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    #1

    Hannspree HZ281 - black screen

    SUMMARY
    ======
    This is a Hannspree HZ281 28" LCD monitor. It has a black screen. The power LED is green, if connected to a computer.

    THINGS TESTED
    ==========
    1) All caps look good
    2) Fuses F211, F220 and F720 on the PSU are ok
    3) The connector between PSU and logic board (CN201) has +5V SB. If the computer is connected PS(ON) has 5V, but there is no voltage at pin 9 and 10 (+12V), and no voltage at pin 5 and 6 (+5V).
    4) If I remove fuse F211 (which apparently connects the 12V rail to the inverter unit), I have the correct 12V and 5V on CN201. Thus it seems that the fault is in the inverter unit.
    5) I removed and measured the 2 lying caps on the inverter unit (C834 and C835). My unit tester shows 1000uF and an ESR of 0. I assume they are ok.
    6) There is no short blinking of the screen, if I switch it on.


    PROBLEM
    ======
    No power, if inverter unit is connected.


    QUESTION
    =======
    How could I track down the fault in the inverter unit?
    Attached Files
    Tags: None
    #2
    Re: Hannspree HZ281 - black screen

    Start by isolating the inverter: remove fuse F211.

    After this, check resistance to ground (on the inverter board) from where 12V power comes in from fuse F211. Report here what you get.

    Next, check all of the large power-related components on the inverter. That would be IC830, IC831, IC832, and IC833 (looks like possibly dual/complimentary MOSFETs?) I can't quite read the part numbers on them to tell you how to check them, so you will need to do that for us.

    Let us know what you find and then we can continue from there.

    Comment

      #3
      Re: Hannspree HZ281 - black screen

      Between 12V power input to the inverter board and ground on the PSU its 5.4MOhm. Is there a different ground on the inverter board, and if yes, how can I find it?

      IC830..833 are STU407DH (Dual Enhancement Mode FETs, N and P Channel)

      Comment

        #4
        Re: Hannspree HZ281 - black screen

        Originally posted by amelie2805 View Post
        Between 12V power input to the inverter board and ground on the PSU its 5.4MOhm. Is there a different ground on the inverter board, and if yes, how can I find it?
        Yes, use the negative lead on either of the two 35V, 1000 uF capacitors on the inverter board as a ground reference for the inverter. Or alternatively, keep the inverter board power cable connected to the PSU board and use ground on the PSU board. 5.4 MOhms seems a bit too high. At the very least you would have seen a lower resistance that then went up as the larger caps on the PSU board and/or inverter board charged.

        Originally posted by amelie2805 View Post
        IC830..833 are STU407DH (Dual Enhancement Mode FETs, N and P Channel)
        Ok, then test like you would an individual MOSFET - i.e. check resistance between G1 and S1, D1 and S1, and G1 and D1, then G2 & S2, D2 & S2, and G2 & D2 for each MOSFET. After that, repeat these measurements for all of dual MOSFETs. Report your findings here in the format:
        ICxxx
        measurements

        ICyyy
        measurements
        .
        .
        and so on.
        Last edited by momaka; 10-02-2020, 04:13 PM.

        Comment

          #5
          Re: Hannspree HZ281 - black screen

          If I separate the inverter unit entirely from the PSU (+12V and GND), I measure 6MOhm between the 12V input of the inverter unit and GND of the inverter unit (i.e. neg. lead of the 1000uF cap).

          The resistance of the MOSFETs is:

          IC830
          G1/S1 33kOhm
          D1/S1 6.1MOhm
          G1/D2 20kOhm
          G2/S2 2.5Ohm
          D2/S2 0Ohm
          G2/D2 2.6Ohm

          IC831
          G1/S1 33kOhm
          D1/S1 4.6MOhm
          G1/D2 4.6MOhm
          G2/S2 2.6Ohm
          D2/S2 0Ohm
          G2/D2 2.7Ohm

          IC832
          G1/S1 60kOhm
          D1/S1 57kOhm
          G1/D2 6.4MOhm
          G2/S2 20kOhm
          D2/S2 5.5MOhm
          G2/D2 5.2MOhm

          IC833
          G1/S1 1.8MOhm
          D1/S1 3.5MOhm
          G1/D2 6.4MOhm
          G2/S2 20kOhm
          D2/S2 5MOhm
          G2/D2 5.5MOhm

          Comment

            #6
            Re: Hannspree HZ281 - black screen

            Originally posted by amelie2805 View Post
            If I separate the inverter unit entirely from the PSU (+12V and GND), I measure 6MOhm between the 12V input of the inverter unit and GND of the inverter unit (i.e. neg. lead of the 1000uF cap).
            Good.
            That means at least there isn't a direct short-circuit on the 12V line of the inverter.

            However:

            Originally posted by amelie2805 View Post
            The resistance of the MOSFETs is:

            IC830
            ...
            G2/S2 2.5Ohm
            D2/S2 0Ohm
            G2/D2 2.6Ohm

            IC831
            ...
            G2/S2 2.6Ohm
            D2/S2 0Ohm
            G2/D2 2.7Ohm
            These suggest the 2nd MOSFET in IC830 and/or IC831 is blown.

            Looks like IC830 and IC831 are in parallel. But either way, remove both IC830 and IC831 and test them out of the circuit. Basically, check resistance across their pins again. If you still get those low resistance readings, especially between G2 and any of the other pins, then that MOSFET is bad.
            Last edited by momaka; 10-06-2020, 07:59 PM.

            Comment

              #7
              Re: Hannspree HZ281 - black screen

              Thanks very much.

              Are their any alternative types of MOSFETs I could use, as these are difficult to get.

              Comment

                #8
                Re: Hannspree HZ281 - black screen

                One of the two MOSFETs is blown.

                As an alternative I ordered two AOD609. Can I combine one old STU407DH and one new AOD609, or should I take the two new AOD609?

                Comment

                  #9
                  Re: Hannspree HZ281 - black screen

                  Probably best to use the two new AOD609 MOSFETs, so that they are matched (and in case the old supposedly "OK" STU407DH has a fault that doesn't show on the multimeter.)

                  Comment

                    #10
                    Re: Hannspree HZ281 - black screen

                    I have exchanged both STU407DH with the new AOD609. Now the monitor runs again. Hurray.

                    Dear momaka, thank you very much for your kind support.

                    Comment

                      #11
                      Re: Hannspree HZ281 - black screen

                      Nice job! Glad to hear you got it working again.

                      Comment

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