Glen,
I measured 0v at the primary of the transformer when the microwave was running. However, when I disconnected the wires to the primary and measured the voltage at the wire ends with the microwave running, it was 120v. It would appear that the transformer or something on the other side of the primary is pulling the voltage to 0v.

One other note .... occassionally, when running the microwave, there is a lower sounding hum than normal and the voltage measures 23v at the primary. But most of the time, it is 0v.

How would I check whether the transformer, high voltage capacitor, diode or magnetron is the problem?

:eek: NO!
Keep away from the high voltage.
I was clearly referring to the primary voltage.
If you don't know how to check it then call someone who does.

First keep in mind that the electricity in a MW can
reach thousands of volts even when turned off.

Something I would want to see is if there is 120 volts AC
reaching the primary of the transformer while running.
If not then you could be looking at anything from a
bad wire connection to the circuit board itself that has
to send primary voltage to the transformer or any other
component in between. If the voltage at the primary winding
does exist then there most likely is a high voltage component
itself such as the transformer itself or one of the other high
voltage components.