3 microwave ovens in 18 months - all won't heat up...

Question

First model (Nu-Tec) lasted well over 3 years then it popped (almost the same noise one gets when metal is put in by mistake) while heating food one day and never heated again. Everything seems to be normal apart from the main task it is supposed to perform - heating. Bought a new microwave - Defy (DM0289) - this lasted just over a year and then exactly the same thing happened. Bought another one - same model - this time it only took 2 months for it to happen again. I live in Namibia in the middle of nowhere and there is a real shortage of experts in this field. After reading up on your website how to fix this problem I am not going to risk my life as the nearest hospital is an hours drive away. I just want to know why this is happening so frequently and if there is anything I can do to prevent it from happening to the next new microwave oven.
Please help

Carla