Is my microwave fried?

Factual Questions
1

I have a relatively new (~3-4 months probably), cheap microwave in which I was cooking some chicken on high for about 12 minutes. After about 11 minutes, I was startled by an extremely loud buzzing sound accompanied by blue sparks emitting from the vent from inside the microwave. I immediately turned it off, but my ears are still ringing. There aren’t any black marks from inside the microwave. Went ahead and ate the chicken and have yet to grow extraneous limbs.

What could cause this? Is it safe to try and use again?

2

Grease or food buildup in the path from the magnetron to the food can cause this, especially if the chasis has any rough spots on it.
I’ve fixed the problem you describe by removing and cleaning the mica-like cover over the magnetron path. I also found and filed down a pointy bit of metal that was the immediate source of sparks between the chasis and the mica-like cover.

3

You cooked chicken for TWELVE MINUTES?

4

The instructions said 10 minutes for a 1200 W oven. Since mine is 1000 W, I figured adding an extra few minutes wouldn’t be of much concern.

Can buildup really already be a problem on such a new microwave?

5

All you need is one drop of grease spattered in the wrong place, or a pointy bit on the case where it’s exposed to microwaves.