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Samsung UN40F5500 - PSU not working

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    #1

    Samsung UN40F5500 - PSU not working

    Hi,

    I have this tv where PSU is not working as expected and tv is not turning on. Backlights are not on where mainboard is disconnected. Tested all the strips and looks like one of the strip has bad LEDs. However even after disconnecting mainboard and led strips, the voltage on the main caps is not stable and jumping from 171v to 230v. Below are the reading on the power connector to mainboard -

    Pin1 (B13v) - Jumping between 5.9v to 6.5v
    Pin2 (P Dim) - Jumping between 2.1v to 2.5v
    Pin3 (B13v) - Jumping between 5.9v to 6.5v
    Pin4 (ECO) - 0V
    Pin5 (Vamp) - Jumping between 5.9v to 6.5v
    Pin6 (BLU) - Jumping between 1.9v to 2.3v
    Pin7 (Vamp) - Jumping between 5.9v to 6.5v
    Pin8 (GND)
    Pin9 (GND)
    Pin10 (GND)
    Pin11 (B5v) - Jumping between 3.7v to 4.2v
    Pin12 (A5v) - 5.3v
    Pin13 (B5v) - Jumping between 3.7v to 4.2v
    Pin14 (PS_ON) - 3.3v

    Can somebody give me starting point to investigate the issue?
    Attached Files
    Tags: None
    #2
    Re: Samsung UN40F5500 - PSU not working

    What do you get on main cap. when you push power button.
    I assume no responsibility for any stupid suggestions I might post.

    Comment

      #3
      Re: Samsung UN40F5500 - PSU not working

      It's same, jumping voltage from as mentioned in post#1.

      Comment

        #4
        Re: Samsung UN40F5500 - PSU not working

        Start by checking all the diodes in the secondary side, It looks like the power supply sis shutting down to to a high load.

        Comment

          #5
          Re: Samsung UN40F5500 - PSU not working

          May also have a PFC fault.
          I assume no responsibility for any stupid suggestions I might post.

          Comment

            #6
            Re: Samsung UN40F5500 - PSU not working

            If all the diodes check out ok, I might suspect one of those 3 electrolytic caps right there by the connector, try heating them up with a hairdyer, see if tv comes on for a bit.

            Comment

              #7
              Re: Samsung UN40F5500 - PSU not working

              I tried to heat up the caps however no change. I observed the diodes DM861C, DM862C, DM863C and DM864C read 0.24v and 0.28v from both sides when tested with multimeter on DOIODE mode. However I tested them on the board itself. Could that be issue?

              Comment

                #8
                Re: Samsung UN40F5500 - PSU not working

                Depends on the circuit. Lift one leg and check them again

                Comment

                  #9
                  Re: Samsung UN40F5500 - PSU not working

                  Tested all the diodes by removing one leg from the board and looks good. The voltage is jumping on 13V line which is causing jumping on B5V line as well. Not sure if it's primary or secondary side at fault. I suspect the primary as the voltage is not stable on main caps also. After bridge rectifier it jumps on all the components.
                  Found this schematic of similar board not the exact same but those are replacement for each other on few tv models. https://cdn.badcaps-static.com/pdfs/...a77e44cdba.pdf

                  Update : I get 0v on the Source and Drain of the IC QP801CS (part# 60R360P).
                  Last edited by anupkg1985; 05-04-2022, 04:22 PM.

                  Comment

                    #10
                    Re: Samsung UN40F5500 - PSU not working

                    maybe check all the diodes and resistors in that area and look up the data sheet on that IC.

                    Comment

                      #11
                      Re: Samsung UN40F5500 - PSU not working

                      Originally posted by anupkg1985 View Post
                      Update : I get 0v on the Source and Drain of the IC QP801CS (part# 60R360P).
                      On the schematic, QP801CS is the pfc mosfet, if you get 0 volts (reference to HOT ground) on the drain, check for bad connections tp801cs and BBP801, I suspect you have a wrong voltage reading

                      Comment

                        #12
                        Re: Samsung UN40F5500 - PSU not working

                        Are all the power rails stable with the main board disconnected and only the backlights on?

