Microwave oven collectors?

Generally the reason that it functions correctly but does not heat is that the poitn that 110v is sent to the 2K+ transformer to power the magnetron (RF transmitter) has blown. Two types are generally used, either a relay or a triac (semiconductor form of a relay). Trace the primary wires back to where they meet the next component and you'll run into either a relay or a triac. Either is cheap to change (if they are the fault) at an appliance parts supplier. I generally make sure it's a gate/relay problem by first hooking to the primary wired with a voltmeter (in ACV range of course) and see if when it kicks in that there is 110 there or not. If not it's likely the relay/triac, or may even be the board that supplies either triac gate power or relay pulldown power. If 110 is present on the primary then you likely have a dead magnetron. They are replaceable too but dependant on the model they might make the unit obsolete by cost. If the relay or triac have no trigger power then check for burnt resistors first in the board's traces back from the relay or triac trigger point.