Dario2 wrote:

A diagram will not help too much. Check the capacitors around FBT. If
they are OK probably your FBT needs to be replaced.

Well, I already changed all capacitors arount it. I'm going to change all
capacitors on the power supply part. I don't know why a diagram shouldn't
help. What FBT fault could blow the line output transistor only during
power on and not during hours of work?

Thanks

Francesco IZ5DWF