed
- The video signal path from receiver to display has a fault (Correct answer)
- Antenna is not rotating
- PRF generator has failed
Correct answer: The video signal path from receiver to display has a fault
A visible sweep with no video indicates the timing and display hardware work but the video signal from the receiver is not reaching the display.
Question 2: When performing a radar power output measurement using a calibrated power meter, what safety precaution is MOST important?
- Wear rubber gloves at all times
- Ensure the power meter is rated for the radar's peak pulse power, not just average power (Correct answer)
- Disconnect the GPS receiver before measuring
- Only measure power during daylight hours
Correct answer: Ensure the power meter is rated for the radar's peak pulse power, not just average power
Radar peak pulse power can be hundreds of kilowatts even though average power is low; using an undersized power meter will damage it and give incorrect readings.
Question 3: A technician finds corrosion inside a coaxial connector between the transceiver and antenna. After cleaning, what test should be performed before returning the system to service?
- Doppler frequency shift measurement
- VSWR measurement to confirm acceptable impedance match (Correct answer)
- Noise figure measurement of the display unit
- Azimuth encoder pulse count verification
Correct answer: VSWR measurement to confirm acceptable impedance match
After cleaning and reconnecting a coaxial connector, VSWR must be measured to verify the connector provides a good impedance match and has not introduced reflections.
Question 4: A radar technician is using a substitution method to isolate a fault. This technique involves:
- Measuring voltages at each stage sequentially
- Replacing suspected faulty modules with known-good units to identify the defective stage (Correct answer)
- Using a signal generator to inject test signals at each stage
- Comparing the radar output to a reference radar on another vessel
Correct answer: Replacing suspected faulty modules with known-good units to identify the defective stage
Substitution troubleshooting replaces suspect modules with confirmed-good spares to isolate the defective stage quickly without detailed circuit analysis.
Question 5: A radar's performance monitor shows a decrease in the sensitivity level (MDS — minimum detectable signal) over time. What is the most common cause?
- Antenna rotation speed increase
- Gradual magnetron frequency drift or aging reducing receiver-to-transmitter coupling efficiency (Correct answer)
- GPS signal degradation
- Gyrocompass precession error accumulation
Correct answer: Gradual magnetron frequency drift or aging reducing receiver-to-transmitter coupling efficiency
As a magnetron ages, its frequency can drift, reducing AFC lock quality and degrading receiver sensitivity as the IF passband becomes misaligned.
Question 6: After completing radar repairs, a technician must perform a sea trial to verify performance. Which parameter is NOT typically verified during a performance check at sea?
- Detection range on a known target at a specific range
- Rain and sea clutter suppression effectiveness
- Magnetron anode voltage under load (Correct answer)
- ARPA target acquisition and tracking accuracy
Correct answer: Magnetron anode voltage under load
Magnetron anode voltage under load is an internal bench measurement and is not a standard operational sea trial performance parameter.
Question 7: A technician uses an oscilloscope to examine the radar's trigger pulses and finds they are irregular with varying intervals. What is the most likely faulty component?
- Receiver noise figure attenuator
- PRF (pulse repetition frequency) generator or timer circuit (Correct answer)
- Antenna drive motor capacitor
- Video bandwidth filter
Correct answer: PRF (pulse repetition frequency) generator or timer circuit
Irregular trigger pulse intervals indicate a fault in the PRF generator or timing circuit, which establishes the repetition rate for all radar timing.
CMRT Troubleshooting & Repair Techniques 5
A radar technician is troubleshooting a system where the display shows no video but the timebase sweep is visible.
What does this indicate?