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    #1

    42inch Touch Screen wont power on

    Hey guys,

    I just pulled apart my 42inch touch screen LCD that suddenly just stopped working.

    I have had a quick look and cant see any bad caps.

    Wondering where i should start?

    Just waiting on a multi meter was going to check the power suply and if that checks out would i start with the main board or the Tcon board?

    Thanks for your help people.

    Not sure of the Brand. anyway to find out? no markings on the tv.
    Tags: None
    #2
    Re: 42inch Touch Screen wont power on

    Originally posted by epicfatigue View Post
    Hey guys,

    I just pulled apart my 42inch touch screen LCD that suddenly just stopped working.

    I have had a quick look and cant see any bad caps.

    Wondering where i should start?

    Just waiting on a multi meter was going to check the power suply and if that checks out would i start with the main board or the Tcon board?

    Thanks for your help people.

    Not sure of the Brand. anyway to find out? no markings on the tv.
    No label on the back of the TV? Nothing on the front? How about on the circuit boards?

    Since we can't see the TV, perhaps if you supplied pictures... Attach them using 'Manage Attachments' - below the text entry area. Start with one covering the entire back of the TV, a second of the power supply, and a third of the main board.

    PlainBill
    For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

    Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

    Comment

      #3
      Re: 42inch Touch Screen wont power on

      Sorry for my lack of information just got the tv.

      Here is a little more information.

      The unit is 7000 and is just out of warrenty.It is a industrial unit it was used in a show case.
      It is unknown because it is a tv inside a tv.
      My guess is its either a samgsung or an LG.
      As the power suply is only used in those models.
      The tcon card is an LG the power invertor is an lg.

      The main board has fujitsu & sumgsung ics on it and a Thompson TV tuner.
      The tvsceen has been pulled out of its case anD esembled inside a IR touchscreen unit.

      Hence why its so expensive I don't have the recipt because the company I work with gave it to me and upgraded to 80 inch ones. There was 8 of these in the case they all stopped working at the same time.
      My guess is a surge of some kind.

      The power invertor looks like all the solder joints have gotten hot at one stage. Is this a common problem or could it be cased by a faulty component.

      LG invertor = LC420WU
      Power Supply = PVP-3140
      Mainboard has not identifiable marks on it.
      Attached Files

      Comment

        #4
        Re: 42inch Touch Screen wont power on

        Originally posted by epicfatigue View Post
        There was 8 of these in the case they all stopped working at the same time.
        My guess is a surge of some kind.
        Hmm, if all 8 stopped working, then a power surge would be a good guess.

        1) Once you get your multimeter, check all the fuses on all boards. A good fuse should measure less than 1.0 Ohms.

        2) Then check the bridge rectifier for shorts. Number the pins 1, 2, 3, and 4. Measure the resistance between pins 1-2, 1-3, 1-4, 2-3, 2-4, and 3-4. You can perform this measurement in circuit. Any reading under 30 Ohms suggests a short and warrants desoldering the component to verify out of circuit.
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          #5
          Re: 42inch Touch Screen wont power on

          3) If the fuse and bridge rectifier test out okay, then measure the DC voltage of all the connectors on the bottom right hand corner on the power board. We want to see it reported like the following.

          CN800
          pin 1 - legend says 12V - my reading is 11.99 V
          pin 2 - legend says 5V - my reading is 5.01 V
          etc
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            #6
            Re: 42inch Touch Screen wont power on

            Originally posted by retiredcaps View Post
            3) If the fuse and bridge rectifier test out okay, then measure the DC voltage of all the connectors on the bottom right hand corner on the power board. We want to see it reported like the following.

            CN800
            pin 1 - legend says 12V - my reading is 11.99 V
            pin 2 - legend says 5V - my reading is 5.01 V
            etc
            Shall do,
            Just waiting on my mate to return my multimeter.

