AcousticSolutions LCD42761F1080P dead

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  • tom66
    EVs Rule
    • Apr 2011
    • 32560
    • UK
    #1

    AcousticSolutions LCD42761F1080P dead

    Got this one in with blown mains fuse no standby.
    Vestel 17PW26-3 PSU with 17MB35-1 main board.

    Found two 7N50 FETs dead short. The 10R gate resistors measuring 35R&50R each. The FAN7711 controller IC appears OK but I will replace it too. Luckily the gate circuit is fairly simple -- nothing special. Looks like a few quid max to repair.

    This is a 1080p 100Hz motion set. Not a bad set apart from the power board. If it had some better components on the power board, it wouldn't be too bad.

    Oddly, the FAN7711 is intended for CFLs, not for the main power controller for a 42" LCD TV. They appear to modulate the RT and CT inputs to regulate the output. Rather bizarre.
    Last edited by tom66; 09-21-2013, 11:41 AM.
    Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
    For service manual, schematic, boardview (board view), datasheet, cad - use our search.
    Tags: None
    • ben7
      Capaholic
      • Jan 2011
      • 4059
      • USA
      #2
      Re: AcousticSolutions LCD42761F1080P dead

      Originally posted by tom66
      Oddly, the FAN7711 is intended for CFLs, not for the main power controller for a 42" LCD TV. They appear to modulate the RT and CT inputs to regulate the output. Rather bizarre.
      Doesn't sound too strange - maybe the chip operates in a resonant, zvs mode? I haven't looked at the datasheet yet though.
      Muh-soggy-knee

        Comment

        • tom66
          EVs Rule
          • Apr 2011
          • 32560
          • UK
          #3
          Re: AcousticSolutions LCD42761F1080P dead

          The driving scheme is very strange. It appears to be variable frequency variable pulse width or perhaps constant pulse width drive with variable duty cycle/frequency.

          Schematic


          It's strange to see something used to drive a 15W CFL as a controller for a 200W converter, but I guess it's 800mA gate output drive is fairly high for such a low cost chip.

          I think I'll possibly replace both FETs with 20N60s because the 7N60s are cutting it close.
          Last edited by tom66; 09-21-2013, 03:43 PM.
          Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
          For service manual, schematic, boardview (board view), datasheet, cad - use our search.

            Comment

            • tom66
              EVs Rule
              • Apr 2011
              • 32560
              • UK
              #4
              Re: AcousticSolutions LCD42761F1080P dead

              OK: Replaced both FETs, driver IC, SMD resistors, plus a diode and MLCC for charge pump.

              Turned on, 24V measures 4.9V, 12V measures 2.4V... Hmm. I listen carefully to the PSU and hear it's chirping between high and low frequency, so I go looking for shorts. So, I found a shorted diode on 24V_AUDIO line, UF5402 type. Replaced with UF5404. Turn it back on, now I get 24V at about 19V and 12V at around 8V. Still too low...

              Checked PFC voltage, getting 385V there which is fine.

              Replaced C844 (as recommended in other tips), no change in 12V/24V. Then I check the Vcc to the FAN7711 chip. It's 12.5V. The ON threshold of the IC is 11~13V, so this is very close to not working.

              I eventually found there's a transistor on the power board which switches the 18V from the standby transformer (VF_MAIN out) to the VCC1/VCC2 rail. I measured the transistor, a little BD138 PNP transistor. I found the emitter-base resistance to be 3 ohms -- not good. The emitter collector measured 500 ohms, and collector-base measured 3 ohms too. I replaced it with a TIP42C, and the 24V then hit 24.5V and the TV worked. (Vcc rose to 18V.)

              I'm guessing that the 12.8V was enough to support the IC and allow it to switch OK, but the 500 ohm resistance will have significantly reduced the pulses into the transistors. Hence the output would not have regulated properly.

              Except... I only got audio out of one channel. On the right. The left speaker was silent. I found the right channel output to be modulating fine (I got a good square wave.) But the left channel was dead. When I disconnected the speaker connections, the square wave for the left channel appeared... hmm. I found that disconnecting the tweeter for the left channel, the noise came back. So I disconnected both tweeters on both sides. Upon testing the tweeter, I could not find any short. But connecting it does cause the audio for the left to drop out. I figure that it probably has a shorted turn, inside the speaker. Very odd.

              What a crapshoot of the TV. At least it was free.
              Last edited by tom66; 09-27-2013, 02:32 PM.
              Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
              For service manual, schematic, boardview (board view), datasheet, cad - use our search.

                Comment

                • attainteddragon
                  Badcaps Veteran
                  • Apr 2013
                  • 764
                  • australia
                  #5
                  Re: AcousticSolutions LCD42761F1080P dead

                  did you do a vid of this one.
                  heaps of us would love to see this repair i think!
                  WHY CAN'T PHILIPS USE PHILLIPS HEAD SCREWS?

                    Comment

                    • tom66
                      EVs Rule
                      • Apr 2011
                      • 32560
                      • UK
                      #6
                      Re: AcousticSolutions LCD42761F1080P dead

                      I did, but it only goes over exactly what I did in this post. I've not got my nice Samsung camera (I've lost it unfortunately) so I don't make so many videos ATM. I need to get a new one. I've made a few bucks from YouTube ads, so I'll be posting more videos soon.
                      Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
                      For service manual, schematic, boardview (board view), datasheet, cad - use our search.

                        Comment

                        • attainteddragon
                          Badcaps Veteran
                          • Apr 2013
                          • 764
                          • australia
                          #7
                          Re: AcousticSolutions LCD42761F1080P dead

                          just seeing the thought process you go through to trouble shoot the problems is what i like to see.
                          at least you started using a tripod, compared to your earlier vids. :P
                          at least samsung cameras are better built than their tv's.
                          i look forward to your future uploads of, FIX THIS DAM TV!
                          WHY CAN'T PHILIPS USE PHILLIPS HEAD SCREWS?

                            Comment

                            • tom66
                              EVs Rule
                              • Apr 2011
                              • 32560
                              • UK
                              #8
                              Re: AcousticSolutions LCD42761F1080P dead

                              Forgot to link to the video on this repair
                              https://www.youtube.com/watch?v=u1gWvF1erzU
                              Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
                              For service manual, schematic, boardview (board view), datasheet, cad - use our search.

                                Comment

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