Kenmore Microwave blows internal fuse

Based on you information, if you disconnected magnatron and unit ran with no fuse problem, I would suspect magnatron. You should consider the age of unit it, may be be a better alternative to replace it.

You would better served replacing unit. Throwing parts at it is an unnecessary step.

Yes, if it really is operating the magnetron and generating microwaves, but that is "virtually" (nearly; almost) impossible, because all microwave ovens come with a special door switch (called a monitor switch) whose job it is to cause the internal fuse to blow and kill power to the microwave. It is more likely that just the fan and light stay on, but not the magnetron, and that is caused by a different defective door switch. (All microwave ovens come with 3 or 4 door switches, and they all must work right for your microwave oven to operate.) You better call a professional repairman to have that microwave checked & repaired.

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Check its transformer so that you will know its fuse size

(Fuse is a ceramic fast blow 20A Little Fuse 0314020 or Eton ABC-20) Melting I²t 631
If the Fuse blows when the door is opened. (not even energizing the microwave) The bottom micro switch (monitor micro switch) is normally closed, (open when the door is closed) however if it remains in the closed state when the door is open you will have a short circuit with the Primary Micro switch. The circuit diagram shows that one of the two switches must be open. I was able to test with an ohm meter. With a bad switch the reading was less than an ohm with the door open and 290 ohms with the door closed - Across the plug with a good fuse. With the new micro switch the resistance shouldn't change by much with the door open or closed.

Hi and welcome to FixYa, I am Kelly.
You have a bad door interlock switch or bent switch actuation arm. Should someone open the door while the microwave is operating without pressing pause/stop when the unit has this symptom will cause the internal fuse to blow and it will not operate until the fuse is replaced. The switches are inexpensive but takes someone with some reasonable mechanical skills and knowledge of using a meter to isolate which of the 3 door interlock switches is causing the problem.
Thanks for choosing FixYa,
Kelly

does the fuse in the plug blow or the internal fuse if it blows the internal fuse it could be one of the door switches two of the switches have a red button on them and one has an orange button its normally the orange one thats at fault 

Usually, when your microwave oven doesn't work it all, the internal fuse (thermal fuse) is blown. This fuse protects both you and the oven. When it blows, you need to have a qualified appliance repair technician replace it.

Door switch If the fuse isn't the problem, check the door hooks and the door-release button. When these are broken, the door switch doesn't activate, so the oven can't work.

SWITCHES ARE THERE FOR A PURPOSE-----SAFETY,,SAFETY...ALWAYS CHECK A MICROWAVE WITH THE POWER CORD REMOVED---YOUR MESSIN WITH A RF VOLTAGE THAT WILL KILL YOU!!!! UNPLUG,,UNPLUG,,,UNPLUG
TRY TO CHECK THE FOLLOWING...SINCE YOU HAVE THE COVER OFF,,LOOK AT THE DOOR LATCHES WHEN YOU CLOSE THE DOOR AND MAKE SURE THEY MAKE CONTACT ON THE DOOR SWITCHES,,3 OF THEM,,,--AN OHM METER IS THE TRICK FOR THIS - SOMETIMES YOU CAN SEE PLASTIC BROKEN ON THE SWITCH ITSELF...ALSO LOOK AT THE COMPLETE SWITCH HARNESS (CHECK FOR BROKEN PLASTIC PIECES) THAT HOLD THE SAFETY SWITCHES...THE MIDDLE SWITCH (MONITOR SWITCH) CLOSES WHEN YOU "OPEN" THE DOOR,,YES "OPEN "THE DOOR AND IT WILL BLOW THE MAIN FUSE..IF IT'S SHORTED.

THERMAL PROTECTORS RARELY GO BAD--IF YOU HAVE A PROBLEM WHERE IT TAKES 25 MINUTES TO COOK A BAKE" POTATOE"--JUST LIKE THE VICE PRESIDENT--THAT'S A THERMAL PROTECTOR..
PLEASE BE CAREFULLLLLL TUNICA

I've had this problem twice with our microwave. The hint to where the problem lies is that the breaker blows when you open the door at times. Inside the microwave are three microswitches; primary and secondary interlock micro switches and an Interlock Monitor switch. The switches are supposed to shut down the microwave if it is running and the door is opened. But, I've found the upper primary switch can stick internal at times and when the door is opened the stuck switch will, because of the way it is wired as a safety switch, cause either the fuse or possibly the main circuit breaker for the oven to pop. The reason the stuck switch blows the breaker or fuse is when the door is opened the interlock switch will cause the neutral wire from the AC power to be applied to the Upper Primary Interlock switch which normally should be open when the door is opened, but if the switch is intermittent or the contacts weld themselves shut the neutral line is connected direct to the "hot" side of the power line through the fuse and thermostat. The first time my microwave failed the switch had melted the contacts together inside the switch and the second time the switch became intermittent and would blow the breaker to the over ever so often. I suspected the switch the second time this happened and utilizing an ohm meter I checked the switch several times by opening and closing the door and once in a while it would remain closed instead of opening when the door was opened. Of course while trouble shooting the power cord to the microwave must be disconnected and use all safety precautions when working around the high voltage areas inside the microwave. I wrote Jenn-Air about the bad switch and the way it is wired into the circuit, but never received any acknowledgement. I suspect these microswitches are under rated for the amount of current that passes through the switch, thus they overheat and eventually arc the contacts together. I hope this helps explain the intermittent problem and could explain many of the intermittent blown fuse problems I see in these internet help sessions