            Is it common for a Powerinvertor to show the discolored solder?
            and if the powersupply tests fine what is gennerally the weaker link the Mainboard or the invertor board? (know this is probebly a unrealistic question)

            Comment

              #7
              Re: 42inch Touch Screen wont power on

              Originally posted by epicfatigue View Post
              Is it common for a Powerinvertor to show the discolored solder?
              That is probably left over flux from the soldering/manufacturing process. You can probably remove it with iso alcohol and q-tip.
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                #8
                Re: 42inch Touch Screen wont power on

                Originally posted by retiredcaps View Post
                That is probably left over flux from the soldering/manufacturing process. You can probably remove it with iso alcohol and q-tip.
                Kool thanks for your swift reply, i have done basic electornics so i understand some of what i am dioing here but i do have one big question in reguarding the HOT and Cool side of this board.

                The Hot sie am i able to check components on this part of the board?
                Last thing i want to do is fry my 400$ multimeter ?
                Is there a way to discharge the hot side to safley work on it?

                Basically i am asking for best practise of working on hot/cold side is there a PDF book or link in this formum for explaining the do's and donts of a TV.

                A friendly point in the right direction would be awesome, as working on electornics has always been a passion but sadly not the area of work i landed in.

                Comment

                  #9
                  Re: 42inch Touch Screen wont power on

                  Don't worry, we won't deliberately tell you to do something that will damage the TV, your DMM, or yourself. Enough people here have done it without mal-direction, we don't need that type of entertainment.

                  Normally the hot side capacitor will be discharged within a few seconds. It might take a little longer if the SMPS isn't starting.

                  Retiredcaps missed one obvious starting point - measure the AC input.

                  The output connectors of the power supply seem too be labeled. A clearer shot of those connectors would be helpful.

                  PlainBill
                  For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

                  Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

                  Comment

                    #10
                    Re: 42inch Touch Screen wont power on

                    Originally posted by PlainBill View Post
                    Normally the hot side capacitor will be discharged within a few seconds. It might take a little longer if the SMPS isn't starting.
                    Back in April 2010, I knew nothing about electronics. So I started with a healthy respect for the hot side. In fact, it took me several months to muster up the courage to measure the main filter cap because I knew it was in the 165V DC range. During those early months, I also tried to keep one hand in the pocket too.

                    So when I write instructions, I try to be very explicit in where to place the probes and give clear WARNINGs if you are in the high voltage area.

                    Retiredcaps missed one obvious starting point - measure the AC input.
                    When I don't know the poster, I purposely start with the low voltage/cold side and work backwards to the hot side if necessary. That is how I do it personally. I try to avoid high voltage situations if there is another avenue.

                    We have had people use the wrong setting (namely confusing DC and AC) or use the wrong ground. Not to mention, probes in the wrong jack, and lately a lot of people mis-understanding how to use a manual multimeter and what the meaning of "1" means when it is on the left hand side of the display.
                    Last edited by retiredcaps; 12-01-2011, 11:08 PM.
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                      #11
                      Re: 42inch Touch Screen wont power on

                      Originally posted by epicfatigue View Post
                      Last thing i want to do is fry my 400$ multimeter ?
                      What brand? And you lend it away? I would be more worried about the friend using it incorrectly and coming back with blown fuses?

                      The $400 multimeters are more likely to survive the high voltage side if anything goes wrong than the cheap $5 ebay multimeters.

                      The one area I NEVER measure is the inverter transformer voltages. It can easily be > 1,000 V which will likely kill any multimeter except the cream of the crop.

                      A couple of personal rules wrt to high voltage. I never work when tired and never work after sunset (I lose a lot of light and the ability to see everything clearly). I also plug things into a power bar so I can power the device off with a flick of the switch.
                      Last edited by retiredcaps; 12-01-2011, 11:47 PM.
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                        #12
                        Re: 42inch Touch Screen wont power on

                        Originally posted by retiredcaps View Post
                        <SNIP>


                        When I don't know the poster, I purposely start with the low voltage/cold side and work backwards to the hot side if necessary. That is how I do it personally. I try to avoid high voltage situations if there is another avenue.
                        <SNIP>
                        Not a bad idea, and I don't even consider using the ability to measure line voltage as a Darwinian test. However, consider that there was some sort of power 'event' that either took out 8 TVs, or took out the AC supply to 8 TVs.

