Question 15B2

When a receiver produces no AF (audio frequency) output, it means the sound signal isn't reaching the speaker. The crucial diagnostic clue here is the "near zero" ohmmeter reading between the speaker leads. A speaker itself has a specific, low impedance (e.g., 4, 8, or 16 ohms). A reading "near zero" indicates a short circuit. Let's evaluate the options: * **A) The 3 – 30 MHz amplifier stage has failed.** This stage is in the RF (radio frequency) front end, long before the audio amplifier. Its failure would result in no AF output but would *not* cause a near-zero resistance reading at the speaker terminals, as the audio output stage and speaker would still have their normal impedance. * **B) The speaker is shorted out.** If the speaker's voice coil or its connecting wires were internally shorted, an ohmmeter reading would indeed be near zero. This would prevent sound output. However, a shorted *audio amplifier output stage* is also a very common point of failure and would present the same low resistance to the speaker terminals while also explaining the lack of audio. * **C) Excess negative feedback is reducing AF gain.** While this would reduce or eliminate AF output, it would *not* cause a near-zero resistance reading across the speaker leads. The speaker's impedance would remain normal. * **D) The audio amplifier stage has failed.** This is the most likely cause. A common failure mode in audio amplifier output stages, especially power amplifiers, involves shorted components like output transistors. If these components fail by shorting, they would effectively present a near-zero resistance across the speaker terminals (which are connected to the amplifier output). This simultaneously explains both the complete lack of AF output and the observed near-zero ohmmeter reading. Therefore, a failed audio amplifier stage that has shorted components in its output section comprehensively explains both symptoms.