LCD Monitor power circuit

Discussion in 'General Electronics Chat' started by redart, Sep 29, 2010.

  1. redart

    Thread Starter New Member

    Sep 29, 2010
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    Hi,
    Am trying to fix an LCD monitor who's power board is playing up - circuit diagram attached.
    The output from the Schottky rectifiers should be +12V and +3.3V, circled in red on the diagram, however instead the voltages are fluctuating between 3.5 and 5 (instead of 12), and 1.6 and 1.8 (instead of 3.3). Can anyone suggest where I can look to start troubleshooting it ?.
    Thanks.
     
  2. tom66

    Senior Member

    May 9, 2009
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    First things to suspect in any power supply are the caps on the output side.

    Check if the electrolytic caps are bulging or not. If they are bulging, they are probably leaky and causing problems for the rest of the PSU.
     
  3. redart

    Thread Starter New Member

    Sep 29, 2010
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    Thanks for the suggestion Tom. The output caps all look good - no bulges or signs of leakage. I've replaced a few bad caps on other boards, but in my experience (pretty limited admittedly) the voltage is fluctuating too consistently and smoothly to fit with failed caps. I may be wrong, but in the absence of an ESR meter I have no way of testing them. Hoping to find fault elsewhere until I can lay my hands on some new caps.
     
    Last edited: Sep 29, 2010
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Take a look at the capacitors 2112 and 2113.
    These will power the regulation IC.

    Bertus
     
  5. redart

    Thread Starter New Member

    Sep 29, 2010
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    Thanks Bertus.....physically they look fine also. Voltage across 2113 is fluctuating steadily between 1.6 and 3V. Does this suggest anything ?.
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,649
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    Hello,

    Be carefull with measuring on that capacitor.
    It is on the live side of the circuit (connected to tha mains).
    It looks like there is a overload, the circuit goes into protection.

    Bertus
     
  7. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I'll agree, a constant fluctuation on more than one output is a sure sign the circuit is trying to start then it senses sometjhing's wrong, shuts down then retries.

    Try measururing the ohms to ground on the power supply rails (once they're discharged, of course) then see if something fishy shows up.

    If nothing there try to isolate the lines and put some dummy loads on them, that should tell you if it's the power supply itself or not.
     
  8. debe

    Well-Known Member

    Sep 21, 2010
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    Hi Redart, The Triac 7109 is an over voltage protector I would start by checking that & 6114 15V zenner this senses the 12V rail. 6116 3v9 Zenner senses the 3.3V rail. This is probably a good area to start checking. Have had simmilar problems with over voltage circuits in TVs causing the powersuply to pulse &have low voltage. Daryl
     
  9. redart

    Thread Starter New Member

    Sep 29, 2010
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    Thanks for the suggestions. It was indeed an overload problem, and eerr...mea culpa....it was due to an earlier bodged repair attempt. I had replaced surface mount resistor 3134 with a standard resistor and soldered it to the cathode of the 6114 zener instead of to ground :(. The power board is now operating, however......the repair I was attempting was from a Philips service bulletin which describes the original problem with the monitor - everything working until the video signal is applied at which point the screen goes black and the power LED flashes. The backlights are definitely on. Philips identify diode 6115 as being at fault. It's a 1N5347B, and measures 170 ohms one way and 70 the other. The datasheets don't specify forward and backward resistance - do these values sound right or do they confirm the diode as faulty?.
    I have now done the mods as per the service bulletin but it hasn't made a difference. For anyone kind enough to help me further, the Philips service bulletin can be found here http://www.loadhigh.khv.ru/Monitor/Philips/peredelka%20170S4.pdf Any ideas why this screen doesn't like video ?
     
  10. tom66

    Senior Member

    May 9, 2009
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    The diode is failed. A diode should have almost infinite resistance one way and almost zero resistance the other way.

    If your meter has a diode check function, use that.
     
  11. redart

    Thread Starter New Member

    Sep 29, 2010
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    0
    Thanks Tom. The circuit symbol shows a normal diode, but it's actually a zener (according to the datasheet - my apologies if I'm stating the obvious). Anyway, I confirmed it's shorted by biasing it forward and backwards and observing current flow both times.
    The outputs of 12V and 3.3V are holding up with the mods I have made but the screen still goes blank when a video signal is applied. Could a shorted 6115 have damaged anything else?
     
  12. tom66

    Senior Member

    May 9, 2009
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    The meter will only apply 0.3V or so on the resistance range, so should not bias the diode either way. (This is done for a reason, so you can measure a resistor paralleled with a diode.) Replace the zener, it'll cost you all of a few cents.
     
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