A very simple check for the fet is meter pins 2 and 3, it will read a dead short if the fet is dead, but this is only a basic test and may not show anything up.
Also check the above items on mvs's list, and then check the transformer primary, last time I had this and the time before the transformer was shot, apllying mains for anything more than a few uS toasts the primary.
The 3843 is a smpsu controller and should be easily obtainable, changing it as a matter of course is prudent.