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Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

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    #21
    Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

    Originally posted by R_J View Post
    I can't see how the voltage would drop from 160vdc to 116vdc. What make/model meter are you using to check this voltage? Try using a different meter if you can.
    It's a Harbor Freight voltmeter, and you are probably right R_J. I bet it isn't reading the right voltage either. Here's why:

    After about 2hrs worth of testing the PS out of the TV with stable voltages I decided to put it back in to the TV and try it, mainly to apply a live load and get the board to operating temperature, trying to get it to fault again.

    The TV has now been working properly since, far as I can tell. It's been on a total of 5hrs with the back off so I can check voltages, which for now are appropriate and stable. I guess I'll keep using it until it faults again so I can perform further testing.

    I've scratched my head so much on this one I now have bald spots! As soon as it faults again I will re-test and report. I do not believe the rectifier diode is faulty.
    They call me......."threadkiller"

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      #22
      Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

      I had an LG/Goldstar meter that I would use on the bench, one time I went to check the Vs voltage on a working plasma tv and the meter suggested the 200v was actually over the meters range of 1000v, I took my fluke out of my tool case and checked the voltage and it was where it should be, The LG meter was picking up the stray high frequency from the Ysus board and causing it to show an over voltage.

      I suspect this is what is happening with your meter, with the power supply in standby, the pfc circuit is not running, so it shows 160vdc across the bridge, Once the power supply is ON the pfc circuit, which operates at high frequency to boost the voltage on the main filters to 385vdc. some of that high frequency finds its way back through the two blocking diodes and causes the meter to show a reduced voltage across the bridge.

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        #23
        Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

        Originally posted by R_J View Post
        I had an LG/Goldstar meter that I would use on the bench, one time I went to check the Vs voltage on a working plasma tv and the meter suggested the 200v was actually over the meters range of 1000v, I took my fluke out of my tool case and checked the voltage and it was where it should be, The LG meter was picking up the stray high frequency from the Ysus board and causing it to show an over voltage.

        I suspect this is what is happening with your meter, with the power supply in standby, the pfc circuit is not running, so it shows 160vdc across the bridge, Once the power supply is ON the pfc circuit, which operates at high frequency to boost the voltage on the main filters to 385vdc. some of that high frequency finds its way back through the two blocking diodes and causes the meter to show a reduced voltage across the bridge.
        That makes sense, and I'm pretty confident you nailed it right on the head. I'm now looking at the Fluke brand meters and plan to get one this week. Do you have any suggestions for a meter in the $150.00 range?
        They call me......."threadkiller"

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          #24
          Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

          Originally posted by R_J View Post
          I can't see how the voltage would drop from 160vdc to 116vdc. What make/model meter are you using to check this voltage? Try using a different meter if you can.
          I was curious because I had noted this on a RCA power supply I was working on and the designer actually monitors the rectified output and sums it with the feedback on a FAN7930 PFC controller. I am still puzzled as to just what the intent was there, but I do know it causes that RCA Power Supply to output less than expected PFC voltage like in the 340 VDC range while also causing the PFC voltage to vary with load requirements.

          And to further satisfy some of that curiosity I just checked a Samsung power supply rectifier output that measures 167 VDC with PFC off and 108 VDC with PFC running. I got virtually the same reading when using a Fluke or a Klein DMM. I would like to look at the output using a scope but since I am still a bit new to the SMPS theory and possess only a cheap isolation transformer I have been a bit hesitant to try that for fear of damaging the scope.

          And one more thing to ponder measuring from the + output of the rectifier to the + input of the large caps results in a reading of 280 VDC.
          Last edited by RDC55; 06-14-2021, 05:47 AM.

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            #25
            Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

            Originally posted by RDC55 View Post
            I was curious because I had noted this on a RCA power supply I was working on and the designer actually monitors the rectified output and sums it with the feedback on a FAN7930 PFC controller. I am still puzzled as to just what the intent was there, but I do know it causes that RCA Power Supply to output less than expected PFC voltage like in the 340 VDC range while also causing the PFC voltage to vary with load requirements.

            And to further satisfy some of that curiosity I just checked a Samsung power supply rectifier output that measures 167 VDC with PFC off and 108 VDC with PFC running. I got virtually the same reading when using a Fluke or a Klein DMM. I would like to look at the output using a scope but since I am still a bit new to the SMPS theory and possess only a cheap isolation transformer I have been a bit hesitant to try that for fear of damaging the scope.

            And one more thing to ponder measuring from the + output of the rectifier to the + input of the large caps results in a reading of 280 VDC.
            This is interesting.....I'm guessing it has to have something to do with the DC side of the bridge being isolated from the PFC circuit by DP808 and DP809? I tested from both the + and - side of the rectifier diode and got 280V from the + side and 397V from the - side when measured to the - side of the main caps.

            Thanks for taking that measurement RDC55!
            They call me......."threadkiller"

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              #26
              Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

              Originally posted by tvtimmy View Post
              Must be a tricky power supply. Let us know what happens with the new parts.
              Originally posted by dick_barton View Post
              You could check you have the supply voltage on SPC1012T and if there is voltage at Vref and also the resistance to ground of the gate drive outputs

              There is a pinout at
              Originally posted by R_J View Post
              What is the voltage across the main caps? Did you check the gate drive resistors/diodes on the two mosfets that were shorted? Try working on just the power supply board without it connected in the tv and check the output voltages, VS Va etc.
              Originally posted by RDC55 View Post
              I was curious because I had noted this on a RCA power supply I was working on and the designer actually monitors the rectified output and sums it with the feedback on a FAN7930 PFC controller. I am still puzzled as to just what the intent was there, but I do know it causes that RCA Power Supply to output less than expected PFC voltage like in the 340 VDC range while also causing the PFC voltage to vary with load requirements.

              And to further satisfy some of that curiosity I just checked a Samsung power supply rectifier output that measures 167 VDC with PFC off and 108 VDC with PFC running. I got virtually the same reading when using a Fluke or a Klein DMM. I would like to look at the output using a scope but since I am still a bit new to the SMPS theory and possess only a cheap isolation transformer I have been a bit hesitant to try that for fear of damaging the scope.

              And one more thing to ponder measuring from the + output of the rectifier to the + input of the large caps results in a reading of 280 VDC.
              So 4 days after removing the solder from and re-soldering QP802S, QP804S and DP802C over concerns of contaminated solder joints and the subsequent unstable voltage issue that ceased seemingly on it's own the TV has not faulted again. I also carefully cleaned all flux from the board. In fact the picture and overall operation of the unit are FAR BETTER than they ever were.

              I did adjust VS and VA up 1 volt and adjusted Vsc up .8V, all to match the recommended settings upon the panel plus I tightened the screws upon all of the boards, some of which were fairly loose.

              It took a week for the TV to fault after the initial replacement of QP802S and QP804S. I guess we'll see what happens.
              They call me......."threadkiller"

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                #27
                Re: Samsung PN51F8500AFXZA, PSU BN44-00619A, Short QP802S, ICP801S testing/sourcing

                Hello again all, this will likely be the last update on this TV.

                Since my last post this TV has been in use in my living room with no faults or problems. At this time I consider the repairs I performed a success.

                Thanks to everyone who participated in this thread and I appreciate it!

                They call me......."threadkiller"

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