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cap plague

Discussion in 'Electronic Design' started by John Popelish, Apr 7, 2007.

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  1. Last year, I was asked to recap a an obsolete and non
    replaceable motherboard (would have required upgrading an
    old system with a completely new version for big bucks and
    new software). Several caps were bulged as shown in this
    article and the values were low, making the regulators
    unstable, but at least, none had yet vented.

    Getting those leads out of a 6 layer, board without damage
    was a real challenge. But new, high quality caps did the
    trick and the board has been back in service for about a
    year without any more problems.
     
  2. Jeff L

    Jeff L Guest

    Yes, that is / was a BIG problem. I've recapped several motherboards over
    the years, as it was easier then replacing the motherboard with another
    model and having to spend hours installing software.

    I also recapped all the servo amplifiers (x 7?) in one of the machines we
    have, as they all started leaking (120 some odd caps in total, some in very
    difficult areas to reach, as large components had to be removed / desoldered
    to get at them)

    Another interesting failure I sometimes see is resistors with a large
    voltage differential going open! Sometimes the resistor is dissipating
    several watts, other times it's dissipating several uW. Removing the
    resistors coating shows that the resistive material had disappeared in a
    small area!
     
  3. Phil Allison

    Phil Allison Guest

    "John Larkin"

    ** Had to perform some emergency surgery on my PC last Friday morning.

    It refused to complete boot up and presented a totally black screen when
    switched off & re-tried.

    Same scenario repeated exactly after 20 minutes of being off.

    Suspecting a " heat "issue - I got out the vac and gave the insides a
    good clean, was pretty bad around the CPU heatsink & fan.

    Tried again - same scenario.

    Damn !!

    Then, I noticed some VERY hot mosfets in the CPU regulators and some wonky
    looking electros next to them - one 1000uF, 6.3 volt Sanyo and two 1200
    uF, 6.3 volt " G S C " brand.

    Extracted some similar electros from another ( unused) motherboard and
    replaced the ones in my PC - these new caps tested extremely low ESR,
    about 30 - 40 milliohms.

    Voila - the PC works fine again and those mosfets now run only warm.

    Checked out the faulty electros and found they were all open circuit, the
    best showed an ESR of about 10 kohms !!!

    How the heck could it function with O/C caps like that ??


    Moral:

    Check those hard working regulator electros from time to time and replace
    them all at the first sign of cap bulging or hot mosfets.



    ........ Phil
     
  4. Robert Baer

    Robert Baer Guest

    Happened to my VIA P4PB 400-FL (rev B2) motherboard after around 2
    years of service.
    Had a bitch of a time finding a replacement MB that would run my
    video card (AllInWonder TV in etc).
    After trying many other vid cards in one of the MBs, research
    indicated that *NOBODY* specifies which protocol / spec the video card
    supports (most MBs seem to do that).
    Real bitchy.
    But i was lucky in that i found (new looking and sealed) VIA P4PB
    400-FL (rev C2) MBs and got two.
    Bought some chip caps to parallel the same-size large caps that went
    bad on the original board.
    Despite the 2-year time frame and knowledge of the bad cap problem,
    VIA chose to use the *same* brand caps (GSC, one of the known offenders
    as well).
    Seems to me that a parallel chip cap will bypass those switching
    spikes and so keep that stress away from the poor aluminums - and so
    they should last a lot longer.
    I posted the fix in ViaArena, attempting to use ASCII graphics.
    Turns out their system ignores spaces between (?non-alfa?)
    characters, so i tried another one that looked like a space in the
    source,but got converted to a wierd character; so i gave up on that.
    They do not allow pictures so too damn bad; if anyone in ViaArena
    wants a picture they can either find me or attempt to complain to VIA.
     
  5. Robert Baer

    Robert Baer Guest

    Teleportation?
     
  6. Robert Baer

    Robert Baer Guest

    Better yet, on a new system (or after replacement), parallel themwith
    the largest physical chip cap that will "reach" the cap pins on the
    bottom of the PCB.
    Materially decreases switching (spike) currents thru the 'lytics.
     
  7. Eeyore

    Eeyore Guest

    How large ?

    Electro-migration presumably.

