Fuse is blowing of

Are other appliances running on the same circuit? Does it happen in the summer with the A/C on?
It may be starved for voltage. I've seen this happen on homes where the beaker panel is undersized or has a loose connection.

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2 PCS 20A 125V WB27X11138 Microwave Oven Fuses for ...

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Having 20 year experience fixing microwaves i know that when the fuse goes that if you can't find a melted wire or overheated components on the circuit board that the high voltage capacitor at the back needs to be replaced. Sometimes you have to replace the magnetron too. You can replace the door switches if you like, microswitches are inexpensive but would only be bad if arced inside so bad that you read over 1 ohm.

A short likely in the wiring for the light or at the safety switch embedded on the side where the door closes----be careful as a 20 amp fuse takes something major to take out.
(for it to blow)

This might be best left to a qualified service person.

The magnetron is shorted, the keypad control board is bad, blowing fuses. If less than 7 years old it has a inverter board that can be trouble, But with the level of work and parts it will easily exceed the cost of a new microwave oven.

If the 20A fuse is blowing, it is often the high voltage transformer or magnetron. If this is the case, and the microwave is not under warranty, replace it. Residential microwaves are generally considered disposable. Parts and labor are just too expensive. A magntron or transformer can cost more that half the cost of a new microwave.

That means your microwave is broken. Replace it or go on a service.

Disconnect the connector to the primary winding of the high voltage transformer. Put a new fuse . If the fuse is not blowing the problem may be in the secondary side of the high voltage transformer. Chek the highvoltage diode, capacitor, magnetron respectively. you can also check the primary winding of the low voltage transformer.

The Interlock Monitor Switch is an added
safety check on the Primary and
Secondary Interlock Switches. If the
Primary and Secondary Interlock Switches
allow the oven to operate with the door
open, the Monitor Switch will blow the
fuse.

You can get more information about adjuting the interlock switch from this link

http://136.166.4.200/contents/Microwave/LMVM1935xx/LMVM1935xx_Service_Manual.pdf

sections 7-11 thru 7-12 discuss the door interlock adjustment that will cause the 20A fuse to blow if it isn't set up right. I hope this helps you...

sounds to me like the magnatron has shorted out. i don't know of a way to test them unless you have some high dollar equipment. a continuity test prob wont tell you anything. what reading did you get on the cap when you tested it?