CMRT Troubleshooting & Repair Techniques 4 — Questions and Answers
Question 1: A technician notices radar performance degrades in rain but sea clutter appears normal. Which subsystem should be investigated first?
- Sea clutter (STC) control circuit
- FTC (Fast Time Constant) / rain clutter circuit (Correct answer)
- Heading sensor interface
- Antenna motor speed controller
Correct answer: FTC (Fast Time Constant) / rain clutter circuit
Rain clutter suppression is handled by the FTC (differentiation) circuit; if it fails, rain returns are not suppressed while sea clutter performance remains normal.
Question 2: A ARPA (Automatic Radar Plotting Aid) is generating erratic target tracks. The radar bearings and ranges appear accurate. What is the most likely cause?
- Defective magnetron
- Speed or course input data error from the ship's sensors (Correct answer)
- IF amplifier saturation
- Corrupted radar display firmware
Correct answer: Speed or course input data error from the ship's sensors
ARPA target tracking relies on accurate speed and course inputs; erratic tracks with correct radar video indicate a sensor data error, not a radar RF problem.
Question 3: A radar technician suspects a failing diode in the duplexer (circulator). Which symptom would best confirm this diagnosis?
- Radar operates normally but heading line is offset
- Receiver noise figure is degraded and transmitter power leaks into receiver path (Correct answer)
- Antenna rotates in the wrong direction
- Display shows inverse video
Correct answer: Receiver noise figure is degraded and transmitter power leaks into receiver path
A failed circulator diode allows transmit energy to leak into the receiver and degrades the noise figure, potentially damaging the receiver front end.
Question 4: During a radar alignment, a technician finds the antenna's horizontal beamwidth is wider than specification. What component is most likely at fault?
- Magnetron output frequency drift
- Damaged or corroded slotted waveguide array elements (Correct answer)
- Modulator PFN capacitor failure
- TR limiter degradation
Correct answer: Damaged or corroded slotted waveguide array elements
The slotted waveguide array determines the horizontal beamwidth; damage or corrosion to the slots degrades the antenna pattern and widens the beam.
Question 5: A radar shows strong target returns but all targets appear at double their actual range. What is the most likely cause?
- PRF (pulse repetition frequency) is set too low, causing second-trace echoes to appear at half-range
- Timebase generator is running at half the correct frequency (Correct answer)
- Video amplifier gain is doubled
- Antenna rotates at twice normal speed
Correct answer: Timebase generator is running at half the correct frequency
If the timebase generator runs at half frequency, the sweep takes twice as long, making targets appear at double their actual range.
Question 6: A vessel's radar shows intermittent loss of synchronization between the sweep and the antenna bearing. What component should be inspected?
- Heading sensor cable
- Azimuth encoder or bearing pulse generator on the antenna pedestal (Correct answer)
- Video processor board
- STC control potentiometer
Correct answer: Azimuth encoder or bearing pulse generator on the antenna pedestal
The azimuth encoder generates bearing pulses that synchronize the display sweep with antenna rotation; a faulty encoder causes bearing sync loss.
Question 7: After a lightning strike near a vessel, the radar receiver is completely dead but the transmitter still functions. Which component is most likely damaged?
- Magnetron
- Low-noise amplifier (LNA) or receiver front-end diode mixer (Correct answer)
- Modulator thyratron
- Display CRT or LCD panel
Correct answer: Low-noise amplifier (LNA) or receiver front-end diode mixer
Lightning-induced transients most commonly destroy the sensitive LNA or front-end mixer diodes in the receiver, which are not protected by the TR limiter against an external surge.
CMRT Troubleshooting & Repair Techniques 4
A technician notices radar performance degrades in rain but sea clutter appears normal.
Which subsystem should be investigated first?