Announcement

Collapse
No announcement yet.

Samsung BN44-0851A powersupply problem

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    Samsung BN44-0851A powersupply problem

    My 40 inch Samsung TV suddenly went off.

    I check the powersupply and has blown fuse. I checked the bridge rectifier and found shorted so i removed it but it was good out of the board. I noticed the Mosfet K10a60w was shorted and i replaced it. Also checked the output diodes and found 2 in parallel shorted. I removed both legs and determined only 1 of them is shorted hence i replaced it.

    I tried to plug it and good the fuse did not blow. I get 310V from the primary caps. However i dont get any voltage from the secondary output.

    What could be the problem and if you have schemati please share. Thanks
    Attached Files

    #2
    Re: Samsung BN44-0851A powersupply problem

    Pwm controller with problems. It's on the back of the board. Including resistors and diodes related to its operation. Even optocoupler.

    Comment


      #3
      Re: Samsung BN44-0851A powersupply problem

      Check voltage on drain to neg. of your mosfet. it should have around 25v higher than main cap. if working.
      I assume no responsibility for any stupid suggestions I might post.

      Comment


        #4
        Re: Samsung BN44-0851A powersupply problem

        so you have no voltage on jp808? what voltages are you getting on cnm803?

        Comment


          #5
          Re: Samsung BN44-0851A powersupply problem

          Originally posted by nomoresonys View Post
          so you have no voltage on jp808? what voltages are you getting on cnm803?

          Hi thank you for the reply, Yes there is no voltage on JP808. actually i tried removing the photocoupler since there is slight sign of " i am not sure - if it's a meltded part" some markings are gone. I see it has 2 photocouplers with exact same part number and when i checked out of the board both of them are shorted.

          I searched the net for the replacement photocouplers but could not find any. It has a marking on top 1008.w i dont know what it meas so i decided to send the powersupply board to the TV repair shop and after 3 days the technician said he cannot repair it. He needs some donor boards because it is probably some of the smaller ICs.

          well, i dont really believe him and i am still trying to fix the board by myself if you can guide me. (i am a newbie but i know a little how to solder, how to use the DMM). i also have a chinese mini SMD station with hot air if it requires to remove some chips.

          could i just put in any photocoupler and what could be the alternative part number?

          Comment


            #6
            Re: Samsung BN44-0851A powersupply problem

            Originally posted by yokoono View Post
            Pwm controller with problems. It's on the back of the board. Including resistors and diodes related to its operation. Even optocoupler.
            all diodes are fine... 2 optocouplers are shorted but i dont have any replacement part. could you tell me any alternative part for the optocouplers. thanks a lot

            Comment


              #7
              Re: Samsung BN44-0851A powersupply problem

              Originally posted by dskall View Post
              Check voltage on drain to neg. of your mosfet. it should have around 25v higher than main cap. if working.
              already changed mosfet and working fine. before it was shorted hence it blows the main fuse everytime i plug. only it has no secondary output. primary line is good

              Comment


                #8
                Re: Samsung BN44-0851A powersupply problem

                How sure are you that both opto couplers are shorted out? I have only seen opto's that go open circuit.
                How did you test them?
                Let me tell you what i do to test these optos. It might not be 100% fool proof but so far i have had good success.
                Lets take a PC817 opto coupler for example and i have added a picture to better explain what i am talking about.
                For this test we need to forward bias the diode part of the opto. I normally use a analog multimeter where i put the unit into ohms scale, normally "x1". This adds a very small voltage on the probes which is just enough to forward bias the diode part. Then i guess you would have to solder 4 wires on the optos legs to do this test. So on the other side of the opto we have the transistor junction... Here you can connect a digital multimeter and put the dial into ohms mode. Now all you need to do is remove one wire on the analog multimeter and observe what reading you get on the digital. If you have connected it up correctly you will notice on the digital a resistance of about 90 ohms when the analog meter is biasing the opto and when you disconnect the analog multimeters wire the digital should go to infinity. Swap the digital multimeters probes and redo the test. The digital multimeter ohms should be around the same resistance.
                Normally when its faulty you get infinity on the digital even though the analog is biasing this diode part. Please note that if you are not familiar with a analog multimeter then i must tell you that once you have the analog meter properly connected the indicating needle "dial" should deflect to about half way to maximum, I hope you understand??? If you don't have a analog multimeter then you will have to find another way of biasing the diode part without damaging it. We have to limit current here in order not to short out the diode in it. About 1.2V is required to bias it with current limited. Perhaps a resistor i series with a battery could work but you need to ensure that only milliamps can feed through. I never bothered figuring out a different way of biasing since my trusty old analog meter works just fine.
                Attached Files

                Comment


                  #9
                  Re: Samsung BN44-0851A powersupply problem

                  Originally posted by rayfloyd170 View Post
                  already changed mosfet and working fine. before it was shorted hence it blows the main fuse everytime i plug. only it has no secondary output. primary line is good
                  How did you determine the mosfet is switching?
                  I assume no responsibility for any stupid suggestions I might post.

