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TV DAEWOO 21T1ST chasis CP380 - tweet tweet....

Discussion in 'Electronic Repair' started by b, May 8, 2009.

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  1. b

    b Guest

    This set had an arcing LOPTfmr. I replaced it and the line transistor
    but still tweeting, with the LED flashing in sync. Did the same with a
    bulb across the c-e and no transistor. No voltage was there.

    checked all primary and LOPTfmr secondary outputs, no short found.
    I replaced the STRS5707 in the psu - but still tweeting.

    I suspect the EEPROM has got corrupted by the arcing. Can I just use a
    'blank' one or must it be the preprogrammed one for this set /chassis?

    any other ideas are welcome!
    regards, B.
     
  2. Franc Zabkar

    Franc Zabkar Guest

    Service Manual for Daewoo CP380 chassis:
    http://www.eserviceinfo.com/download.php?fileid=972

    Page 3 of the service manual talks about the EEPROM settings but these
    unfortunately appear to require a Service Remocon. :-(
    Could D811 (near T802) be leaky or S/C?

    Are there any shorts on the 14V secondaries of the switchmode
    transformer?

    BTW, I'm having trouble understanding how optocoupler I804 inhibits
    the supply because in doing so it would turn off the +5V to the uP
    which controls it.

    - Franc Zabkar
     
  3. b

    b Guest

    Hi Franc,

    Thanks for the info - very useful.

    I probed a bit more earlier today. The D811 tested Ok, I replaced it
    and the supply still kept up its rapid tweeting/LED flashing.

    Out of curiosity, I desoldered the eeprom and powered the set up -
    symptoms stayed exactly the same. So maybe thre's just some short I've
    overlooked.

    I had also isloated the vertical/frame circuit by desoldering one leg
    of a rectifier diode feeding it from the line transformer.

    I should point out that I'm using a dummy load of a 60w bulb in place
    of/across c-e of the line transistor. No power gets there.....

    -B.
     
  4. b

    b Guest

    a few more tests:
    the 123v (B+ feed to the line stage) output of the psu transformer
    never reaches more than about 50v, even unloaded. all the others have
    negligible volts on. could i802 be the cause?

    here's a part of the psu schematic: http://www.fileden.com/files/2007/6/4/1143893/cp380 psu.jpeg

    I'm all ears to any suggestions.
    B
     
  5. b

    b Guest

    The psu stops its cyling/tweeting when the 123v line is isolated, but
    still does not reach over 50v on that pin, which led me to suspect
    some regulation problem. Not only that, but the line stage seems fine:
    I'm using a bulb in place of the transistor, and I have not found any
    shorts on the line transformer secondary loads.

    I'm beginning to suspect the replacement line transformer may not be
    correct. the original was an 1142.5057 but I was sold an ELDOR
    1142.5077 (which , it says on the box, will sub. a .5057 or 30473 -
    This may not be the case! Will contact them and check.

    ..
     
  6. Franc Zabkar

    Franc Zabkar Guest

    I have seen Sony sets where the B+ cap has gone open and caused this
    type of problem. However, it would seem unlikely that both C816 and
    C817 would be faulty.

    What are the voltages on C819 and C818? It would seem to me that if
    the B+ voltage has dropped from 123V to 50V, then the 14V supplies
    would be around 5V.

    Here is the datasheet for the TDA8138 (note that there are three
    versions):
    http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXuuzsu.pdf

    AFAICS, you can detect the current draw of the +5V rail by measuring
    the voltage across R831 or R832. That's assuming pin 4 is not
    disabled.

    It might also be worth seeing what optocoupler I804 is doing to the
    Inhibit pin of the SMPS regulator IC.
    The HR Diemen equivalent for both the 1142.5057 and the 1142.5077 is
    an HR 7880:
    http://www.hrdiemen.es/products/index.php?command=viewSchema&filename=./img/esquemas/HR7880.gif

    - Franc Zabkar
     
  7. Franc Zabkar

    Franc Zabkar Guest

    R703 and R704 present a resistance of about 11K to a 123V supply. If
    IC701 was shorted, then the current draw would be about 11mA. I don't
    see why this would load your supply.

    - Franc Zabkar
     
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