                        In most cases these TVs are designed to operate at input voltage of over 250 VAC; and will draw over 2 Amps at 120 VAC. Either this is a power supply problem (blown fuse?) or something took out the main boards.

                        PlainBill
                        For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

                        Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

                        Comment

                          #13
                          Re: 42inch Touch Screen wont power on

                          Okay Guys finally got this bugger of a multi meter back.

                          Iv been doing a little bit of reading to help me as its been years since i have touched electronics as i moved into a different field.

                          Now first Up I tested the fuse for continuity it checked okay.
                          The only fuse i could fine was the one as the power goes from the wall into the power supply, seems unusual to me shouldn't there be more ?

                          Next Up Redcaps you asked me to measure the bridge rectifier.
                          Now i had a look and i have a couple of questions before doing this.

                          If i remember correctly the bridge rectifier has 4 prongs and is generally attached to the heatsinc ?

                          This is located on the hot side is this the correct one? or can the bridge rectifier be 3 prongs? as there are many of these located on hot and cold side?
                          Marked on IMG 4 Prong(A) 3 Prong (B)
                          (Note there are 1x (A) and 12x(B) )

                          Second when you say measure the power comming out of the DC power supply in the bottom right hand corner do you mean (C) or (D) or both?

                          And do i measure these all from a common ground?
                          Attached Files

                          Comment

                            #14
                            Re: 42inch Touch Screen wont power on

                            Originally posted by epicfatigue View Post
                            Now first Up I tested the fuse for continuity it checked okay.
                            I suggest getting an actual Ohms reading. Multimeter vary greatly and beep "continuous" for readings anywhere from < 30 Ohms to < 1800 Ohms. That means if the fuse readings 29 Ohms, it will be continuous. Really crappy multimeters will be continuous if the reading is 1799 Ohms.

                            In my early days, I measured a picofuse at 33 Ohms and thought it was good because it beeped. Now I know a good fuse should measure less than 1.0 Ohms.

                            I like to see people respond, "the fuse measures 0.6 Ohms".

                            The only fuse i could fine was the one as the power goes from the wall into the power supply, seems unusual to me shouldn't there be more ?
                            Fuses start with the letter "F" on the board. See

                            http://en.wikipedia.org/wiki/Electro...e_designations

                            Fuses come in all shape and sizes so I rely on the "F" to help me spot them.

                            If i remember correctly the bridge rectifier has 4 prongs and is generally attached to the heatsinc ?
                            Yes and see the above URL (bridge rectifier is BR or sometimes BD).

                            This is located on the hot side is this the correct one?
                            Yes.

                            or can the bridge rectifier be 3 prongs?
                            That might be your power mosfet. You can also check it for shorts by measuring 1-2, 1-3, and 2-3. Report the Ohms reading.

                            Second when you say measure the power comming out of the DC power supply in the bottom right hand corner do you mean (C) or (D) or both?

                            And do i measure these all from a common ground?
                            Both C and D. Use the ground screw on the bottom right hand corner for ground.

                            If every pins measures 0.0 V DC, then we need check the hot side.

                            To check the hot side, measure the DC voltage largest capacitor (the Samxon 220uF 450V). Black probe goes on the negative leg of the cap. Red probe goes on the positive leg of the cap. If you are in North America, the DC voltage reading should be around 165 V DC.

                            This is 120 V AC x 1.414 (rectified) = roughly 165 V DC.
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                              #15
                              Re: 42inch Touch Screen wont power on

                              If the rectifier is shorted, then there is no need to do all the DC voltage measurements on the hot and cold side.
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                                #16
                                Re: 42inch Touch Screen wont power on

                                Main Fuse checks = .1 ohms

                                Bridge rectifier
                                1-2 = .916 M
                                1-3 = 3.62 M
                                1-4 = 1.13 M
                                2-3 = 381.1 k
                                2-4 = 4.50 M
                                3-4 = 4.50 M

                                CN06 (C on my diagram)

