Microwave not heating up

Magnatron is suspect. Need to put a voltmeter on the input (primary) side to the high-voltage transformer (big heavy iron thing in there) Should be getting 120Vac when it goes to cook cycle. Don't touch the other (high voltage) side of this transformer. If the power is there, you are looking at replacing either the magnatron or the high-voltage capacitor and diode. Its affordable if you do it yourself - buy the part on EBAY - lookup "universal magnatron"