Explanation: The most likely cause is a shorted Push-To-Talk (PTT) circuit in the microphone. The PTT switch is designed to place the transceiver into transmit mode. If this circuit becomes shorted, the radio will continuously operate in transmit, regardless of whether the PTT button is physically pressed. When a transceiver is in transmit mode: 1. The "TX" indicator illuminates to show transmission is active. 2. The receiver section, including the AF output stage, is automatically disabled to prevent audio feedback and to route all power to the transmitter. Therefore, a continuously shorted PTT circuit perfectly explains why the radio is powered on, the "TX" light is illuminated, and there is no AF output from the receiver. * A shorted speaker (A) or a failed AF amplifier (D) would prevent AF output but would not cause the "TX" light to illuminate. The radio would remain in receive mode. * A "crowbar" circuit (C) in the power supply is a protection mechanism that would typically shut down power to the radio entirely, meaning it wouldn't be powered on or illuminate "TX."