Dell S199WFP Works for a few hours, then turns off

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  • el3ctroded
    Senior Member
    • Nov 2008
    • 198
    • USA
    #1

    Dell S199WFP Works for a few hours, then turns off

    It's a widescreen. The backlight turns off after a while of working fine. When it turns off, it blinks a bit and then goes off.

    No obvious caps, and if it was caps, I would think it would stay on once warmed up, not turn off.

    Once it turns off, I can turn it on again, but it's on-time progressively gets shorted until it won't stay on any longer than a second.

    Seems to me to be a thermal issue, but nothing I can feel gets hot, and blowing on either side of the power supply pcb or the logic PCB does not seem to keep it on any longer once it gets to the 1sec to off mode. However if I let it sit off for a minute, it will stay on for longer.

    Thoughts?
    Tags: None
    • Th3_uN1Qu3
      Believe in
      • Jul 2010
      • 6031
      • Romania
      #2
      Re: Dell S199WFP Works for a few hours, then turns off

      Resolder the inverter transformers.
      Originally posted by PeteS in CA
      Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
      A working TV? How boring!

        Comment

        • el3ctroded
          Senior Member
          • Nov 2008
          • 198
          • USA
          #3
          Re: Dell S199WFP Works for a few hours, then turns off

          Sorry, forgot to mention I already soldered the inverter transformers and the PWM IC and most of the stuff on the high voltage side of the inverter circuit, plus all the high-power components.

            Comment

            • smason
              Badcaps Legend
              • Feb 2010
              • 1652
              • Canada
              #4
              Re: Dell S199WFP Works for a few hours, then turns off

              Does the monitor turn off, or just the backlights?
              36 Monitors, 3 TVs, 4 Laptops, 1 motherboard, 1 Printer, 1 iMac, 2 hard drive docks and one IP Phone repaired so far....

                Comment

                • el3ctroded
                  Senior Member
                  • Nov 2008
                  • 198
                  • USA
                  #5
                  Re: Dell S199WFP Works for a few hours, then turns off

                  Screen is on, backlight goes out. I also just noticed that often the LCD gets brighter, like it surges, right before it turns off. Could that indicate an intermittent short in the transformer?

                  I don't see any visible indicators of short in the transformer, but that could explain why it takes so long to short; the transformers don't get very hot, and there is a substantial amount of mass to warm up, but once warm it shorts.
                  Last edited by el3ctroded; 01-31-2011, 02:44 PM.

                    Comment

                    • Th3_uN1Qu3
                      Believe in
                      • Jul 2010
                      • 6031
                      • Romania
                      #6
                      Re: Dell S199WFP Works for a few hours, then turns off

                      Could be. But you'd need to monitor the 12v line that the transformers feed from to know for sure. If there's a sudden drop in that line before it shuts down then you may be correct.
                      Originally posted by PeteS in CA
                      Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                      A working TV? How boring!

                        Comment

                        • retiredcaps
                          Badcaps Legend
                          • Apr 2010
                          • 9271
                          #7
                          Re: Dell S199WFP Works for a few hours, then turns off

                          Originally posted by el3ctroded
                          Could that indicate an intermittent short in the transformer?
                          Post a clear focused picture of the inverter transformers (front and back) using manage attachments. Please don't post inline.
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                          We respectfully ask that you make some time and effort to read some of the guides available for basic troubleshooting. After you have read through them, then ask clarification questions or report your findings.

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                            Comment

                            • el3ctroded
                              Senior Member
                              • Nov 2008
                              • 198
                              • USA
                              #8
                              Re: Dell S199WFP Works for a few hours, then turns off

                              Yes, the supply drops. Not a lot, but some. It's regularly at 16.4, and it drops to 16.3. That's the best I can tell using the DMM. After the backlight goes off, it jumps to 20v.

                              I also noticed that one of the transformers is a little warmer than the other, and when it gets to 40deg c is when the whole thing shuts down...
                              Attached Files

                                Comment

                                • retiredcaps
                                  Badcaps Legend
                                  • Apr 2010
                                  • 9271
                                  #9
                                  Re: Dell S199WFP Works for a few hours, then turns off

                                  Just for fun, with power off and lcd unplugged, measure the resistance of secondary inverter transformer pins.

