Question 18B5

A **crowbar circuit** (C) is a protection mechanism in a power supply designed to detect an overvoltage condition. When activated, it rapidly creates a short circuit across the power supply's output terminals. This intentionally disables the power supply, cutting off operating voltage to the transmitter's RF stages and ensuring **zero RF output** to prevent catastrophic damage to sensitive components. * **A) The connection of a transceiver is connected to a “Dummy Load” antenna.** A dummy load is designed to absorb RF power, allowing the transmitter to operate normally without radiating a signal. The transmitter *would* still produce RF output, which is then dissipated as heat in the dummy load, not zero output. * **B) The failure of the voice bandpass filter in the stage preceding the RF amplifier.** F1B FEC is a digital mode (like RTTY) using frequency-shift keying (FSK) of tones, not voice. While a filter failure could degrade or distort the signal, it typically would not cause a complete cessation of RF output from the transmitter. * **D) The generation of a positive VCO feedback signal to a Phase Locked Loop circuit in the transmitter.** Phase-Locked Loops (PLLs) use feedback to maintain frequency stability. While a PLL malfunction could lead to incorrect frequency or instability, "generation of a positive VCO feedback signal" does not describe a condition that would inherently cause a complete shutdown resulting in zero RF output.