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Dell ST2421LB Dead monitor

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    Dell ST2421LB Dead monitor

    Hi there,
    got this monitor that looks great cosmetically, but no life. After a bit of a struggle, I opened it up. I am getting 160 V across large cap. No voltages on the connector that goes to main board. If this is anything like TVs, I'd expect some standby voltage to the main board.

    I am not familiar with this power supply board 4H.17B02.A00

    I found an IC that's close to the mainboard connector, thinking it might be the standby regulator. Has the folowing numbers on it
    101058
    1T10231

    No luck with any datasheets on this.
    help!
    Attached Files

    #2
    Re: Dell ST2421LB Dead monitor

    Correction, under a microscope, the I closer to the main board connector is 210105BA (not 101058) But this IC is not getting any voltage, so I don't believe it's the problem.
    Thank you

    Comment


      #3
      Re: Dell ST2421LB Dead monitor

      It's the TOP258PN next to the main cap.

      Comment


        #4
        Re: Dell ST2421LB Dead monitor

        Thanks. That IC has 160 V on pin 4 (Bottom left - below the missing pin) and all zero volts (reference to negative leg of large cap) on all other pins
        Last edited by jason123; 08-26-2020, 01:41 PM.

        Comment


          #5
          Re: Dell ST2421LB Dead monitor

          You can check that ZD601 is'nt shorted, but it's likely the TOP258 ic is bad, the internal soft start is likely bad. You should have around 5.8v on pin2
          Last edited by R_J; 08-26-2020, 07:09 PM.

          Comment


            #6
            Re: Dell ST2421LB Dead monitor

            Thank you. Diode is not shorted. IC is on order

            Comment


              #7
              Re: Dell ST2421LB Dead monitor

              Hi . Got a new ic in place and same result. Probing around, I found that C611 was shorted - Replaced and Monitor is back up and running. Thank you for the help.
              Last edited by jason123; 08-28-2020, 01:27 PM.

              Comment


                #8
                Re: Dell ST2421LB Dead monitor

                does anyone know what the zener diode ZD601 is on this ps board

                Comment


                  #9
                  Re: Dell ST2421LB Dead monitor

                  Originally posted by Rodoz View Post
                  does anyone know what the zener diode ZD601 is on this ps board
                  I believe it is a 9.1 volt, BZV55-C9V1

                  Comment


                    #10
                    Re: Dell ST2421LB Dead monitor

                    Originally posted by R_J View Post
                    I believe it is a 9.1 volt, BZV55-C9V1
                    Hi R_J, many thanks for your resonse. The ZD601 on mine is S/C and naturally the supply wont run. I could not find the schematic anywhere and there is no info printed on this surface mount diode so your information is very handy. Much appreciated, regards from Rodoz

                    Comment


                      #11
                      Re: Dell ST2421LB Dead monitor

                      ZD601 - 20v

                      Comment


                        #12
                        Re: Dell ST2421LB Dead monitor

                        I don't have the exact schematic but this one is similar and uses the same top258pn
                        You may need to clean the top of the diode with alcohol to see the marking code.
                        Attached Files

                        Comment


                          #13
                          Re: Dell ST2421LB Dead monitor

                          Yes, you need to look where it (ZD) stands according to the scheme and its marking.
                          Attached Files

                          Comment


                            #14
                            Re: Dell ST2421LB Dead monitor

                            Could anyone help with the value for C607 please. I have the same board 4H.17B02, zener ZD601 appear shorted in circuit but once taken off PCB appear OK. Took off C607 but found pads for ZD601 still shorted eventually found that it was C608 47uF 25V that was shorted. However in the process of taking off C607 end terminals was destroyed.

                            Anyone would could help with value of C607 would be much appreciated.

                            Tried 80nF which gave approximately 5.8V at pin2, ~1.26V at pin1 and 330V at pin4 of TOP258PN.

                            Thanks.
                            Last edited by llphat; 02-25-2022, 10:01 AM.

                            Comment


                              #15
                              Re: Dell ST2421LB Dead monitor

                              C607 seems to be 0.1µf. If C608 was shorted, check if R606 is open or the opto ic could be bad.I would also replace C611. C607 is just to remove any high frequency noise on the line, its not that critical and should operate without it.

                              5.8v on C (pin2) is supplied by the ic, Once the circuit starts, the control voltage for pin 2 comes from D602/C611 through the optocoupler.
                              Last edited by R_J; 02-25-2022, 05:07 PM.

                              Comment


                                #16
                                Re: Dell ST2421LB Dead monitor

                                R606 measures 6.6ohm in circuit. C611 checked OK approx 10uF out of PCB.

                                I could see 340V (against negative leg of C605 large filter cap) on the Pin 1,2 and 0V on Pin 4,5 on primary side of the transformer T601, but no output voltage on the secondary side (pin 6/7, pin8/9 and pin10), does this mean transformer is broken?

                                Comment


                                  #17
                                  Re: Dell ST2421LB Dead monitor

                                  It does not mean the transformer is broken, it means the circuit is not operating. Have you looked at the datasheet to understand how the ic operates? Replace the two capacitors I suggested
                                  Check the voltage on pins 3 & 4 of the optocoupler (using hot ground as meter ground)
                                  Last edited by R_J; 02-28-2022, 01:33 PM.

                                  Comment


                                    #18
                                    Re: Dell ST2421LB Dead monitor

                                    Unfortunately I do not know enough about how the circuit works.

                                    pin3 on the optocoupler measures 5.84V (against hot ground) which is the same as pin2 on the TOP258PN IC which it is connected to. pin4 on the optocoupler measures 7.96V against hot ground, so about 2.12V when measured across pin3-pin4 of the optocoupler. Across pin1-2 of the optocoupler measures about 1.126V.

                                    On the TOP258PN IC, pin4 measures ~337V, pin2 (=pin3 on the optocoupler) is 5.84V, pin1 is about 1.26V.

                                    Thanks for your patience.

                                    Comment

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