Got what I think is a failed psu from a 3 year old computer. The cpu is an athlon xp 2600 (from memory) and the psu connector is 20 pin with a white wire - pin 20. I've got a spare psu that works 24 pin without the white wire, is it likely to cause a problem using the spare psu?
No, they're standard thankfully enough. 24pin PSU's are exactly the same except obviously you'll have 4 pins hanging over. The wire colours do differ sometimes however I suspect you might actually be looking at pin 18 which is optionally +5v. Safe to use.
Wasn't the psu, well don't think so anyway. Remembered I had one of those diagnostic cards so put that in and low and behold computer now booting.
Symptoms - green and orange indicator lights on tower flashing manically when the computer on button pressed, all fans running, seemingly no power to hard drive or cd drive, no display. I removed everything except cpu put in video card in case onboard display failed, still nothing. Tried psu in a different computer same symptoms so started to suspect psu. Replaced everything computer now working.
Totally confused and wondering if there's an intermittant short on the motherboard.
Happens often - many whitebox cases have little imperfections especially near the PCI slots and can cause shorts as a result. One of the first things I do with whitebox PC's is to run it out of the case to rule out a short, space/time permitted :D