    Graham
     
  8. Phil Allison

    Phil Allison Guest

    "Jeff L"
    ** There are two cases of ( usually open cct) failure I see quite a lot:

    1. Metal and carbon film resistors ( 1/2W & 1 W) of from 22k to 220k with
    applied DC voltages of from 50 to 250 volts - as found in power amps, tube
    amps, scope vertical amps, SMPS kick starts etc.

    2. Metal and carbon film resistors of 1M ohm to 22 Mohms with applied
    voltages of 500 to 2000 volts - ie monitor focus dividers.





    ........ Phil
     
  9. Eeyore

    Eeyore Guest

    I recall that original VOX AC30s commonly suffered from open 100k anode load
    resistors. I have no idea if that was simply poor quality components though.
    This is from memory from about 30 yrs back. Since the amps couldn't have been
    older than 1961 vintage IIRC, that would have made them no more than 15 yrs old
    at the time.

    Their Hunts coupling 0.01uF paper caps' black plastic encapsulation used to
    crumble too !

    Graham
     
  10. Phil Allison

    Phil Allison Guest

    "Eeysore"

    ** I saw a bunch of early VOXs where 220kohm anode resistors had all gone
    high - up to 1 or 2 Mohms.

    But they were the notorious, C&N, carbon composition types.



    ** Small matters compared to early VOX's penchant for delivering 300 volts
    DC to the input jack ground connection.

    Had a customer bring me one in that lethal condition complaining of no
    sound....

    A similar condition VOX laid Beatle John Lennon out flat on his back once
    at a live concert - while the other Beatles played on in puzzlement.

    Would have been very bad for business to have electrocuted a Beatle 20 years
    before a demented Yank did so with a revolver.





    ........ Phil
     
  11. MassiveProng

    MassiveProng Guest


    The trick to de-soldering such components in multi-layer, power and
    ground plane included PCB assemblies is pre-heating the assembly so
    the heat of your soldering station tip doesn't get "soaked out" too
    quickly to do the job. About 180 F will offset from ambient enough to
    allow you to do through-hole work. You can go higher, if you are
    careful to observe what other parts are on the assembly that may not
    like temperatures that are higher...

    Working with mil, conduction cooled, ultra high density assemblies
    has allowed me to learn a few tricks. This is but one.
     
  12. MassiveProng

    MassiveProng Guest


    I have owned (still do) several ATI AIW cards of several different
    busses, and have NEVER had a MOBO that they wouldn't run in.

    The only problem is that ATI are snobs about providing fucking
    drivers. So I quit buying their CRAP, because without comprehensive
    driver coverage (Linux), the best piece of hardware is nothing more
    than a total piece of shit when it comes to PCs.
     
  13. MassiveProng

    MassiveProng Guest


    The world would certainly be a different place if the Beatles had
    been taken from us at that time.

    Damned shame that some stupid brit ambulance jock let Jimi drown in
    his puke too. The world would be a different place if Jimi were still
    around as well.

    Today... thank god for wireless hooks on stage.
     
  14. MassiveProng

    MassiveProng Guest


    Better still, pull the fucking EL cans and replace them with mil
    hermetically sealed versions found at the surplus store!

    They have 50 plus year life spans and NEVER... I repeat NEVER vent!
     
  15. kell

    kell Guest

    On Apr 8, 5:43 am, MassiveProng
    Jimi was dead long before anybody called an ambulance. The chick he
    was staying with that night was a twitchy freak that ran away, and
    spent the rest of her life in total denial.
     
  16. Phil Allison

    Phil Allison Guest

    "kell"


    ** See:

    http://en.wikipedia.org/wiki/Monika_Dannemann


    One hell of a mess.......



    ........ Phil
     
  17. MassiveProng

    MassiveProng Guest


    You may be thinking of Jim Morrison.

    JIMI was still breathing when he was placed in the ambulance, and
    regurgitated IN the ambulance, and drowned on it IN the ambulance.

    That is the NEWS I remember hearing.
     
  18. MassiveProng

    MassiveProng Guest

    She probably was murdered then, and what person would take NINE
    sleeping pills, so she probably was complicit in his death as well.
     
  19. I believe it was Nichcon that had posted a warning on it web site about
    cheap copies of its electrolytics. Not sure its related but the two seem to
    go hand in hand.

    Cheers
     
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