What is a high-voltage diode?

The diode and capacitor together roughly double the transformer’s output to the tens of thousands of volts the magnetron needs. When the diode shorts, the transformer labours into the fault and produces a distinctive loud hum with no heating, often taking the line fuse with it. It is one of the cheapest parts in the machine and one of the more common no-heat causes — but it sits next to the capacitor, so it is not owner-serviceable.