Aging is the most common and costly source of electronic problem.
The amplifier take place where no infiltration of humidity occurs.
I can't say (and measure) if the transistor auto-oscillate.
1) I didn't consider this.
2) Surely there's a bad thermal connection: you could see when I'll post a pic next week.
3) No built in AGC here, but the gain of the antenna is far to overdrive the input of the amp: without amp I can receive few signals.
5) The SMPS is voltage regulated, 12V measured. I also tried a linear PSU with the original CLCC filter but the transistor is always hot.
6) I'll buy a new BFG135.
Take a look of the picture. This is the solder layer, in the component layer there is the "burnt area"
Thanks you, I'll leave the amplifier working for the rest of its days.