Av någon anledning läste jag förbi "9 år gammal mikrovågsugn" ... väl här går du:
Bad magnetron
Eller så här: (Personligen skulle jag vara lite mindre lax på att säga, eh ... det är bra)
Sparking and scorch marks in microwave. What's going on here?
It's possible that either the glass or the [frozen food] allowed charge to build up to the point of arcing. It's also possible that there is something wrong with the microwave. Try putting a cup of water in the microwave and running it for about 30 seconds. If there is no arcing then the microwave itself is probably fine. If you continue to have arcing then replace the microwave.
-engineer_comp_geek, straightdope.com
Mikrovågor gillar inte is. - thenakedscientists.com