GE Microwave--Everything works but heat

Microwave Repair
,
1

I have a GE JVM1870SF04 microwave. Everything works except heat. On partial power I can hear it switching on and off, but still no heat. Panel works fine, vent fan, light, under-light, turntable–all work. My best guesses are:

  1. The line fuse (but wouldn’t it not power at all then?)
  2. Secondary door switch
  3. Magnetron

Is this correct, and is it worth replacing 1 and 2 on an 11 year old microwave before trying a new magnetron? Or is it possible it’s something else? Seems like at $200 for a new magnetron it’s the equivalent of putting a new transmission on a '78 Pinto.

Thanks in advance!