PSU with FAN8754 won't torn on

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  • fejlong
    Senior Member
    • Dec 2013
    • 55
    • United Kingdom

    #21
    Re: PSU with FAN8754 won't torn on

    At the other end of the series resistor-diode I measured 102.4V DC regarding to IC ground pin 1

    Comment

    • budm
      Badcaps Legend
      • Feb 2010
      • 40746
      • USA

      #22
      Re: PSU with FAN8754 won't torn on

      Looking at the pictures:
      https://www.badcaps.net/forum/attach...6&d=1474884739

      So what happen to the diode and the two resistors connected to pin 4 for the HV pin, some one replace those parts? D101 R102, R???
      Never stop learning
      Basic LCD TV and Monitor troubleshooting guides.
      http://www.badcaps.net/forum/showthr...956#post305956

      Voltage Regulator (LDO) testing:
      http://www.badcaps.net/forum/showthr...999#post300999

      Inverter testing using old CFL:
      http://www.badcaps.net/forum/showthr...er+testing+cfl

      Tear down pictures : Hit the ">" Show Albums and stories" on the left side
      http://s807.photobucket.com/user/budm/library/

      TV Factory reset codes listing:
      http://www.badcaps.net/forum/showthread.php?t=24809

      Comment

      • fejlong
        Senior Member
        • Dec 2013
        • 55
        • United Kingdom

        #23
        Re: PSU with FAN8754 won't torn on

        Yes, replaced them. But no change

        Comment

        • budm
          Badcaps Legend
          • Feb 2010
          • 40746
          • USA

          #24
          Re: PSU with FAN8754 won't torn on

          Originally posted by fejlong
          Yes, replaced them. But no change
          What ACV are you getting at the Anode of D101 ref to GND pin of the IC?
          Never stop learning
          Basic LCD TV and Monitor troubleshooting guides.
          http://www.badcaps.net/forum/showthr...956#post305956

          Voltage Regulator (LDO) testing:
          http://www.badcaps.net/forum/showthr...999#post300999

          Inverter testing using old CFL:
          http://www.badcaps.net/forum/showthr...er+testing+cfl

          Tear down pictures : Hit the ">" Show Albums and stories" on the left side
          http://s807.photobucket.com/user/budm/library/

          TV Factory reset codes listing:
          http://www.badcaps.net/forum/showthread.php?t=24809

          Comment

          • fejlong
            Senior Member
            • Dec 2013
            • 55
            • United Kingdom

            #25
            Re: PSU with FAN8754 won't torn on

            At D101 anode is 119V AC regarding to ground.

            Comment

            • dick_barton
              Badcaps Legend
              • Aug 2015
              • 6642
              • Wales

              #26
              Re: PSU with FAN8754 won't torn on

              I'm wondering if the mains input supply which should be 230V is low. In post #1 it was reported that there was 300V DC across the HV capacitors but I think it should be about 325V which may be why the HV voltage at pin 4 is just below the minimum Brownin voltage.
              Willing to help but I'm no expert.

              Comment

              • rievax_60
                Badcaps Veteran
                • May 2012
                • 897
                • australia

                #27
                Re: PSU with FAN8754 won't torn on

                The 2v on the FB pin 2 is close to the possible zero duty threshold. There could be a leaky component on pin 2 or the optocoupler is being turned on. What is the LED voltage?
                Last edited by rievax_60; 09-27-2016, 04:13 PM.

                Comment

                • fejlong
                  Senior Member
                  • Dec 2013
                  • 55
                  • United Kingdom

                  #28
                  Re: PSU with FAN8754 won't torn on

                  Leaky component, you mean caps ?
                  The only cap on that line is C112 - 4u7 / 50V I don't know his role.
                  The multimeter cannot measure resistance on this cap, nor capacitance. I will desolder it to check it more thouroughly.
                  Attached Files

                  Comment

                  • rievax_60
                    Badcaps Veteran
                    • May 2012
                    • 897
                    • australia

                    #29
                    Re: PSU with FAN8754 won't torn on

                    I was talking about pin 2 of the IC, U101. Do a resistance measurement to live ground (pin 1) first. Do not try to apply power with any pin of the optocoupler disconnected.
                    What is the voltage across the opto's LED pins?

                    Comment

                    • fejlong
                      Senior Member
                      • Dec 2013
                      • 55
                      • United Kingdom

                      #30
                      Re: PSU with FAN8754 won't torn on

                      Resistance between pin 1 and 2 of IC U101, with no power applied, is 100 Ohms.
                      Voltage drop on optcoupler pins 1 and 2 is 1.05V
                      With power on, led voltage of the optocoupler between pin 1 and 2 is 1.05V .

                      Comment

                      • rievax_60
                        Badcaps Veteran
                        • May 2012
                        • 897
                        • australia

                        #31
                        Re: PSU with FAN8754 won't torn on

                        The bit that doesn't make sense is the 1.05v across the opto's pins 1 and 2. Is that with no power applied also? Do you measure any voltage at the 24v output?
                        I should have mentioned that we don't normally make resistance measurements with power applied. It is not safe for you and the TV. You must have had power applied to get the 100 ohm reading.
                        Last edited by rievax_60; 09-28-2016, 04:34 AM.

