Your question suggests you don't have much of a clue about eletrical repair. Which "dodes"? Why do you "Want to fit diodes" Why "in the power circuit"?
Do you think something has failed? How do you know it is "the diodes"? Have you checked the ones installed at the moment? Otherwise why do you want to "fit diodes"?
I would respectfully suggest you find somepone who knows what they are doing - or just buy a new £50 microwave - before you kill someone important - or yourself.
XXX
December 2015