SMPS repair - fuse blows

Discussion in 'Technical Repair' started by dimakis_f, Oct 28, 2016.

  1. dimakis_f

    Thread Starter New Member

    Oct 28, 2016
    2
    0
    Hello everyone.
    I am working on an old CRT TV.
    The TV set does not start at all.

    I have checked the smps, and the 800mA fuse ( SI624 ) on the shematic , near the main capacitor , keeps blowing .
    I changed the electrolitic capacitors on the primary side,
    changed the TDA4605 and the PowerMOS transistor.
    All diodes are OK and 300 volt is measured on main capacitor ( C626 ).
    The PowerMOS transistor seems short circuit and the fuse blows.

    I have attached the shematic of the board.
    Any help will be appreciated.

    Thank you very much.
    Fotis D.
     
  2. wayneh

    Expert

    Sep 9, 2010
    14,719
    5,235
    Where on the schematic?
    Try removing the transistor for testing off the board.
     
  3. dimakis_f

    Thread Starter New Member

    Oct 28, 2016
    2
    0
    The transistor is on the first page, T644 , and is the BUZ90A.
    When i remove it and test , off the board , it's ok.
    Then put it back, turn on the tv set, fuse blows and T644 seems short circuit again.

    Is there any chance that the SMPS transformer TR651 causes the problem ?
    When i first checked this tv set, i could measure some voltages on the secondary of the SMPS,
    but was half of the appropriate values (for example 4 Volts instead of 8.5 at point +E on the schematic).
    Thank you.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    The only guy here I know that might be able to help is @R!f@@ . My last attempt to repair a CRT didn't end well.
     
  5. Lyonspride

    Member

    Jan 6, 2014
    127
    19
    Is there an MOV in the circuit for over voltage protection? Has this gone full short cct? Quite a common problem here in the UK with devices being made in China for 220v and then shipped out to the UK where we have a 240v (+/-10%) supply, the MOV gradually becomes more and more sensitive until it becomes a complete short and blows the fuses.
     
  6. wayneh

    Expert

    Sep 9, 2010
    14,719
    5,235
    I don't understand what you mean. If the voltage on the gate is enough, that transistor will turn on and conduct. That's what it's supposed to do, under control of the gate voltage. I would not call that a short.

    If the control of the gate voltage is at fault, you need to trace that down. You might try disconnecting the gate and controlling it yourself, by connecting it to its source pin. It should stop conducting.
     
  7. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    7,423
    1,223
    Check D647/8 flyback diodes Byv16 for s/c, also remove the transformer winding outputs, there could be shorts on the supply lines causing the fuse to blow.
     
    Last edited: Nov 4, 2016
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