Re: [LAD] [Somewhat OT] Strange failure mode of a PC

On Saturday, July 17, 2010 10:47:58 pm fons@email-addr-hidden did opine:

> Hello all,
>
> Early this week one of the three 'rendering' PCs of the WFS
> system in the Sala Bianca failed. It just appeared completely
> dead and didn't even try to boot when the power button was
> pressed, but the standby power (for the network interface)
> was available.
>
> I suspected the power supply, so ordered a new one which
> arrived two days ago. Installed it and things worked again.
>
> But now comes the interesting part. While installing the new
> PS, I also disconnected the wires to the power button, and
> to test I just used a screwdriver to short the two pins that
> normally connect to it. But when I reconnected the power button
> the PC switched off after a few seconds. So it seemed as if the
> power button was permanently being pressed. I again installed
> the old PS, and things worked as long as the power button was
> not connected.
>
> Measuring the power button switch with a multimeter showed
> an unstable resistance value of between 1 and 3k while it
> was not pressed. So I removed the thing, which turned out
> to be a cheap miniature switch, a little cube of around 8
> mm size. I opened and disassembled it, and noticed that the
> contacts had some black dirt on them. Cleaned with aceton
> and reassembled, and things worked perfectly again.
>
> What I don't understand is how the contacts got so dirty.
> If a resistance of a few kOhm is enough to make it look
> as a closed contact then it can't be handling large currents,
> so there should not be any arcing. And the construction of
> the thing is such that it is virtually closed, no dust or
> whatever could ever creep in.
>
> Still it's quite sobering that this cheap 0.30 Euro thing
> was capable of bringing down a 1600 Euro workstation...
> Who would suspect a switch to fail in this way ?
>
> Ciao,

Some lubricant obviously made its way in there, and one would have to
assume that the switched current was sufficient to cause a microscopic
arcing, which will in time create a smoke film on the internals, which is
what you were measuring, and which caused the PSU to think the button was
pressed full time. Lubricants are a good idea only on a circuit that can
be called a dry circuit, where dry=microamp currents at very low voltages.

Washing it out with acetone was ok, but I would probably have reached for a
can of paint thinning alcohol as it is less likely to degrade the plastics.

Yup, I am a C.E.T., with 60 years of that sort of trouble shooting.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
God is subtle, but he is not malicious.
                -- Albert Einstein
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