Question E4E10:
Which of the following can create intermittent loud roaring or buzzing AC line interference?  

Choices:  
A: Arcing contacts in a thermostatically controlled device  
B: A defective doorbell or doorbell transformer inside a nearby residence  
C: A malfunctioning illuminated advertising display  
D: All these choices are correct    

Correct Answer is  D: All these choices are correct  

To identify AC line interference sources, remember that anything with arcing contacts, transformers/ballasts, or high-voltage/power electronics can radiate broadband noise that sounds like a loud roaring or 60/120 Hz buzz on HF.

Why the correct answer (D) is correct:
- All listed devices can produce intermittent broadband noise that couples onto the AC line and radiates from wiring and devices, sounding like a harsh buzz or roar in your receiver.

Why each listed source (A–C) can cause the noise:
- A: Arcing contacts in a thermostatically controlled device
  - Thermostats and relays open/close under load; microscopic arcs act like a spark transmitter, creating wideband RF noise. Cycling is intermittent (e.g., HVAC on/off).
- B: A defective doorbell or doorbell transformer
  - Failing small transformers or rectifiers can arc or saturate, producing a rough 60/120 Hz buzz with strong harmonics that rides the power lines. Intermittent as the transformer heats/cools or when the doorbell is used.
- C: A malfunctioning illuminated advertising display
  - Neon/fluorescent signs (bad ballasts/ignitors) and LED signs (noisy switching supplies) can generate intense broadband hash and line-borne interference, often intermittent as they start/restart or flicker.

Why the other individual choices are not the best single answer:
- Each is a valid cause, but selecting only one ignores the others. The most complete answer is “All these choices are correct.”

Useful rules of thumb:
- Power-line/arc buzz often has 60/120 Hz components and many harmonics:
  $$f_n = n \times 60\ \text{Hz} \quad \text{or} \quad f_n = n \times 120\ \text{Hz}$$
- Arc noise is broadband (often HF through VHF) and can be very strong across bands.
- Troubleshooting tips:
  - Use an AM receiver tuned off-station; the buzz gets louder near the source.
  - Check correlation with HVAC cycles (thermostat), doorbell usage/transformer warmth, or sign on/off times.
  - Kill circuits at the breaker to isolate the offending branch; noise often drops dramatically when the culprit circuit is opened.