Syncmaster T190 - a headache

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  • 50AE
    New Member
    • Aug 2015
    • 5
    • Bulgaria

    #1

    Syncmaster T190 - a headache

    Hello to everyone,

    I recently bought a used display – Samsung Syncmaster T190. When I brought it home and plugged it into my computer, I found out it's completely dead:
    -No backlight
    -No image
    -No power indicator LED glowing
    -No reaction from the touch sensor button or any button at all.

    So I decided to try repairing it. I have repaired a few LCD screens in the past and a few SMPS by recapping them, so I thought “why not”?
    I took off the power supply board and examined it. There were two bumpy leaky capacitors visible. I took them off, measured their ESR, it was 1.1R and 1.8R.
    I decided to replace all capacitors, even those on the processor board. I used Fujicons low ESR series 105C, except on a place where 3 caps where in parallel. Because I had much higher voltage Fujicons for there and they couldn't fit. I decided to solder 3 no-name brand ones.

    When I plugged everything like it was and turned the power on again – nothing happened. It was dead again, the same way it was before, when I bought it.
    Then I measured the voltages coming from the power supply board to the processor board:
    - 15,8V measured on the +14VDC rail
    - 5,2V measured on the +5VDC rail

    I also examined the processor board and its datasheet. The processor's name is SE758MRH-LF.It turned out, it has three local regulators: 1,8V ; 3,3V ; 3,3V. They are feeded from the +5VDC rail. It turns out, the first local regs 1,8V and 3,3V are feeded only when a mosfet switch saturates when its GATE connected to KEY1 pin goes to positive 3,3V

    I tried measuring all the local regs with the KEY1 pin to 3.3V and the voltages are good. But I also found out, that the power LED never turns. I even soldered a local LED near the Vled out pin, but I had no result.

    On the power board, there is a ON/OFF pin that when connected to PIN1, turns the backlight on, but no image outputs. Also, the power sensor button works in this situation. I also tried connecting ON/OFF to PIN1 and 3.3V – still only the backlight exists, but I get no image.

    So, in summary:
    -The voltages measured with a DMM measure okay, except for the 15,8V on the +14VDC rail.
    -The power LED doesn't turn on.
    -The ribbon cables seem okay.
    -The CFL work
    -The ON/OFF connected to KEY1 brings the backlights


    And I have some things to try before going desperate about it:
    -Measure each voltage rail with an oscilloscope
    -Change the no-name parallel new caps I put with low esr fujicons.
  • addytzu123
    Senior Member
    • Mar 2013
    • 54
    • ROMANIA

    #2
    Re: Syncmaster T190 - a headache

    Hi,
    See the attached service manual, try to use the troubleshooting guide.
    Attached Files
    In a transistor circuit protected by a fuse, the transistor will always blow to protect the fuse.

    Comment

    • addytzu123
      Senior Member
      • Mar 2013
      • 54
      • ROMANIA

      #3
      Re: Syncmaster T190 - a headache

      See also the wiring diagram...
      Attached Files
      In a transistor circuit protected by a fuse, the transistor will always blow to protect the fuse.

      Comment

      • 50AE
        New Member
        • Aug 2015
        • 5
        • Bulgaria

        #4
        Re: Syncmaster T190 - a headache

        Many thanks!

        So judging from the troubleshooting guide, the problem is still in my power supply board.

        Comment

        • 50AE
          New Member
          • Aug 2015
          • 5
          • Bulgaria

          #5
          Re: Syncmaster T190 - a headache

          I changed the 3 capacitors with Fujicon low ESR and no luvk - still dead. I'm going for the osciloscope.

          Comment

          • Porto
            Senior Member
            • May 2015
            • 109
            • NL

            #6
            Re: Syncmaster T190 - a headache

            Good chance some mosfet / driver or diode is shorted.

            Comment

            • 50AE
              New Member
              • Aug 2015
              • 5
              • Bulgaria

              #7
              Re: Syncmaster T190 - a headache

              With the main board on (KEY1 to 3.3V), I get 50mV of ripple on the 5Vdc line. The other lines seem okay.

              On photo 1, there's the voltage line
              On photo 2, the small peak at the highest ripple amplitude is zoomed at 20us/div
              Photo 3 reveals the ~10 MHz harmonic content, provably caused by the digital circuitry.
              Attached Files

              Comment

              • addytzu123
                Senior Member
                • Mar 2013
                • 54
                • ROMANIA

                #8
                Re: Syncmaster T190 - a headache

                Hi,
                Have you found a solution to the problem ? I am not sure the PS is at fault here.
                Can you insert 5V from another source (like a bench power supply) into the main board, (monitor''s PS disconnected) and check again the voltage at the local regulators, memory IC's and if the 5V DC is present at the LVDS (CN400 )cable, pins 1 and 7 ?
                In a transistor circuit protected by a fuse, the transistor will always blow to protect the fuse.

                Comment

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