Announcement

Collapse
No announcement yet.

Samsung PSU low voltage.

Collapse
X
Collapse
 
  • Page of 1
  • Filter
  • Time
  • Show
Clear All
new posts
Previous template Next
    #1

    Samsung PSU low voltage.

    LA26B450 - N66D Chassis
    BN44-00259A power/inverter board.

    Unit does not power up.
    If I unplug the logic board, the backlight turns on.
    Checked the voltages at the powerboard connector and they all seem slightly too low which is I suspect is the issue. eg (5v rail is measuring around 4.7v. 13v rail is measuring about 12.7)
    Caps look ok

    I have the schematic, but cant seem to decyper the various parts. They definatly not as good as the sony diagrams I have used in the past.

    Not sure where to look, but suspect around the regulating area, so any assistance is appreciated.

    Thanks

    TEX
    Tags: None
    #2
    Re: Samsung PSU low voltage.

    Originally posted by zedexnz View Post
    LA26B450 - N66D Chassis
    BN44-00259A power/inverter board.

    Unit does not power up.
    If I unplug the logic board, the backlight turns on.
    Checked the voltages at the powerboard connector and they all seem slightly too low which is I suspect is the issue. eg (5v rail is measuring around 4.7v. 13v rail is measuring about 12.7)
    Caps look ok

    I have the schematic, but cant seem to decyper the various parts. They definatly not as good as the sony diagrams I have used in the past.

    Not sure where to look, but suspect around the regulating area, so any assistance is appreciated.

    Thanks

    TEX
    This is a very interesting design. I don't fully understand all of the schematic, but it appears the 5.3 volts is switched. Here's a couple of places to check. With the TV switched off, but plugged in, what is the voltage across the large cap - CP815? What is the voltage on pin 3 of CNM802 (STB 5.2V)? What is the voltage on pin 15 of CNM802? Use pin 10 of CNM802 as the ground point for the last two measurements. With the TV turned on what are the voltages at the same three points?

    The STB 5.2V is regulated, UM851 and PC801S provide the regulating signal to UM801S, the SMPS controller. What is very interesting is that the 5.3 volt supply is derived from the STB 5.2V supply!!!

    PlainBill
    For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

    Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

    Comment

      #3
      Re: Samsung PSU low voltage.

      Thanks for the insight Bill.

      I can really turn the unit on as such. I dont even get a red light. When the logic board is unplugged the power board at least lights up the backlight of the LCD and this is how I have been doing the testing, with the logic board disconnected.

      Pin 3 and pin 15 measure 5.16v with respects to the Gnds on pin 10. The 13V rails with respects to the same Gnd are measuring 12.47v.

      The main cap is 340V DC

      It is like the reference voltage for the regualting circuit is causing a lower output.
      TEX

      Comment

        #4
        Re: Samsung PSU low voltage.

        I'll be brief, when I give instructions for certain tests, it's because I need additional information.

        Hook up the main board. Plug the power cord into the power supply and make the measurements I requested, plus measure the voltage on CNM802, pin 1. Hit the power button and repeat them, including CNM802. Report the 'Off' and 'On' readings. With that information I can decide what additional testing is required. The voltage are slightly low, but not all that much.

        PlainBill
        For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

        Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

        Comment

          #5
          Re: Samsung PSU low voltage.

          Sorry Bill you probably see stupid questions like this every day so I appreciate you patience and help.

          But yes the values are the same with the logic board plugged in and turned on and off.

          Pin1 on/off 0.7V
          Pin3 on/off 5.17v
          Pin15 on/off 5.13v
          Main cap on/off 386vdc

          Comment

            #6
            Re: Samsung PSU low voltage.

            Originally posted by zedexnz View Post
            Sorry Bill you probably see stupid questions like this every day so I appreciate you patience and help.

            But yes the values are the same with the logic board plugged in and turned on and off.

            Pin1 on/off 0.7V
            Pin3 on/off 5.17v
            Pin15 on/off 5.13v
            Main cap on/off 386vdc
            Speaking of stupid questions, where are you located? The voltage across the main cap would indicate either the PFC is active, or your line voltage is 275 volts. Pin 1 indicates the PFC should be off, but as I've indicated, this supply design is very unusual.

            By teh way, the service manual is available at Elektrotanya.