                                  Measure the ohms between the 2 yellow dots. Report measurement. Do the same for 2 red dots. They should be within 3% of each other.
                                  Attached Files
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                                  If you are new to this forum, we can help a lot more if you please post clear focused pictures (max resolution 2000x2000 and 2MB) of your boards using the manage attachments button so they are hosted here. Information and picture clarity compositions should look like this post.

                                  We respectfully ask that you make some time and effort to read some of the guides available for basic troubleshooting. After you have read through them, then ask clarification questions or report your findings.

                                  Please do not post inline and offsite as they slow down the loading of pages.

                                  --- end sig file ---

                                    Comment

                                    • Th3_uN1Qu3
                                      Believe in
                                      • Jul 2010
                                      • 6031
                                      • Romania
                                      #10
                                      Re: Dell S199WFP Works for a few hours, then turns off

                                      You may have a problem with the supply feeding them. 16v to 20v is a big difference. What voltage are the caps on that rail and is it spec'd at some voltage anywhere? It's way too high to be a 12 volt. It could be a 18 volt rail... but the 16v reading is bugging me.
                                      Originally posted by PeteS in CA
                                      Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                                      A working TV? How boring!

                                        Comment

                                        • el3ctroded
                                          Senior Member
                                          • Nov 2008
                                          • 198
                                          • USA
                                          #11
                                          Re: Dell S199WFP Works for a few hours, then turns off

                                          0.494 and 0.498

                                          All the caps on that rail are 35V

                                            Comment

                                            • Th3_uN1Qu3
                                              Believe in
                                              • Jul 2010
                                              • 6031
                                              • Romania
                                              #12
                                              Re: Dell S199WFP Works for a few hours, then turns off

                                              Close enough. Do measure them immediately after it's turned off too, but i don't think the transformers are the problem.
                                              Originally posted by PeteS in CA
                                              Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                                              A working TV? How boring!

                                                Comment

                                                • Rtech
                                                  Badcaps Legend
                                                  • Jul 2010
                                                  • 1095
                                                  #13
                                                  Re: Dell S199WFP Works for a few hours, then turns off

                                                  Try some test CCFL's,instead of the originals,just to confirm that the fault is on the PSU/Inverter PCB.Funny things can happen with both lamps and lamp wiring,and it takes very little time to confirm.

                                                    Comment

                                                    • el3ctroded
                                                      Senior Member
                                                      • Nov 2008
                                                      • 198
                                                      • USA
                                                      #14
                                                      Re: Dell S199WFP Works for a few hours, then turns off

                                                      coils were .500 and .503 after turned off.

                                                      Trying with different CCFLs.

                                                      The voltage rail that supplies the inverter circuit appears to be a switching boost regulator that is fed from a 12v rail. odd.

                                                        Comment

                                                        • jetadm123
                                                          Badcaps Legend
                                                          • Feb 2010
                                                          • 2169
                                                          #15
                                                          Re: Dell S199WFP Works for a few hours, then turns off

                                                          If swapping CCFL's doesn't resolve the problem, try acclerating the shutdown process by heating up parts of the power board using a hair dryer. I would concentrate around the CCFL controller area, inverter transformers and the two 8-pin mosfet chips between the transformers.

                                                          It also could be a bad solder joint. Try gently pushing down on various parts of the board with a wooden dowel or some other insulated object.

                                                          Also, a photo of the entire power/inverter board would be helpful.
                                                          Last edited by jetadm123; 01-31-2011, 07:02 PM.

                                                            Comment

                                                            • el3ctroded
                                                              Senior Member
                                                              • Nov 2008
                                                              • 198
                                                              • USA
                                                              #16
                                                              Re: Dell S199WFP Works for a few hours, then turns off

                                                              It looks like the ccfls did the trick, but I could only run it for 2 hrs before I had to leave. Will test more tomorrow.

                                                              Thx for the hair dryer tip and dowel suggestions, will try if diff. ccfls don't last for a long haul...

                                                                Comment

                                                                • el3ctroded
                                                                  Senior Member
                                                                  • Nov 2008
                                                                  • 198
                                                                  • USA
                                                                  #17
                                                                  Re: Dell S199WFP Works for a few hours, then turns off

                                                                  Got the screen apart and one of the return wires on one of the CCFLs was broken off. Soldering it back on and I'll soon see how it works.

                                                                    Comment

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