If you're lucky there will be a wiring diagram on the inside of the cover. If you're very lucky, it will match the actual wiring.
As the lights and fan work, this would indicate a fault on the secondary side of the transformer. There are only four components in the secondary circuit of most microwave ovens: the HT fuse, the diode, the capacitor and the magnetron.
The fuse will be a special high-voltage fuse contained in a white plastic tube. One end will be connected to the transformer, the other end to the capacitor. Disconnect both push-on terminals and test the fuse for continuity with an AVO.
The rectifier diode sits between the capacitor and chassis. This has a much higher voltage drop than a normal 1N4007-type diode, so you can't just test it with an AVO. Connect it in series with a 9V smoke alarm battery and 1kΩ resistor as shown:
bat (+) ---*---/\/\/\/---*---diode------ (-) bat
Measure between both points * with an AVO set to voltage, and repeat with the diode connected the other way around. Different readings each way (about 2V or so with the cathode (marked end) of the diode to battery (-) and next to nothing the other way) mean the diode is fine.
You can't test the capacitor with a normal AVO unless it has a capacitance range; but they rarely fail and when they do, they make a mess.
If the faulty part blows again straight away on replacement, the magnetron is probably faulty. This means the oven is a write-off.
NB. Leave the µwave unplugged for at least an hour before opening it up! This should give the capacitor time to discharge -- there may be up to 2000V sitting across it, with enough energy to be painful or fatal.
Good luck with it!
AJS
December 2008
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