                                Pin 1 Legand Says 24V - My reading 0
                                Pin 2 Legand Says 24V - My reading 0
                                Pin 3 Legand Says 24V - My reading 0
                                Pin 4 Legand Says 24V - My reading 0
                                Pin 5 Legand Says 24V - My reading 0
                                Pin 6 Ground
                                Pin 7 Ground
                                Pin 8 Ground
                                Pin 9 Ground
                                Pin 10 Ground
                                Pin 11 Blank
                                Pin 12 Blank
                                Pin 13 my reading 0
                                Pin 14 My Reading 0

                                CN04 (D on my diagram)
                                Pin 1 LEgand Power - My reading - 2.947V
                                Pin 2 LEgand GND - My reading - 0
                                Pin 3 LEgand GND - My reading - 0
                                Pin 4 LEgand 5V - My reading - 157.9mV
                                Pin 5 LEgand 5V - My reading - 157.9mV
                                Pin 6 LEgand 5Vs - My reading - 5.23V
                                Pin 7 LEgand GND - My reading - 0
                                Pin 8 LEgand GND - My reading - 0
                                Pin 9 LEgand 12v_C - My reading - 49.4mV
                                Pin 10 LEgand 12v_C - My reading - 49mV
                                Pin 11 LEgand INV_d1 - My reading - 0
                                Pin 12 LEgand unknown - My reading - 0
                                Pin 12 LEgand unknown

                                When i measured this i ensured all lead are unpluged except the voltage comming from power socket.

                                When all leads are plugged in a relay clicks in power supply and nothing happens.

                                I looked high and low for another fuse marked F and could not see one.

                                What is my next step something is clearly wrong here
                                Last edited by epicfatigue; 12-05-2011, 03:45 AM.

                                Comment

                                  #17
                                  Re: 42inch Touch Screen wont power on

                                  Originally posted by epicfatigue View Post
                                  CN06 (C on my diagram)
                                  Pin 1 Legand Says 24V - My reading 0
                                  Pin 2 Legand Says 24V - My reading 0
                                  Pin 3 Legand Says 24V - My reading 0
                                  Pin 4 Legand Says 24V - My reading 0
                                  Pin 5 Legand Says 24V - My reading 0
                                  CN06 (all the colors) looks like it powers the inverter board. No power means no backlight.

                                  CN04 (D on my diagram)
                                  Pin 1 LEgand Power - My reading - 2.947V
                                  Pin 4 LEgand 5V - My reading - 157.9mV
                                  Pin 5 LEgand 5V - My reading - 157.9mV
                                  Pin 6 LEgand 5Vs - My reading - 5.23V
                                  5Vs is your standby which is working.

                                  Pin 1 = power could be working. Some TVs use 0 V and some use > 2 V to signal power on. Let's assume it is working.

                                  So now the problem is that the SMPS is not starting since we are missing 5, 12, and 24 V DC.

                                  We will need a picture of the backside of the power board. We are looking for a 8 legged IC (or more) that is the smps chip. We need the part number and its datasheet.

                                  This chip could be bad or the startup cap (usually a little one in the range of 22uF to 50uF) is bad.

                                  PS1. There is a small white SMD fuse on the inverter board just next to the yellow colored cap.
                                  PS2. What type of multimeter (I'm curious).
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                                    #18
                                    Re: 42inch Touch Screen wont power on

                                    looks like FPG68H (8 could be B) FAN75278 (8 could be B)
                                    Attached Files

                                    Comment

                                      #19
                                      Re: 42inch Touch Screen wont power on

                                      1) It is Fan6527b which is a PFC controller.

                                      2) Please post a picture of the entire backside as well.

                                      3) Anymore chips on the back?

                                      4) If there are no more chips on the backside, the SMPS could be integrated with the power mosfet. This would be one of the transistors mounted on a heatsink on the front side.
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                                        #20
                                        Re: 42inch Touch Screen wont power on

                                        Hey yes there is another chip on the back, has 16 legs and is located near the cold side of the board.

                                        Ill just get the Camera out and take a close up of it only way i can read the markings.
                                        Attached Files

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