It depends of what the history of the box was. Suppose it has become defective after some bad weather as a day with some lighting strikes, then the most logic thing to do is to replace the cpu for another. But finding a new one, as also one which should work with these boxes is not so easy to find, and also the price may to high which makes it not so interesting to do. If it's known that it became defective after using it, than it could be maybe a solder ball short circuit. Then it may maybe, with the right equipment and knowledge to perform it all as removing/reballing/re-solder, solve it. And yes, I've done it several times with vu+ and dreamboxes with different results because it is never certain whether it will work after such an operation. Another way it may happen, is due insects. I had some cases which damaged the cpu due microscopic small insects under the cpu, and some cases which were repaired after destroying them after a reflow. In any case, there is no way to say what the best method is, sometimes you win, and sometimes you lose to repair a box. Even with the most expensive or best equipment there is no guarantee that it always succeed.