                        Comment

                        • fejlong
                          Senior Member
                          • Dec 2013
                          • 55
                          • United Kingdom

                          #32
                          Re: PSU with FAN8754 won't torn on

                          I was not fully understanding you question in post nr 29, so I measured voltage between opto's pin 1 and 2. It results 1.05V with power applied and Pson.
                          And also desoldered pin 1 and 2 of the opto, and with the meter set to diode test, it shows 1.05V voltage drop. I also measured other opto's wich I have as spare, they all read about the same 1V.

                          At the 24V output I can measure 3V.

                          There is also a ratling sound from the transormer, it's about 6-10 Hertz.

                          Comment

                          • dick_barton
                            Badcaps Legend
                            • Aug 2015
                            • 6642
                            • Wales

                            #33
                            Re: PSU with FAN8754 won't torn on

                            From the information supplied the voltage across the capacitor indicates that the PFC circuit is not operating.

                            The voltage at the HV input (94.5V) is below the minimum threshold (104V) to maintain the PWM output drive of the FAN6754. However, these figures are given in the data sheet for a 200K resistor/diode combination and not 156K/diode combination as in this circuit.

                            The VCC (pin 7) is at 18.5V. D103 connected to the VCC input to ground is an 18V Zener so that voltage looks correct. This voltage is generated by U201 (VIPER17) which also supplies the 3.3V standby which is also present.

                            You could lift out diode DS1 to isolate any problems with the 24V supply side and check if voltage at Vb rises > 300V.

                            Can’t think of anything else at this time.
                            Willing to help but I'm no expert.

                            Comment

                            • fejlong
                              Senior Member
                              • Dec 2013
                              • 55
                              • United Kingdom

                              #34
                              Re: PSU with FAN8754 won't torn on

                              PFC ? You mean Power Factor Corrector ? I don't know if this has any PFC
                              Last edited by fejlong; 09-28-2016, 05:01 AM.

                              Comment

                              • rievax_60
                                Badcaps Veteran
                                • May 2012
                                • 897
                                • australia

                                #35
                                Re: PSU with FAN8754 won't torn on

                                The 1.05v across the LED indicates that it is passing forward bias current and emitting light causing the opto's internal transistor to turn on, in turn causing U101 to throttle almost fully down. There will be bursts of very brief pulses applied to he MOSFET's gate causing the rattling and very little power flow through the transformer.
                                The circuitry involving the TL431 and the opto's LED isn't supposed to turn on the LED until the output has climbed to 24v.
                                Difficult to figure out the reason without an understanding of how the TL431 works. Do some research on the TL431. Never apply power while the regulation loop is open. The output will go over voltage and cause more damage. That capacitor you circled can possibly be the cause if leaky.
                                Last edited by rievax_60; 09-28-2016, 05:06 AM.

                                Comment

                                • dick_barton
                                  Badcaps Legend
                                  • Aug 2015
                                  • 6642
                                  • Wales

                                  #36
                                  Re: PSU with FAN8754 won't torn on

                                  OK, with DS1 removed what is the ac voltage at Pins 9 & 10 of TS1
                                  Willing to help but I'm no expert.

                                  Comment

                                  • fejlong
                                    Senior Member
                                    • Dec 2013
                                    • 55
                                    • United Kingdom

                                    #37
                                    Re: PSU with FAN8754 won't torn on

                                    Pins 9 and 10 are joined. if you mean output of the transformer coils, is zero volts. And the rattling disappeared, the feedback killed the supply totally. I will try study TL431 on this board to see what causes the high feedback. Thank you rievax_60 , i will be back with the feedback

                                    Comment

                                    • rievax_60
                                      Badcaps Veteran
                                      • May 2012
                                      • 897
                                      • australia

                                      #38
                                      Re: PSU with FAN8754 won't torn on

                                      Originally posted by dick_barton
                                      OK, with DS1 removed what is the ac voltage at Pins 9 & 10 of TS1
                                      Doing that will likely cause the MOSFET to fail.

                                      Comment

                                      • rievax_60
                                        Badcaps Veteran
                                        • May 2012
                                        • 897
                                        • australia

                                        #39
                                        Re: PSU with FAN8754 won't torn on

                                        Originally posted by fejlong
                                        Pins 9 and 10 are joined. if you mean output of the transformer coils, is zero volts. And the rattling disappeared, the feedback killed the supply totally. I will try study TL431 on this board to see what causes the high feedback. Thank you rievax_60 , i will be back with the feedback
                                        I hope that you did not remove DS1 and apply power?

                                        Comment

                                        • fejlong
                                          Senior Member
                                          • Dec 2013
                                          • 55
                                          • United Kingdom

                                          #40
                                          Re: PSU with FAN8754 won't torn on

                                          Actually I removed only 1 pin, the cathode. Nothing happened as i can see. Soldered it back, and still rattles.

                                          Comment

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