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Microwave makes humming noise then blows main fuse when door is closed

The microwave in question is: Kenmore 363.62709200 'microwave/hood combo'

This problem originated a few months ago when the main (20A .025/1.25in) fuse blew. I replaced the fuse without issue and the unit worked fine for 2-3 months, but then had to replace the fuse again. This time the new fuse only lasted for about a week.

Now if I put a new fuse in it (while the door is latched), the internal fan cuts on and there is a humming noise (not sure if it's the transformer or magnetron or just the sound of the fan running, but the humming noise is coming from that general area) and after 30 seconds or so, the fuse blows again. If I open the door, however, the humming noise ceases, the fan cuts off, and the fuse does not blow (I currently have the interior light bulb removed and the door taped slightly ajar so I can use the range fan and light).

I checked the door switches, and the button on the secondary switch looked a bit worn so I replaced all the switches. Unfortunately, this did not remedy the problem and I'm not sure what to test next. I have tested the thermal fuses with the multimeter and they all show continuity.

Does this indicate there is a problem with the control board? Or maybe something within the high voltage circuit?

Please help

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crwdns2886778:01crwdne2886778:0

Squiggle, the door switch interrupts power to the hi-voltage/maggie units. 20A is a lot of current. Blowing this up only when the door is closed indicates a short circuit somewhere in the high power usage circuitry. Did you try using your ohmmeter to see if the resistance from the line voltage wire to ground is low when the power is disconnected? (Be careful when working around the HV circuitry; the capacitors - which could be the problem - can zap you even when power has been removed for some time.) Start disconnecting things until the resistance goes up appreciably. (Note: I don’t have one of these particular units, so this is just general advice, not specific to your unit.)

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