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

[Ralf Mardorf]

On Sat, 2010-07-17 at 22:46 +0200, fons at kokkinizita.net wrote:
> 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.

Particulate matter!

>  
> 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,

The Yamaha SPX 90 II has got mechanical relays for the audio signal. One
channel had a loss of high frequencies. The relays seem to be ok, but
one relay wasn't ok.

I add switches to all those devices that will keep on standby mode.
Sometimes those switches don't come through 2 month.

If something fails today I always check mechanical elements first.

Btw. in Germany the following three products of the company 'Kontakt
Chemie' sometimes even can repair fader.

First use Kontakt Chemie 60 to clean, then use Kontakt Chemie WL to
flush out and after this, to avoid what's called 'fader death', make the
fader move smoothly again by using Kontakt Chemie 61. And only buy small
aerosol cans, the content crawl through the folds of the cans.

Debouncing and live time of switches always is a gambling game.