CMRT Troubleshooting & Repair Techniques 3 — Questions and Answers
Question 1: During radar maintenance, a technician discovers the modulator's pulse-forming network (PFN) capacitors are bulging. What action is required?
- Discharge and replace the PFN capacitors immediately (Correct answer)
- Increase the charging voltage to compensate
- Replace only the damaged capacitor and retest
- Reduce the PRF to extend capacitor life
Correct answer: Discharge and replace the PFN capacitors immediately
Bulging PFN capacitors indicate failure and pose an explosion hazard; all capacitors must be safely discharged and replaced immediately.
Question 2: A radar technician uses a spectrum analyzer on the radar's IF output and finds the IF signal is broader than specification. What does this indicate?
- Antenna rotation speed is too fast
- IF filter bandwidth is too wide or a filter component has failed (Correct answer)
- The magnetron is over-powered
- Video amplifier clipping is occurring
Correct answer: IF filter bandwidth is too wide or a filter component has failed
A broader-than-specified IF bandwidth indicates a failed or degraded IF filter component allowing excessive noise into the receiver.
Question 3: What is the safest method to discharge a high-voltage radar modulator capacitor bank before performing maintenance?
- Short the terminals with an insulated screwdriver
- Use an approved bleeder resistor discharge tool rated for the voltage (Correct answer)
- Allow the capacitors to self-discharge overnight
- Apply reverse polarity voltage to cancel the charge
Correct answer: Use an approved bleeder resistor discharge tool rated for the voltage
A rated bleeder resistor discharge tool safely dissipates stored energy in a controlled manner without risk of arc flash or equipment damage.
Question 4: A radar shows strong returns at all bearings at a fixed range, forming a complete circle. This phenomenon is called:
- Indirect echoes
- Interference from another radar
- Range ring calibration error
- Fixed-range false echo or 'ring' due to internal reflection (Correct answer)
Correct answer: Fixed-range false echo or 'ring' due to internal reflection
A bright circle at a fixed range at all bearings indicates an internal reflection or standing wave within the radar's waveguide system.
Question 5: A technician replaces an S-band radar magnetron but the transmitter output power remains low. What should be checked next?
- Video amplifier gain settings
- Modulator output voltage and pulse parameters (Correct answer)
- Display brightness controls
- Antenna rotation motor speed
Correct answer: Modulator output voltage and pulse parameters
If output remains low after magnetron replacement, the modulator must be checked to ensure it delivers correct pulse voltage and width to drive the new magnetron.
Question 6: A radar system's automatic frequency control (AFC) circuit is not locking to the magnetron frequency. What is the most likely effect?
- Antenna fails to rotate
- Local oscillator drifts, causing IF to shift and reducing receiver sensitivity (Correct answer)
- Bearing error increases proportionally
- PRF becomes unstable
Correct answer: Local oscillator drifts, causing IF to shift and reducing receiver sensitivity
If AFC fails to lock, the local oscillator drifts away from the magnetron frequency, shifting the IF out of the filter passband and reducing sensitivity.
Question 7: When replacing a waveguide section on a marine radar, what must be verified after reassembly to ensure proper operation?
- That the waveguide is painted with anti-corrosion paint
- VSWR (voltage standing wave ratio) is within specification (Correct answer)
- The waveguide is grounded at both ends to the ship's hull
- Waveguide orientation matches magnetic north
Correct answer: VSWR (voltage standing wave ratio) is within specification
After waveguide repair, VSWR must be measured to confirm proper impedance match and absence of reflections that would reduce transmitted power.
CMRT Troubleshooting & Repair Techniques 3
During radar maintenance, a technician discovers the modulator's pulse-forming network (PFN) capacitors are bulging.
What action is required?