There are several interlocks for the main AC line, follow the L1 side of the wall plug from the plug through the filter board, door main and secondary switches and any and all disk OT components. In addition look for brown or burned connectors on the door interlocks as they fail for loose contact. If any of the door micro switches are defective make sure us replace with the correct amp rating. The unit might have failed initially in the high voltage area, common failure, check the Magnetron, High voltage cap or diode. The diode can only be tested with a dc voltage, like a 9 volt battery, as it must be forward biased on the test it. The tech sheet in the unit is a good source of information. Best of success.