Panasonic Dimension 4 Microwave?

The microwave function on my 4 year old NNA772 Panasonic Dimension 4 oven turns itself off after 20 seconds. The grill and convection part still work OK. I can't see anything obviously wrong inside; fuses, connectors and wiring are OK. Maybe it's a faulty relay. Is there any test I can safely perform on the microwave generator before I consign it to the skip?

Al
September 2006