What alot of users don't know is a psu can get pretty darn hot. The movement after shutting down can allow a weaken component to flex and/or move abit and break its electrical contact and/or soldered area. Which means it was poor to begin with or it was really under a lot of stress, heat build-up. Also componets can crack with similar results. The 300W pus depending on its manufacturer were prone to failure until better wattage units came out. There was a rash of bad psu's for awhile but that was years ago or when the 300W was the norm for many a new system. Designs have changed but low wattage psu tend to be stressed and/or come to failure in short order vs. newer high wattage psu that can get into the 1KW range. Some psu do have a build-in "power fail" circuit which causes the psu to remain-OFF when it fails or reaches certain damage. It self-detructs in order not to be a source of fire and yet the fuse remains intact, go figure. Open the failed PSU and check it out. It may even power-UP if re-installed if one of the above was the cause now that it has cooled(it happens).
I purchased a new 300 watt PSU (a Sparkle FSP300-60BT) 2 months ago to replace a failing PSU in an older P2 machine that i use primarily for older games and the PSU died the other night in the strangest fashion.
I needed to physically move the computer over to a new area last night. So i shut down, began unplugging, then began replugging. The whole proccess took no more than 5 minutes. After i had everything setup, i went to flip the computer back on and nothing, no lights, no sounds, no spinning up of fans, absolutely nothing. I then swapped in another PSU to check what was wrong and the computer booted up without an issue.
There is no on/off switch in the back of the PSU. Is there some sort fail safe switch with these types of PSU's? How can a PSU suddenly die like this?