Subelement B: MF-HF-DSC-SITOR (NBDP) Equip. & Operations— Topic :
Question 15B1
Element 9 (GMDSS Maintainer)The MF/HF receiver fails to suppress the AF output in the absence of a sufficiently strong input signal. What is the most likely cause?
Explanation
The squelch circuit's primary function is to mute the receiver's audio output when the incoming signal strength falls below a user-adjustable threshold. This prevents the constant hiss and static that would otherwise be heard in the absence of a strong, usable signal.
If the squelch circuit fails, it loses its ability to suppress this background noise. Consequently, the receiver will continuously produce audio (static/hiss) even when no sufficiently strong input signal is present, precisely matching the described symptom.
* **A) AGC (Automatic Gain Control)** adjusts amplifier gain to maintain consistent audio levels for varying signal strengths, but it doesn't mute the receiver.
* **B) The BFO (Beat Frequency Oscillator)** is used for demodulating SSB and CW signals; its failure would prevent proper reception of these modes, not cause continuous noise.
* **D) The AF (Audio Frequency) amplifier** boosts the audio signal for the speaker/headphones. If it failed, there would be no audio output at all, which is the opposite of the problem.
Related Questions
14B5 You are able to hear signals in the SITOR (NBDP) mode but not in the voice/SSB mode. What is the most likely cause?14B6 The AGC function of an MF/HF receiver is inoperative. What is the most likely cause?15B2 The MF/HF receiver fails to produce any AF output. An ohmmeter reading is near zero between speaker leads. What is the most likely cause?15B3 The MF/HF transceiver produces strong AF output but no signals are heard on any frequency. What is the most likely cause?15B4 The MF/HF transceiver produces strong AF output but no signals are heard on any frequency. What is the most likely cause?