@Ty_Eberfest Thanks for all your comments.
@Ian it's totally fine for me.
But please wait one more day until we take some decision about shipping anything....
Today my local electronic shop promised me to order for me IRF640 - should be here tomorrow. Let's check if replacing it by new one will do the job
RNi - I think that's a good plan. If I am right about it being the IRF640 (I'm 90% sure) then you will have it working immediately instead of having to wait for international shipping and everything. Let us know how it goes!
Ian - Thanks for having a look at this thread. I didn't want to suggest "back to base" until you had seen what we've been doing. You're a long way from home! Be safe.
Look into it later when the dust is clearing off the crater.
when you exchange the IRF640 take care of static electricity. These parts don't have any gate protection and are rated for a Gate to Source voltage of +/-20V max. which can easily be reached.
As these Power MOSFETs have a higher gate capacitance than high frequency FETs it will take more Coulombs to reach the critical voltage but nevertheless keep in mind that there is no protection, so you have to take some care!
I don't know which ESD precautions you take when working with such parts...