That's probably a shorted high voltage rectifier diode or a bad magnetron.
It should have a nine-year warranty on the magnetron.
Here are some links you or someone you know can use (in order given) for test help, but read the safety warnings first:
http://www.gallawa.com/microtech/diode.html (if it is NOT shorted, it's probably fine)
http://www.gallawa.com/microtech/mag_test.htmlThere
should also be a "mini-manual" hidden inside the unit behind the
control panel or hidden on the left side behind the grille, which is
very helpful when troubleshooting & testing.
At
our site, we have a video showing how to remove a typical
over the range control panel assembly in under 5 minutes.
You can usually find helpful exploded view diagrams
and order parts by entering your full model number
here.
GE may send you a replacement magnetron. Or you can contact them to find the GE-authorized servicer. GE Customer Service is
here.
We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.