491. If signals are heard with headphones in the detector plate circuit of a receiver, but none are heard with headphones in the first AF stage plate circuit, what might be the cause?

Audio at the detector plate but absent at the first AF stage indicates a fault in the coupling or the AF stage itself. An open transformer winding, a defective coupling capacitor or improper potentials, and a defective tube are all plausible causes that could interrupt the signal or prevent amplification, making 'all of the above' the correct response.