I'm assuming the failed FET is in a metal can, if so the failure could be tin whiskers. Several cures have been discussed in TekScopes recently. The simplest was to rap the can to mechanically dislodge the offending whisker. More in the last resort category was finding a short and fusing it with a voltage low enough to not damage the semi when the short disappears. 1st step is to ohm out the device to see if there is a short. Good luck.
