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10 year old Vizio VO370M has intermittent flickering backlight

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    10 year old Vizio VO370M has intermittent flickering backlight

    I have this Vizio VO370M TV that I'd like to save because it has very low input lag (as good as the most expensive TVs sold today!, https://alantechreview.blogspot.com/...input-lag.html), but it has a very annoying issue: sometimes the backlight flickers (or is just kind of dim). The flicker is slower than 60hz, so we aren't just talking some kind of motion compensation).

    I was hoping maybe it was a power board issue (aka failing caps) but I opened it up and I don't see any that look bad. I have a budget capacitance/ESR meter if it's worth checking those in detail but I was wondering if that's the place to start. I haven't done any diagnostics at all other than taking it apart and looking at the caps with my bare eyes.

    This has a CFL backlight, so maybe the tube is dying of old age? An ebay search did not turn up any separate inverter boards for this model (https://www.ebay.com/sch/i.html?_fro...leDesc=0&rt=nc) so I assume if it's not the tube it's the main power board. Interestingly, there seems to be at least two different power boards listed on ebay for this model, suggesting one model number mapped on to multiple devices.

    I can take pictures of the boards in my specific device if that's helpful.

    I did a search on the board and didn't see any general advice for diagnosing flickering backlights, sorry if I missed the relevant FAQ entry.

    #2
    Re: 10 year old Vizio VO370M has intermittent flickering backlight

    power and mainboard attached.
    Attached Files

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      #3
      Re: 10 year old Vizio VO370M has intermittent flickering backlight

      Take a look at all the capacitors on the power supply, especially the ones under the metal heatsinks for any signs of bowing.

      Also take a look at the solder joints underneath the power supply and make good any that look poor or cracked especially around the transformers.

      Is the voltage when measured between the two legs of the 120uF, 450V cap steady or does that flicker when the screen flickers?
      Last edited by dick_barton; 05-15-2021, 05:51 AM.
      Willing to help but I'm no expert.

      Comment


        #4
        Re: 10 year old Vizio VO370M has intermittent flickering backlight

        Thanks for the suggestion!

        caps all look flat - though the metal heat sink makes it a royal pain to check. I can us my T4 super-budget esr meter to check further but it will take a lot of desoldering given that heatsink placement :-(

        I don't see any solder cracks.

        Voltage looks steady, but my voltmeter has a pretty heavy lowpass filter so it would need to be flickering at like 1hz to see anything. This flicker is more like 20-30hz.

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          #5
          Re: 10 year old Vizio VO370M has intermittent flickering backlight

          I had a Sharp 42" that had flickering backlights (CCFL). The tubes weren't soldered, they were "snapped" into a holder. One of my tubes wasn't fully snapped into its holder on one end, so contact would dance from low resistance to a higher resistance, causing the b/l to dim. All I had to do was first find the culprit, then snap the loose end into its holder. This fix was my first of a flat panel TV.

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            #6
            Re: 10 year old Vizio VO370M has intermittent flickering backlight

            Very interesting. I can confirm that the tubes are snapped into this model as well. But they are snapped tight, no loose ones. In fact it was quite a challenge to remove the CCFL driver board they were so tight.

            Comment


              #7
              Re: 10 year old Vizio VO370M has intermittent flickering backlight

              It would be my bet that one of the tube holders has a fractured solder joint, or a bad crimp connection. Look at the entire backlight for dark areas, which would indicate arcing across a joint.

              When it flickers, is it limited to an area of the screen, or does it cover the entire screen?

              Comment


                #8
                Re: 10 year old Vizio VO370M has intermittent flickering backlight

                entire screen.

                Comment


                  #9
                  Re: 10 year old Vizio VO370M has intermittent flickering backlight

                  Does your power supply monitor the AC detect and if so could that be at it's minimum level and dropping in and out causing the fluctuation
                  Willing to help but I'm no expert.

                  Comment


                    #10
                    Re: 10 year old Vizio VO370M has intermittent flickering backlight

                    I'm trying my best to parse that message but either I'm misunderstanding what you are saying, or perhaps it was voice transcribed and one or more of the words was autocorrected wrong?

                    I think what you mean is the power supply could be the cause and if I measured in the right place I'd see a change in voltage? unfortunately, my voltmeter doesn't see any change in voltage, but it has a serious lowpass filter that would probably hide anything slightly transitory.

                    Comment


                      #11
                      Re: 10 year old Vizio VO370M has intermittent flickering backlight

                      I think you've got what I was trying to say. A voltage somewhere is probably not stable. Do you have access to a digital meter or perhaps loan one from a friend.
                      Do you have a schematic for this supply?
                      Willing to help but I'm no expert.

                      Comment


                        #12
                        Re: 10 year old Vizio VO370M has intermittent flickering backlight

                        I'm not sure how a digital meter differs from a volt meter / multimeter? are you saying something like a low-end oscilloscope?

                        I do not have any schematics.

                        Comment


                          #13
                          Re: 10 year old Vizio VO370M has intermittent flickering backlight

                          You mention earlier that
                          Voltage looks steady, but my voltmeter has a pretty heavy lowpass filter so it would need to be flickering at like 1hz to see anything. This flicker is more like 20-30hz.
                          I was thinking that perhaps a digital multimeter would show any fluctuations in the supply being checked. Beter still if you had access to an oscilloscope.
                          Willing to help but I'm no expert.

                          Comment


                            #14
                            Re: 10 year old Vizio VO370M has intermittent flickering backlight

                            Where would you measure if you had a good meter? The largest cap, or somewhere else?

                            Comment


                              #15
                              Re: 10 year old Vizio VO370M has intermittent flickering backlight

                              Take some close-up pictures of the backside of the power supply board, especially in the area of the two transformers (lower left corner in your top side photo). Also, try to clean all the dust and cobwebs off the top side of the PS so that we can get a better view. A blast from a compressed air line will do just fine.

                              As for measurements, start at the tubes themselves, then at the tube end of the supply lines, then at the PS end of the supply lines, then at the output of the Schottky diode for the tube supply. Somewhere along this chain, you will find either a broken solder joint, a bad wire crimp, or a bad component. My bet would be one of the first two options.

                              Comment


                                #16
                                Re: 10 year old Vizio VO370M has intermittent flickering backlight

                                I took photos by quadrants starting where you requested. In doing so I noticed something that might be an issue: some of the joints looks pitted almost like the solder didn't melt completely? see last two photos for closeup. There's no evidence that these joints are loose and the holes are tiny so I'd be surprised that they don't work, or that they would only work intermittently, but it's a place to start. See anything else worth touching up at the same time? The area with the transformers looks perfect, none of the odd pits.
                                Attached Files

                                Comment


                                  #17
                                  Re: 10 year old Vizio VO370M has intermittent flickering backlight

                                  I can't see any problems with any of the joints. See if you can induce the problem by wiggling the cables to the tubes.

                                  As a last resort, you could buy a single replacement tube and replace the ones in the TV one at a time to see if you have a single bad tube. Watch the TV with the screen removed (just the backlights) and see if a single tube flashes first. You can film it with your camera and slow the playback to better visualize what is happening.

                                  Comment


                                    #18
                                    Re: 10 year old Vizio VO370M has intermittent flickering backlight

                                    The tubes are snapped in with clips soldered directly to the inverter board, and the snaps are definitely making contact - it's very hard to remove each tube from the clip. I think the contacts are good, actually.

                                    Thanks for all the advice. The idea of filming while testing is a great one, I'll try that next time I try to fix something like this. Even if I haven't fixed this one I'm definitely learning some useful skills!

                                    I'm going to call it on this one and send it to the recycler.

                                    Comment

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