You have a very loud low-frequency hum appearing on your transmission. In what part of the transmitter would you first look for the trouble?

A loud, low-frequency hum (typically 60 Hz or 120 Hz) on a transmitted signal is a classic symptom of alternating current (AC) ripple leaking into the direct current (DC) supply. This points directly to a fault within the power supply. The power supply's job is to convert AC from the mains into smooth, stable DC using rectifiers and filter capacitors. If a filter capacitor fails or a rectifier diode blows, the power supply can no longer smooth out the voltage effectively. This unfiltered pulsating DC then makes its way into the transmitter's circuitry, modulating the output signal and causing a characteristic "AC hum" to be heard by anyone receiving your transmission.