Microwave wave is trippin me out dude and dudettte"s

I was given a new looking microwave oven the above the range type with a exhaust fan. I was told it draws too much power and trips house breakers here you take it... well I did. I opened it up and found a bad (open) on the first control board supporting the noise filter and the in line fuse. The fuse was good. Hmmmm. I got another board from a appliance repairman I know. It wasn't the same but matched components and fuse good to go.. I installed it. I energized the oven with 115 vac on a 15 amp circuit in my house. It works !? Nooo but the fan, the timer and the light worked but when I turned the oven onto defrost it tripped my breaker. the board looked none the worse for wear and tear. In fact the fuse is not blown??? How