LG Above range microwave problem out of no where. LMV 1680ST

So basically, I bought this used a year ago and it has been great. 3 days ago my girlfriend put in a bowl of soup, hit cook and after about 6 seconds, the unit popped and lost all power. I checked the breaker (microwave only) in the box down stairs and it was tripped. I swtiched it back on and went up to see the unit was powered again. I set the clock, the vent worked, lights etc... So we selected cook time of 1:54 pressed start cook and the unit immediately went dead again, tripping the breaker. Here I removed the power cord, plugged it in the dinning room with an extension cord and shut everything off in there. Same issue as it tripped the dinning room breaker this time. I then tested a portable microwave on the original microwave receptical and it worked fine. What could my issue be here, I'd really like to keep this unit and dont have a problem replacing a bad part. Where should I begin? UNIT http://www.electronicsz4sale.com/lmv1680st-microwave-oven-p-23214.html