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SyncMaster 151s LCD monitor turns itself completely off after 20 minutes

Discussion in 'Electronic Repair' started by Paul & Fiona, May 6, 2006.

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  1. Paul & Fiona

    Paul & Fiona Guest

    Hi Folks,

    I have a powersupply inside a Samsung LCD monitor that turns itself off
    without fail after 20 minutes.
    All the Switchmode DC rail voltages such as 12V, 5V and 3.3volts all
    completely disappear.
    When you disconnect the mains for about 5 minutes and then plug it in again
    the power LCD monitor goes for another 20 minutes
    before completely losing it's power i.e. No Green power LED and no Picture.
    Even disconnected the load from the power supply, but it still has the same
    symptons.
    Sprayed Rapid Freeze on all the components, but it did make the power supply
    come back on-
    Even changed some Electrolytic caps due to higher than normal ESR readings
    but these have still made no difference.
    If you leave the power supply in standby mode with the green led flashing,
    it will stay on indefinately for days without
    losing the 5V DC rail.
    So if there are some Switchmode power supply experts out there, it would be
    nice to get
    a tip to solve this strange peculiar problem.

    Kind regards

    Paul Bellett
     
  2. Franc Zabkar

    Franc Zabkar Guest

    I'd be doing a little circuit tracing. For example, locate the PWM
    controller IC, find its datasheet, and determine if it has a latching
    shutdown pin. Then work backwards from this pin and determine whether
    it is connected to an overvoltage circuit, a current sense circuit, or
    a on/off pin on the mainboard.

    - Franc Zabkar
     
  3. Paul & Fiona

    Paul & Fiona Guest

    Thanks Franc,

    Sounds like you are right on to it - A good trouble shooting technique for
    analysis.
    I guess, if the PWM controller is anything like a common garden variety of
    UC3842 PWM IC , there are a couple
    methods for shutdown that are accomplished by, either raising pin 3 above 1
    V or pull pin 1 below 1 V,
    which would localize as to where the external fault to the PWM would
    probably lie.
    I'll keep you posted on what particular component is causing the PWM IC a
    bit of grief!

    Cheers

    Paul
     
  4. Paul & Fiona

    Paul & Fiona Guest

    Hi Frank,

    Problem solved - Found that after around 20 minutes, an obsure 5 pin SMPS
    supervisory IC with the only markings
    on it (S0202 / DP502T) had made the heatsink it was mounted on, rather
    hot to touch. In fact even the PCB had some brown heat marks close to where
    the actually IC resided.
    However Samsung did not fool me with their cyptic identification of the
    device! I thought to myself, it is probably a common SMPS IC under a
    different manufacturers name.Yes:-, In fact it was a Fairchild
    KA5Q12656RTYDU IC, and I confirmed this by Fairchild's Semiconductors web
    application note on the KA5Q series, for which Samsung have copied the
    identical circuit including surrounding components for 'their' LCD monitor
    power supply!
    This IC has a built-in thermal shutdown latching circuit for when it is
    operating abnormally.
    Anyway, I replaced it with the Fairchild KA5Q12656RTYDU IC from Mouser
    Electronics ($3.80 USD) and it is now running almost as cool as a cucumber,
    and the fault has of course entirely disappeared.

    Cheers for now

    Paul
     
  5. Franc Zabkar

    Franc Zabkar Guest

    "KA" is/was a prefix used by Samsung. The reason Fairchild also use it
    is because Samsung sold off its power electronics division to them.

    http://query.nytimes.com/gst/fullpage.html?res=980CE6D9123CF931A15751C1A96E958260

    Congratulations on your clever sleuthing, BTW.
    - Franc Zabkar
     
  6. Guest

    Actually this is a Samsung component. Probably Fairchild took over the
    Samsung semiconductor division, or the other way around??
     
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