Computer will not boot - very weird issue

[deadlock]

Hi all,

My 1366 build (about two years old) suddenly would not turn on about six weeks ago. Power was getting to the motherboard (on-board LED was on) but pressing the power button (whether through the case button or on-board power button) did not turn it on.

I started a process of diagnosis, and eventually found that if I unplugged the 3.5" internal card reader from one of the USB headers, the motherboard turned on fine. So, the problem must have been a short within the card reader. Life goes on.

But two weeks ago, with the card reader still unplugged, the same thing happened again.

This time I stripped EVERYTHING from the build, going as far as to remove the entire motherboard from the case with nothing in it but a stick of RAM, the graphics card and a CPU, and it STILL wouldn't turn on.

In a desperate last attempt, I stuck a couple of paperclips into the correct contact points within the ATX 24v power connector and shorted them to "force" the PC to turn on. It turned on fine, but of course as soon as I broke contact between the paperclips, it would turn off again.

However, if I pressed the power button straight after this, it would turn on normally. Everything works and is rock solid. Everything is plugged into the build. No BSODs. No crashes. If I shut the computer down, the only way to turn it back on again is to short those paperclips again and then very soon after that press the power button.

I do not know what is causing this, and why "jumpstarting" it by doing the paperclip-short would cause the issue to temporarily disappear.

Any ideas?

 

[paul878]

Any bulged capacitors on the motherboard?

 

[deadlock]

Just had a good look. All the caps on the board are polymer ones without vents, so I don't think they would display the distinctive "bulge" even if they were failing. I did not see anything out of the ordinary.

Thanks for the pointer though.

 

[notposting]

Have another power supply to try?

 

[Smoblikat]

Little known fact about mobos. The green light is not actually controlled by teh PSU. The motherboard has its ow converter for the LED, all teh PSU does is act like a wire from the wall to the mobo.

 

[grohl]

Power switch bad?

 

[mindless1]

Have you tried clearing CMOS and checking battery voltage? Until I had seen one myself I never knew some boards act dead if the battery is dead. Otherwise, the 5VSB circuit on your PSU may be going bad. Not only would that affect turning on, if it's a progressively worsening fault that would explain why removing the reader running off USB which is potentially 5VSB rail powered, lessened the load back into a stable region.

So as notposting already posted, try a different PSU. If you are using USB keyboard or mouse from 5VSB powered USB ports, you might also see if unplugging those helps any. Measurement of ripple & DC voltage on 5VSB lead with a multimeter wouldn't hurt either if within your capabilities.