Computer Re-boots

[Dr_Ju_ju]

A cautionary\information tale for computer owners...

Background: my main desktop computer, would, at random times re-boot, with event viewer reporting the system had lost power. This could be understandable, as I'm currently rebuilding my kitchen with new wiring etc, and the RCD trips are very sensitive to any hint of cross connection (neutral\earth) even though the MCB are off, which does 'kill' the circuits on that RCD. This is exacerbated by lots of dust in the air.

I've also been sat @ the computer, where it has re-booted in front of me, where no external influence should be causing the re-boots. Whereas the re-boots would occur on a 1\2 times per week basis, last Friday it got to the point, where it started a re-boot loop, i.e. the computer would attempt to boot but a few seconds later (between 5 & 20) it would start the re-boot sequence.

When this occurred, I could hear a 'clicking' sound coming from the power supply, a Corsair RM850, which from what I've read, this clicking would indicate a problem with the power supply. This wouldn't surprise me its probably 5\6 years old, so coming to the end of its life....

Luckily I have a spare Antec supply, which I duly fitted into the computer. But, on switching on the supply, the it was still performing a re-boot sequence !! 

After a little head scratching, one thing I noticed, is that the computer would re-boot as soon as power was applied, i.e. I didn't need to push the 'power\re-set' switches. Removing these switches' connections, from the motherboard, stopped the re-boots after applying power. 

If I then carefully shorted the 'power' pins, on the motherboard header, the computer re-booted as normal, with everything working as expected, YAY !!

So the problem was with the switches, which is a little odd, as this type of switch is a momentary push to make, where the switch is open circuit till pushed, where it shorts the contacts and goes open circuit again, when released.

On checking the switches with a continuity-tester\multimeter, I found that, when pressed the switches did short the contacts, but when released, they did not go fully open circuit, but instead had a residual resistance of 1-5K ohms !! This residual resistance was enough to fool the motherboard into believing that the 'power\re-set' switches had been pressed so initiating the re-boots.....

The encapsulated design of the switches renders them un-repairable, which is preferable, but a bit of a faff sourcing replacements, so I've had to do a quick lash up, with some spare switches...

So if you have a computer that occasionally re-boots, for no reason, don't always blame the OS, motherboard, disks etc. it may be the humble switch that you use to power it on.

 

 

[henry b]

Now that was a very good piece of fault tracing.......Henry b.