                  Comment


                    #10
                    Re: Samsung BN44-0851A powersupply problem

                    The optocouplers are LTV-1008-tp1-g and are likely good, How are you checking them that they show shorted?
                    If the mosfet was shorted, the drive ic has been damaged as well.
                    Last edited by R_J; 05-14-2022, 12:59 PM.

                    Comment


                      #11
                      Re: Samsung BN44-0851A powersupply problem

                      Originally posted by angelos View Post
                      How sure are you that both opto couplers are shorted out? I have only seen opto's that go open circuit.
                      How did you test them?
                      Let me tell you what i do to test these optos. It might not be 100% fool proof but so far i have had good success.
                      Lets take a PC817 opto coupler for example and i have added a picture to better explain what i am talking about.
                      For this test we need to forward bias the diode part of the opto. I normally use a analog multimeter where i put the unit into ohms scale, normally "x1". This adds a very small voltage on the probes which is just enough to forward bias the diode part. Then i guess you would have to solder 4 wires on the optos legs to do this test. So on the other side of the opto we have the transistor junction... Here you can connect a digital multimeter and put the dial into ohms mode. Now all you need to do is remove one wire on the analog multimeter and observe what reading you get on the digital. If you have connected it up correctly you will notice on the digital a resistance of about 90 ohms when the analog meter is biasing the opto and when you disconnect the analog multimeters wire the digital should go to infinity. Swap the digital multimeters probes and redo the test. The digital multimeter ohms should be around the same resistance.
                      Normally when its faulty you get infinity on the digital even though the analog is biasing this diode part. Please note that if you are not familiar with a analog multimeter then i must tell you that once you have the analog meter properly connected the indicating needle "dial" should deflect to about half way to maximum, I hope you understand??? If you don't have a analog multimeter then you will have to find another way of biasing the diode part without damaging it. We have to limit current here in order not to short out the diode in it. About 1.2V is required to bias it with current limited. Perhaps a resistor i series with a battery could work but you need to ensure that only milliamps can feed through. I never bothered figuring out a different way of biasing since my trusty old analog meter works just fine.
                      thanks for the input, i am not that expert in electronics and just have basic understanding of some common parts. good that you showed an illustration. based on terminals 1 and 2 it shuould be a diode if i am correct. The optocoupler i took out was partly melted on the top and showed shorted pins 1 and 2 while checking it out of the board and using diode mode.

                      Comment


                        #12
                        Re: Samsung BN44-0851A powersupply problem

                        Originally posted by R_J View Post
                        The optocouplers are LTV-1008-tp1-g and are likely good, How are you checking them that they show shorted?
                        If the mosfet was shorted, the drive ic has been damaged as well.
                        actually i send this whole board to a technician for TV sets and he cannot fix it because he mentioned to me it could be the drive IC's at the back of the board in which there are 2. he doesnt have samsung donor boards at the moment hence i decided to buy a replacement powersupply board.

                        Comment


                          #13
                          Re: Samsung BN44-0851A powersupply problem

                          I decided to buy a replacement powersupply as i dont have the driver ICs and during installation i found out the LED backlights are working but still without display.

                          I checked several coils from the mainboard and found 0 volts all across even from the starting IC close to the connectors. Then i tried tracing from the connectors and found 1 SMD fuse Y-type which is blown. I searched the net and confirmed its rating at 15Amps. i only have some P fuses from macbook backlight i previously repaired and tried it just for luck. It worked! i will still replace it with the proper Y-fuse later on.

                          Comment


                            #14
                            Re: Samsung BN44-0851A powersupply problem

                            Last edited by nomoresonys; 06-12-2022, 04:11 AM.

                            Comment


                              #15
                              Re: Samsung BN44-0851A powersupply problem

                              15amps seems like a lot. And fuses usually blow for a reason.
                              I assume no responsibility for any stupid suggestions I might post.

                              Comment


                                #16
                                Re: Samsung BN44-0851A powersupply problem

                                If it is marked "P" it is a 3 amp.

                                Comment


                                  #17
                                  Re: Samsung BN44-0851A powersupply problem

                                  Originally posted by R_J View Post
                                  If it is marked "P" it is a 3 amp.
                                  YEah what i have are P fuses for macbook Pro backlight... The one used in samsung mainboard is Y-type which says 15amps 65V.. Anyway, i still used the P-type and it works well.

                                  Thanks

                                  Comment


                                    #18
                                    Re: Samsung BN44-0851A powersupply problem

                                    Originally posted by dskall View Post
                                    15amps seems like a lot. And fuses usually blow for a reason.
                                    That was what i am thinking, the Y-type fuses i replaced with what i have which is P-type... The reason why it blows could be from the failed powersupply.

                                    everything now works fine,

                                    Comment

                                    Working...
                                    X