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I have changed the capacitor 2 times but after a few days of heating, the microwave stops heating. Any idea what could be going wrong? The diode tested fine. And if I put a new capacitor, it works for few days - so I guess that rules out Magnetron and other switches n stuff. So a faulty circuit board probably? Has anyone had this problem?

mrchief, Just because it works again after a capacitor replacement does not mean the magnetron isn’t failing. It could possibly be that the magnetron is burned out bad enough that you are over-driving the capacitor to blow out. There should be more than one cap in the power circuit for the magnetron driver circuit. It’s a turned circuit, so check out an iductor loops in there as well for visible damage to their insulations. Even with a schematic and some thorough circuit testing, finding an impedance/reactance mismatch would be pretty difficult. Your hint might be that the cap is either getting overvolted, or draw much too much current. Hope that helps, best of luck