missing 6 volts from psu

Discussion in 'General Electronics Chat' started by abc_user, Sep 9, 2011.

  1. abc_user

    abc_user Thread Starter New Member

    Joined:
    Aug 24, 2011
    Messages:
    12
    hello,

    i have a lg lcd tv power supply which shows 0v on its 6 volts output. all other voltages supplied by the board are ok, 12v,3.4v,19v, 5v stby, 24v for the inverter.

    i have been struggling with this for 2 days now...any ideas?
    here's an image of the board
  2. Jotto

    Jotto Member

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    Apr 1, 2011
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    Biloxi MS
    Does the unit come on? Missing 6vdc isn't telling much.

    But since I work on a lot of LG units. Look for 100mf @ 25-50v. Also 47mf @ 50v. The primary usually never has a problem on the power supply, it will be on the secondary side the PMPS. Sometimes its a larger value cap, but look at your small ones first.

    I had to help someone. Also look at the PWM chip and replace cap associated with it. Usually 47@50v, but not all the time. It also may have 2 PWM chips.

    Highest I have replaced is 1500mf.
    Last edited: Sep 9, 2011
  3. abc_user

    abc_user Thread Starter New Member

    Joined:
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    thx for the reply

    yes the unit turns on, the back light comes on, there is a no signal across the screen too.

    the thing is that the 6 v is sent to the mainboard, then to a 5v voltage regulator and then to 5v buffer
    i think from looking a circuit diagrams that this 5v is used for the tv tuner, and also goes to a transistor which upon sensing a signal for a composite video signal turns on and forwards the signal to the vctp ic resulting in us seeing a picture.

    because of the 5v not being present the transistor does not work and if i connect a composite video source it still shows no signal. i was able to get a picture by directly connecting the video carrying cable to a smd capacitor near the vctp ic and the other end to ground, with help from schematic diagrams.

    anyway, the mainboard isn't a concern, because there is no 6v output from the psu to begin with. and yes i have tried replacing all 10v,25v even the 50v 47uf but nothing...

    any ideas?
  4. tom66

    tom66 Senior Member

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    Basingstoke, UK
    Test all the diodes on the secondary side of the power supply. An open circuit diode would explain your symptoms. If you post pictures we can advise on which pins of what components to check. [I can't tell anything from the ebay image.]
  5. abc_user

    abc_user Thread Starter New Member

    Joined:
    Aug 24, 2011
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    i will try to upload pics later...

    there are only 4 schottky barrier rectifiers on the secondary side...

    1> this i believe this is for the standby circuit...fch10u10

    2> two fcq10u06 ... 12v and 19v

    3> one mbr20100ct ... 24v

    i have the first 3 out...the method i found for testing them 2 of them test fine with the method in the link but fch10u10 has no ohms reading with the black probe to the center pin and red probes on either pins while the other two do...probes placed the other way, red center and black either, meter goes full scale...on all diodes...

    but i have another schottky salvaged from a dell psu ug10dct which is also a 100v, 10a as the fch10u10 and it, too, has no ohms reading with the black probe to center pin...

    btw meter was set to 10k.......

    any help?
    Last edited: Sep 9, 2011
  6. abc_user

    abc_user Thread Starter New Member

    Joined:
    Aug 24, 2011
    Messages:
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    diodes are removed...

    Attached Files:

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