Has anyone some ever repaired a (partly) defective power supply of a PC.

Mine seems to delever some voltages because the fans are stil working (most likely 12V).
But some are missing because the PC won't start up.

I heard that most of the times only 1 or 2 components are defective (like the switching transisor or some diodes).

I know the cost is a new opne is rather low, but these days It happens more and more that the power supply is unstable or faulty.

Well most power supplies have labels that say you shouldn't open them and all that, and you should be careful when you're opening and changing any component connected directly to the mains supply.
You can often find the circuit diagrams for things like this on the Internet and in various specialist forums. Also, a basic understanding of electronics helps. Most components should be easy to source and fix if you are sure that you know which component is causing the problem. You could always check with a digital multimeter to find out.

It would be a much safer idea to just buy a new one. Opening a power supply can lead to serious damage to you and/or your computer. My old power supply had a message on it saying (WARNING: No serviceable components inside, DO NOT open.). Plus, any un-noticed solder splash could lead to major shorts. Remember, the power supply is where all the electricity from the wall heads. It is not lead through any resistors or anything, it is all there. It takes 0.001 milliamperes to stop ones heart and a typical wall outlet has much more.

Well I studied electronics at school (about 15 years ago).
The problem is that I don't find any circuit diagrams for that power supply.
Maybe I just open it and check the solder points.
There are no parts black and smelling (that's already a good point).
If I can't repair it , I can always reuse the fan, they are easy to remove.

well you could repair but for the time and total effort you would ahve to put in would be enormous so i would recommend buying a new one as these psus are very tempramental and it may be fixed one monet then explode in a cloud of smoke in the next minute, defiantly buy a new oen, also just because its got a label saying do not open that has never stopped me ive opened tons and they all worked lol, any ways with watever yu decide to do good luck with and and i hope it all goes smoothly, s to conclude i reccommend buying a new one or one that is working instead of trying to do a diy job, just go on ebay there are hundreds of the fings, thanks