I have minor knowledge of Microwave ovens, but you have a large current draw when it fails, to dim your kitchen lights and blow a 20amp ceramic fuse (those tend to be slow blow), and you did nothing to turn the microwave tube on (not from the description you gave). So starting from the plug, check the resistance to ground from the high and low sides. If resistance checking doesn¡¯t get you anywhere then check the coloration of any capacitors, resistors, IC¡¯s, etc. I might disconnect the microwave tube and repeat the power up check.
On another note, your microwave issues should not dim your lights, they should be on separate circuits (lighting on 15 amp breakers, 14 ga. wiring, outlets on 20 amp breakers, 12 ga. wiring)
You may also have a problem purchasing repair parts, you may have to show certification in microwave oven repair to buy parts, unless they are generic parts.
Don Bitters
