Ian, thank you, I have some points wishing to be declared:
1) I replace the MOSFET (usually?), does the problem will over?
2) If we say for some reason, the photocoupler ( = feedback) was defective, what is the effect of that on the board? (no output voltage will be produced?, short circuit at the primary side? unregulated output voltage?...etc)
3) I can see a Triack, what its function?
4) Have you ever fixed a SMPW and the problem was the ferrite hight frequency power-transformer?
thank you in advanced!