Didn't go quite the way I had in mind........................

Discussion in 'General Electronics Chat' started by ian field, Nov 23, 2015.

  1. ian field

    Thread Starter Distinguished Member

    Oct 27, 2012
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    When I powered up my other PC last night, the monitor produced a nasty niff instead of anything on the screen.

    More or less resigned to scrapping it, I opened it up to see if there were any bits worth salvaging - there was a nice cluster of toroid inductors that would be handy for a current project.

    Then I noticed the blackened mains fuse. More out of curiosity, I pulled the power board and checked the MOSFET. That was OK so then the bridge rectifier - also OK. Having got this far, I wired a 40W bulb across the fuse clips and fed it mains, the bulb intermittently tried to almost light - and thick grey smoke billowed from the common mode choke.

    Over the years, I've found a small number of common mode chokes damaged by a short downstream - but this is the first I've ever seen one of those chokes actually cause the fault.

    Needless to say, the monitor got repaired and the junk box was deprived of some quite interesting components.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Think of it as junk parts repurposed into a monitor. Seriously, that's effectively what you did by keeping it out of the landfill.

    More than once I've been foiled in my junk collecting by fixing the junk good as new.
     
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  3. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    If the monitor is powered via a UPS that performs a 'diagnostic inverter run' each power-up you may want to check its output (inverter active) -- I've known 'wonky' UPS inverters to do just such damage to PSU line conditioning components...

    Best regards
    HP
     
    Last edited: Nov 23, 2015
  4. ian field

    Thread Starter Distinguished Member

    Oct 27, 2012
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    Its straight off a strip block with MOV protection.

    As it happens, I habitually unplug the whole computer desk strip block when I'm not using it.

    There's no hard mains switch, so unplugging it is a must. At the point I narrowed down the faulty part, it was shifting barely enough fault current to glow a 40W bulb - that could have *SO* easily not blown the T3.15A fuse, and started a fire.

    Its actually a TV that just happens to also have a VGA socket, its also the only flat screen I have with a SCART socket.
     
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  5. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Thanks to the ban on polyhalogenated biphenyls (specifically, recent inclusion of PBBs to 'the list') such fires are increasingly 'in vogue':rolleyes::rolleyes::rolleyes:

    Best regards
    HP
     
  6. Aleph(0)

    Member

    Mar 14, 2015
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    I know PHB more public health than tree hugger issue but reminds me so I say it ironic how loudest voices for enviromentalism and conservation come from confirmed urbanites who not know difference between wilderness and city park:rolleyes:
     
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  7. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    I hear that! -- It happens to me all the time when wishing to salvage back-light inverter transformers from 'condemned' big screen TVs/Monitors:rolleyes:

    Best regards
    HP:)
     
  8. ian field

    Thread Starter Distinguished Member

    Oct 27, 2012
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    It worked by producing such thick smoke that it suffocated its own fire.

    Similar principle to the limit on fermentable alcohol because anything over a certain concentration poisons the yeast.

    Salvaging parts from scrap boards with a blowtorch, I've managed to ignite a board once or twice - but they mostly self extinguished fairly quickly.

    Boards will certainly burn on a bonfire, a continuous arc in an equipment can sustain a fire long enough to ignite other materials.

    When I serviced monitors, there was a Chinese make that I never managed to identify the manufacturer. It had more safety certification stickers on the back than most others put together.

    They used dodgy resistors in the SMPSU sensing divider. When these went high, the output voltage crept upwards. Usually the vertical output died first and the short caused its dodgy PSU rectifier to fail and crack in two - unencumbered by the shorted vertical, the voltage set off upwards again, eventually the flyback voltage in the horizontal stage got high enough to jump PCB tracks.

    The heat usually melted the bottom of the case and welded it to the clip on pedestal - since that had to come off to get at the screws, destroying the case was the only way in. Sometimes the horizontal stage would fail short circuit and finish off the PSU once and for all, but some had been arcing long enough for combustion to happen on the bottom of the plastic case.
     
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