On Saturday, July 17, 2010 10:47:58 pm f...@kokkinizita.net 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 _______________________________________________ Linux-audio-dev mailing list Linux-audio-dev@lists.linuxaudio.org http://lists.linuxaudio.org/listinfo/linux-audio-dev