                        Comment

                          #13
                          Re: Samsung UN40F5500 - PSU not working

                          Originally posted by R_J View Post
                          On the schematic, QP801CS is the pfc mosfet, if you get 0 volts (reference to HOT ground) on the drain, check for bad connections tp801cs and BBP801, I suspect you have a wrong voltage reading
                          My bad! 0v readings are on Gate and Source, 173v present on Drain and it's stable. I'm using main caps -ve terminal as a ground.

                          I have removed the PSU and not not connected to anything, however it should be on in case of Samsung I believe. Also no change if I connect mainboard.

                          Addition: I believe Source is connected to GMD only so 0v is expected (ideally I should not measure that ).

                          Update: ZDP803 tested with Diode mode read 0v and shows continuity while remaining on the board. Could this be issue?
                          Last edited by anupkg1985; 05-04-2022, 07:14 PM.

                          Comment

                            #14
                            Re: Samsung UN40F5500 - PSU not working

                            Check your voltage across CS802, CS805 and then CP812. It may be that CS805 is bad, it is the cap that supplies the vcc voltage to the pfc ic via QS801
                            Last edited by R_J; 05-04-2022, 07:16 PM.

                            Comment

                              #15
                              Re: Samsung UN40F5500 - PSU not working

                              Originally posted by R_J View Post
                              Check your voltage across CS802, CS805 and then CP812. It may be that CS805 is bad, it is the cap that supplies the vcc voltage to the pfc ic via QS801
                              CS805 - 18v
                              CS812 - 9.1v
                              CS802 - 6.3v

                              All voltages are stable.

                              ZDP803 tested with Diode mode read 0v and shows continuity while remaining on the board, as mentioned in post#13. Could this be issue?
                              Last edited by anupkg1985; 05-04-2022, 08:10 PM.

                              Comment

                                #16
                                Re: Samsung UN40F5500 - PSU not working

                                Originally posted by anupkg1985 View Post
                                CP805 - 18v
                                CP812 - 9.1v
                                CP802 - 6.3v

                                All voltages are stable.

                                ZDP803 tested with Diode mode read 0v and shows continuity while remaining on the board, as mentioned in post#13. Could this be issue?
                                What is the voltage across CS805? I don't think ZDP803 is on the board. I think you are likely checking BEP801 (coil)
                                Last edited by R_J; 05-04-2022, 08:01 PM.

                                Comment

                                  #17
                                  Re: Samsung UN40F5500 - PSU not working

                                  It was typo in previous post, actually I measured voltages across CS805, CS812 and CS802 only as mentioned above.

                                  Comment

                                    #18
                                    Re: Samsung UN40F5500 - PSU not working

                                    CS802 is likely ok, CS805 does seem to be a bit low, I would thing it should be above 20v, I would try replacing CS805 47µf/50v
                                    The voltage across CP812 should be around 15v
                                    Last edited by R_J; 05-04-2022, 08:20 PM.

                                    Comment

                                      #19
                                      Re: Samsung UN40F5500 - PSU not working

                                      Originally posted by anupkg1985 View Post
                                      My bad! 0v readings are on Gate and Source, 173v present on Drain and it's stable. I'm using main caps -ve terminal as a ground.

                                      I have removed the PSU and not not connected to anything, however it should be on in case of Samsung I believe. Also no change if I connect mainboard.

                                      Addition: I believe Source is connected to GMD only so 0v is expected (ideally I should not measure that ).

                                      Update: ZDP803 tested with Diode mode read 0v and shows continuity while remaining on the board. Could this be issue?
                                      Your getting stable voltage of 173v on drain and 50v fluctuation on main cap?
                                      I assume no responsibility for any stupid suggestions I might post.

                                      Comment

                                        #20
                                        Re: Samsung UN40F5500 - PSU not working

                                        Originally posted by dskall View Post
                                        Your getting stable voltage of 173v on drain and 50v fluctuation on main cap?
                                        No on main caps it's jumping from 171v to 230v.

                                        Comment

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