This is most often caused by a problem with the door switch, door switch mount, a bad connection at the magnetron, a bad magnetron, or another bad component in the high voltage section - usually in that order!
In my professional opinion, people should not work on a microwave unless they are fully informed and aware of the deadly hazards and necessary precautions.
One can find such information and very specific in our safety and disassermbly text files at http://www.microwavedisplay.com
You can find safety and disassembly text files (as well as a text file which helps in diagnosing and repairing door switch problems) at http://www.microwavedisplay.com
AFTER you read the safety info I referred to above, you can find more info on how they work here: http://www.gallawa.com/microtech/how_work.html
You can find UK part suppliers and authorized servicers via http://www.sharp.co.uk
William Miller † MicrowaveDisplay.com, March 2007
