OK...
First of all: in the end EVERY single component gets its power from the PSU. Maybe not directly but also the mainboard doesn't power itself in some magical way.
Had to say that after reading several times in this thread about the difference if something got its power from PSU or MB. There is no difference if it's the psu which is at fault...
@ STROBE: it's always difficult to make diagnostics on a somehow damaged PC if you don't have it lying before you. For example I have already seen a faulty IDE cable completely preventing a PC from even POSTing. Something like this is impossible to guess from afar...
So if a fault is not obvious to see (like for example everything that should be cooled is covered with dust...) the best thing to do is to unplug things step by step and see if the problem disappears at some point. And if you already stripped the box down to only the very necessary stuff (MB, CPU, RAM, VGA, PSU) and the fault still remains you can only see if you have some parts to temporarily exchange the rest piece by piece. This is necessary since without most of these parts the PC won't POST anyway so just taking away RAM completely will not make you see anything.
But I guess you have a PC yourself, so you might have some parts to exchange - and like suggested many times before I would also go for the PSU at first. Like rabidmaddog already said there is no need to mount it inside the case - just plug everything in where it should be and see if it works. I wonder why you haven't done that already after every second suggestion in this thread told you to see if it's the PSU...
You know, a spinning fan does not necessarily mean that there is enough power to also run a CPU/GPU which need A LOT more power. Also a PSU has different outputs with different voltages and sometimes only one of them can be faulty causing the whole system to fail.
If all of this doesn't get you anywhere, there is still the other way round that you can try. If you have a flawlessly working PC you could exchange its parts piece for piece with the ones from the damaged one and see at which point the working PC stops working...
I know this can be a lot of screwing and mounting and stuff and can take a long time but if you have absolutely no hint why the PC stopped working a simple try-and-error-process is anything you can do.
I hope I could help you and make myself clear because my english is not quite perfect in some of these technical terms...
Anyway - I wish you good luck in finding the error...
P.S.: ah! I forgot! There are parts that are not
directly powered by the PSU - it's the RTC and the CMOS...
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