Trying to fix power supply for Samsung T220HD LCD monitor

Discussion in 'General Electronics Chat' started by meth, May 21, 2016.

  1. meth

    Thread Starter New Member

    May 21, 2016
    4
    0
    Hello to all, I need some help fixing the power supply of the mentioned monitor. I turn on the monitor, it is working fine for a second or two, and then the backlight goes off.

    The problem is with the inverting part of the suply that powers the backlight of the monitor (650 volts AC).

    I have come to the conclusion that the problem is somewhere around the IC SEM2005, which is the driver for the invertor. It gives oscilations to the gates of MOSFETs to convert DC to AC voltage, that is raised from 12V to 650V. My driver was NOT oscilating on the pins 8 and 9 that drive the MOSFETs.
    I thought it was the driver, I changed it but the problem remained.

    Now, somewhere I read that the pin 1 of the IC, which is Shut Down Timing Capacitor, (basicaly the pin is connected to GND with a SMD capacitor) is some kind of protection, and can be bridged with a 82K resistor to GND. I did that and I got the required voltage on the output pins.

    Now, to anyone who has experience with this driver or with these power supplies:

    Is this a permanent solution? If no, what is causing the driver to malfuction, is it the capacitor? I am stuck for days, I cannot realise the purpose of this pin and the capacitor itself.

    Thank you in advantage,
    Meth.
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
  3. meth

    Thread Starter New Member

    May 21, 2016
    4
    0
    Bro I read the datasheet 15 times, my question is, is the capacitor causing the malfunction, and can the supply work normally with a resistor parallel with the capacitor. I do not have experience with backlight power supply and I dont wanna burn em.

    Thanks.
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    Can you see anything on the screen before it shuts down ?
    And is it shutting down or back light shutting down without monitor going to stby ?

    Post some pictures.
     
  5. SLK001

    Well-Known Member

    Nov 29, 2011
    825
    229
    You probably have bad CCFLs. The shutdown is being expressed for a reason.
     
  6. meth

    Thread Starter New Member

    May 21, 2016
    4
    0
    Yes I can see the monitor working fine for a second or two and then it shuts down.

    I dont think the problem is with the CCFLs because I am measuring the voltage of the power supply with the lamps disconected. The part with 3,3V, 5V and 12V is working perfectly, the part with the inverting voltage is malfunctioning. The IC SEM2005 stops giving oscilations to the gates of the FETs after 1-2 seconds. When I connect the Pin 1 to GND with a resistor it is working fine.

    I repeat, these malfunctions happen WITHOUT the CCFLs connected.
     
  7. SLK001

    Well-Known Member

    Nov 29, 2011
    825
    229
    Your shutdown will also occur WITHOUT the CCFLs connected. This is interpeted as an OPEN CCFL and a failure mode is flagged.
     
  8. meth

    Thread Starter New Member

    May 21, 2016
    4
    0
    Yes I thought that might be an option, because what I am doing is bypassing the shutdown mechanism. So what is your suggestion, that I change the CCFLs?

    And I am also not sure about this, if I turn on the monitor with the Pin 1 bypassed with a resistor, can something go terribly wrong? Because the IC will not be able to shut-down itself.
     
  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    You have a Inverter or CCFL issue. Most of the time it's the CCFL
     
  10. SLK001

    Well-Known Member

    Nov 29, 2011
    825
    229
    Tear down your monitor and examine the connections to the CCFLs. I've seen quite a few monitors where the CCFLs had bad solder joints at the ends. You might be lucky and just have to reflow the ends (be careful, as too much heat will damage the tubes).

    This is the equivalent to open heart surgury - a bad surgeon will kill the patient.
     
  11. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    Or get a heart attack instead when you break the CCFL.
     
Loading...