C7 Troubleshooting & Maintenance Procedures 3 — Questions and Answers
Question 1: A technician is called to troubleshoot a telecom system where analog phone lines have loud hum on all circuits. What is the MOST likely cause?
- Excessive cable length
- Ground loop induced 60 Hz interference (Correct answer)
- Incorrect cable pair assignments
- Faulty RJ-11 connectors
Correct answer: Ground loop induced 60 Hz interference
A 60 Hz hum on all analog lines simultaneously is a classic symptom of a ground loop, where differing ground potentials create a current in the signal path.
Question 2: When performing preventive maintenance on a fiber optic network, a technician should clean connectors with:
- Isopropyl alcohol and a lint-free wipe, then inspect with a fiber scope (Correct answer)
- Compressed air only
- Standard cotton swabs dampened with water
- Acetone solution and paper towels
Correct answer: Isopropyl alcohol and a lint-free wipe, then inspect with a fiber scope
Proper fiber connector cleaning uses IPA with lint-free wipes followed by inspection with a fiber scope to confirm no contamination remains.
Question 3: A technician measures a network cable and finds the propagation delay skew between pairs exceeds TIA-568 limits. This will MOST likely cause problems with:
- Power over Ethernet delivery
- MIMO signal processing that relies on pair timing (Correct answer)
- Physical connector alignment
- Cable jacket flame spread rating
Correct answer: MIMO signal processing that relies on pair timing
Excessive delay skew affects systems like Gigabit Ethernet that use all four pairs simultaneously and rely on timed arrival of signals from each pair.
Question 4: A card reader at an exterior door grants access intermittently in cold weather. After ruling out software issues, the MOST likely hardware cause is:
- The Wiegand cable has degraded insulation from UV exposure
- The reader's internal components are contracting in the cold, causing poor solder joints
- The door contact is misaligned due to frame thermal contraction (Correct answer)
- Low battery in the access control server UPS
Correct answer: The door contact is misaligned due to frame thermal contraction
Metal door frames contract in cold weather, which can misalign magnetic door contacts or latch sensors and cause intermittent status reporting to the controller.
Question 5: During a fire alarm panel maintenance inspection, the technician notices the battery float voltage reads 25.5V on a 24V nominal system. This indicates:
- The battery is overcharging and should be replaced
- Normal float charging voltage for a sealed lead-acid battery (Correct answer)
- The charger circuit has failed and the panel is running on battery
- A loose battery terminal connection
Correct answer: Normal float charging voltage for a sealed lead-acid battery
A float voltage of approximately 13.5–13.8V per 12V battery (27–27.6V for 24V systems) is normal; 25.5V is within normal range for a properly maintained sealed lead-acid battery.
Question 6: A data network switch port shows amber link light instead of green. This MOST commonly indicates:
- The port is operating at a reduced speed (10/100 Mbps instead of Gigabit) (Correct answer)
- The cable is unplugged
- PoE power delivery is active on that port
- The switch is in spanning tree blocking state
Correct answer: The port is operating at a reduced speed (10/100 Mbps instead of Gigabit)
On most managed switches, amber link LEDs indicate a 10 or 100 Mbps connection instead of the expected Gigabit, often caused by a cable fault or negotiation mismatch.
Question 7: Which measurement confirms that a coaxial cable has correct characteristic impedance continuity before connecting to a CATV splitter?
- DC resistance with a standard ohmmeter
- Impedance sweep with a vector network analyzer or TDR (Correct answer)
- Voltage measurement with the cable energized
- Insulation resistance test at 500V DC
Correct answer: Impedance sweep with a vector network analyzer or TDR
A VNA or TDR measures the cable's impedance as a function of frequency or distance, confirming the 75-ohm characteristic impedance required for CATV systems.
C7 Troubleshooting & Maintenance Procedures 3
A technician is called to troubleshoot a telecom system where analog phone lines have loud hum on all circuits.
What is the MOST likely cause?