I worked on a very unusual PC:
As soon as I plug mains power into the PC, the power button would not start the PC until I waited for about 20 minutes, and then the PC would start normally.
Ok, looks like a faulty power supply… I replace it with a new standard 680 Watt PSU… The PC seems to start immediately, but I hear strange crackling sounds from the power supply… sound like its struggling to supply enough power.
And the PC just crashes after just 2 minutes.
Fine, I’ll use a 860 Watt PSU. That works a bit better, but it still makes crackling sounds and still crashes, or sometimes it just refuses to start.
At this point, I’m suspecting the video card (an inno 3d 7300GT DDR2), as it needs external power (from the PSU), but when I replace it with a low-power video card, I get similar problems…
I try to test the RAM, but the system doesn’t stay up long enough to run memtest.
I unplug the HDD and the 2 DVD drives, the floppy, remove the sound card and the dial-up modem… and now the PC starts normally.
After a careful process of elimination, I track the problem to one of the DVD drives…
It seems that one DVD drive is faulty (maybe an internal electrical short circuit), such that id draws a large amount of electrical current from the PSU… so much that the rest of the system is affected.
The strange thing is that even when I remove the faulty DVD drive, the original PSU (and my new 680 Watt unit) is still not able to run the system correctly.
In the end, considering the time taken, the cost of a new DVD drive and PSU, the customer decides that its not worth fixing, and will buy a new PC instead.