I've seen guides for replacing it, but it's an expensive PSU specially if you're not in the US (my case). I've read that most PSU problems are a capacitor or something silly that could be easily replaced without having to spend hundreds of dollars.

Is it possible to repair the PSU I already have? Any ideas?

Thanks everybody for your replies. I do not understand much of PSU, and was wondering if it was a matter of simply replacing specific parts, like the ones that are usually faulty, I thought that kind of knowledge would be good to have documented out there.

But given the fact that it might be extremely dangerous, or excessively complicated, I now understand the reason the information is unavailable, and conform to the fact that I will have to replace it via Apple.

On a tangent matter,the problem seems to me, that this sort of information is really not available even to certain technicians. I took the computer to an Apple Authorized Center and they told to replace the logic board. I still have the same problem and it really seems to be the PSU, but I can't pinpoint the diagnostic.