            PlainBill
            Last edited by PlainBill; 10-21-2011, 05:40 AM.
            For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

            Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

            Comment

              #7
              Re: Samsung PSU low voltage.

              I am a factory authorized and trained technician on Samsung TVs. You have actually proven that your power board is good. If unpluging the Logic board from the Power board allows the backlights to come on, the power board is working correctly. This troubleshooting tip came directly from a Samsung tech support rep, and has NEVER lied to me. The voltages you are measuring that are slightly low would not concern me, they are well within the 10% tolerance I would expect. In short, you most likely have a main board problem, but it could be the function/control board. (front panel controls) A quick check you can do is to measure the voltage at each of the pins of the connector going from the main to the function/control board. Even when off, one of them should be most likely 3.3 volts, could be as high as 5 volts. If none of the pins of that connector have a voltage on them, you definitely have a main board problem.

              Comment

                #8
                Re: Samsung PSU low voltage.

                I am located in NZ. 240V is our line voltage.

                I checked the switch/control board as one of the first things I tried and definatly getting 3.3v and 5.12v at the various points, so its something between the power board and main board. Googling has told me that the samsung have either a cap issue on the power board that causes the eprom on the logic board to go, but I can't seem to find any bad caps or a noticable eprom for that matter.

                Ironically the unit has only less than 500 hours use, but was outside of the 1 year warranty :S

                Thanks

                TEX

                Comment

                  #9
                  Re: Samsung PSU low voltage.

                  You are not experiencing a caps issue here. Chances are, something has failed on the main board. Due to the ridiculously incomplete nature of modern tv schematics, you will be EXTREMELY lucky to determine what has failed. Your best bet is probably try to find a used main board.

                  Comment

                    #10
                    Re: Samsung PSU low voltage.

                    This is the same exact problem that I had with a set. I swapped out the mainboard with no success. You can read some of it here:

                    https://www.badcaps.net/forum/showth...010#post192010

                    There was another thread where I went back and forth with a tech saying that there was no way it could be the power board and I was dead wrong!!! A new power board fixed the problem completely. It was expensive and hard to find but.... it worked.
                    The strong-minded rise to the challenge of their goals,the weak-minded BECOME HATERS

                    Comment

                      #11
                      Re: Samsung PSU low voltage.

                      Here is the main thread that I started for this set. You will see that I had thought the same thing about the backlights turning on with the mainboard unplugged. It's a good read before you go and buy a board you dont need.

                      https://www.badcaps.net/forum/showthread.php?t=14644
                      The strong-minded rise to the challenge of their goals,the weak-minded BECOME HATERS

                      Comment

                        #12
                        Re: Samsung PSU low voltage.

                        Thanks Dgtech, I actaully had been reading that thread to see what other solutions others might have found. It looks like the same board is used in quiet a few models and they all share similar faults.

                        So with your issue it was power board rather than a logic board??? Although you were getting yours to show up with a picture with a flashlight. Mine doesnt even do that and red light doesnt even stay on/flash. It goes on for about a second when you first plug the power in and that its, its stays off.

                        TEX

                        Comment

                          #13
                          Re: Samsung PSU low voltage.

                          Post a picture of your power board. There may be diodes on the power board that are failing you. That has been know to happen on these models as well.
                          The strong-minded rise to the challenge of their goals,the weak-minded BECOME HATERS

                          Comment

                            #14
                            Re: Samsung PSU low voltage.

                            pic as attached. Checked all the main diodes as per Wizards post. They are fine. Tried force starting with the 5v -->power on via a resistor on the power board, nothing. From the panel connector its definatly switching, but dont get a power on from the logic board. to the powerboard though. The power on/ stays at 0.7v doesnt seem to switch.

                            Looking more like the logic board....
                            Attached Files

                            Comment

                              #15
                              Re: Samsung PSU low voltage.

                              Zedexnz, everything you have posted in this thread tells me your main board is the problem. I work on 15-20 Samsung flat panel TVs per week. I have a LOT of experience on these. . .

                              Comment

                                #16
                                Re: Samsung PSU low voltage.

                                Thanks all, sounds like it must be the logic board, as most other problems of the same model seem to be power board related and differ slightly. Randtek how many units do you see that have had main board issues??? Any managed to fix one?

                                THanks

                                Comment

                                Previous template Next
                                Working...
                                X