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LCD Power Supply

Discussion in 'Repairing Electronics' started by karyoker, May 23, 2009.

  1. karyoker

    karyoker New Member

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    This Tv is a Polaroid FLM-323B 32". It's blowing fuses in the power supply. Shown is a partial schematic and a data sheet of components involved.

    Q2 and the Zener on the source were shorted. I replaced Q2 with a FVP 11N60 and the zener. I then had 5VSB and led lit. I hit the on/off switch and the led turned blue then in about 5 seconds the fuse blew again. Q2 and the zener are shorted again. The source has 5 resistors to ground all marked 1R5 which I take to be 1.5 ohms.

    I did replace the 2 infamous caps in this power supply BTW this is a 200-P00-MLT666A-AH I do have the full schematic and working on a parts list with Mouser stock numbers and digital macros of trace brd to sell later on.

    There are no other signs of bad components or ohmed shorts.

    schematic http://cabaret-karaoke.com/fix/_1.PDF

    data sheet http://cabaret-karaoke.com/fix/Q2.pdf

    I did find out later Q2 is a 18N60 which is an 18 amp and not 11 amp which I replaced it with. Do you think that is why it shorted again or do you think its a bad inverter board which is puttting too much of a load on the power supply?
     
    Last edited: May 24, 2009
  2. Techyboy

    Techyboy New Member

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    When you say the Zener was shorted, were you measuring with the component in-circuit or removed? If your drawing is correct you are stating that their are "5" 1.5ohm resistors in parallel so using a regular DMM will not give you an accurate reading. If that zener does have a short reading with the component removed, you have to also check those parallel resistors, if even one of them was open your meter still wouldn't be able to detect it, that is, unless you have a milliohm meter which most hobbyists don't posess.
     
  3. karyoker

    karyoker New Member

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    It was removed and using the diode or ohms function it is shorted both ways (no front to back ratio) . The fet is shorted all 3 elements. I did discover a .5 ohm 1/2 watt resistor under the heat sink in parallel with the 1R5,s. After removing it ohms about .5 and the smd's ohm about .5 ohms.

    Here is a jpg you can call it up with paint and magnify. Maybe it will give you a better (feel) for the problem.

    Q1 does have a front to back ratio in circuit so I do think it's good.

    I cant right now but I think I will order the 18 amp from Mouser and replace everything and try with the inverter unplugged and see if it will turn on. Or what are your thoughts?

    photo
     
  4. dave

    Dave New Member

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  5. karyoker

    karyoker New Member

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    The photo was taken with the transformer pins unsoldered in TS but they were soldered before the test.
     
  6. Techyboy

    Techyboy New Member

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    By the way, what is the fuse rating of the power supply that is blowing?
    Those devices that you are replacing can obviously handle alot of current, so they shouldn't be easy to short like they appear to be doing. It has to be something else also, i would check to make sure no other diodes/components are open before that part of the circuit, such as D2. It would be wise to replace the part with OEM rated(current) or it could get expensive.
     
  7. karyoker

    karyoker New Member

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    It is a 5Amp/250V.

    I dont have a clue what IC5 (it triggers Q1) is. I think when I get the replacement parts I will load the power supply and scope out some signals and measure voltages.

    BTW I did get most of the rest of the schematic done and weeding out mistakes etc.

    schematic
     
  8. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Just to perhaps help you out a little, it's common practice to have a power factor correction circuit before the main power supply - IC5 and associated parts may be the PF correction section, and the actual PSU is the part to the right of IC5. The bottom part, as I'm sure you know, is the standby PSU.
     
  9. karyoker

    karyoker New Member

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    Ok Nigel the 12v &24v are driving opto P4 which triggers IC5. Do you think there ia a fault in this circuit which is not switching Q2 and as a result Q2 is in runaway?

    We know the 5VSB works because I had a red led and blue when I turned it on. Thats when the fuse blew again.
     
    Last edited: May 25, 2009
  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    P4 is an overvolts circuit, overvolts on either of the two rails turn P4 ON and that should shut the supply down.
     
  11. chriss123

    chriss123 New Member

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    Hi,

    Can someone tell me the pin number that is activated (and how) when the unit (power supply) is turned on.

    Thanks
     
  12. karyoker

    karyoker New Member

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    It is STB on pin 10. It turns on Q10 which turns on the fet Q13. It also lowers the voltage to the LED on P2.

    I would like to get a part # on !C1 & IC 5.. or generic equivalent..

    Actual designation on chips IC1 62674.. IC5 2F473.. (I think its hard to read) Same chip in a 15" power supply 1D7575RS...
     
    Last edited: May 26, 2009
  13. chriss123

    chriss123 New Member

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    Thank you.

    Is the STB high or low to turn on the PSU?
     
  14. karyoker

    karyoker New Member

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    Typically 3.5 -5volts pos.
     
  15. chriss123

    chriss123 New Member

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    Hi,

    Your schematic does not match my PSU for this TV (Model FLM 323B). You show two 10-pin connectors with the STB being pin 10 of one of them. My PSU board has a 10-pin connector and a 7-pin connector. It looks like the STB pin is pin 7 of the smaller connector. Would you happent to know if this is the case?

    Thank you.
     
  16. karyoker

    karyoker New Member

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    Chris I se these power supplies on Ebay for the FLM-323b

    photo

    Mine is different..

    photo

    In fact there is no wire going to the 5V on the plug. I assume they use a 5V regulator on the control board from the 12V for 5V.

    And this which looks like a mod on the cntrl brd. Yellow wire cut. Is yours like this?
    mod
     
    Last edited: May 26, 2009
  17. chriss123

    chriss123 New Member

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    No, my unit is like the one on ebay. Thank you for the fast response.

    Regards
     
  18. karyoker

    karyoker New Member

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    Chris are you working on that power supply or the FLM-323B?
     
  19. chriss123

    chriss123 New Member

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    I am working on the power supply for the FLM 323B
     
  20. karyoker

    karyoker New Member

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    What are the symptoms? Or what is it doing? Have you read all the stuff on Fixya about Polaroids?
     
    Last edited: May 26, 2009
  21. chriss123

    chriss123 New Member

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    Hi,

    The TV is completely dead, nothing happens when the power switch is pressed. I am a retired electrical engineer, what I need is a schematic diagram so that I can troubleshoot and fix the unit.

    Thanks
     

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