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    #81
    Re: Problems with Samsung 740N monitors

    Originally posted by warragul
    mladen82, am I right in assuming that the FETs are the small SMDs on the underside of the board? Can't find my magnifying glass. If so the fuse doesn't last long enough for it to get warm.
    yes, they are smd, 8pin
    i think they have label 4502C
    http://www.alldatasheet.com/datashee.../AF4502CS.html

    check these for shorts

    Comment


      #82
      Re: Problems with Samsung 740N monitors

      Originally posted by warragul
      Thanks again for the assist.
      I'm a little puzzled, though.
      This is the first SMPS I've encountered with a fuse in the output. These power supplies usually rely on the oscillator stopping if the load gets too much.
      The Topswitch chip manufacturer's suggested circuits I've found don't have output fuses either. Some sort of effect of the sort of load it's driving, perhaps?
      The fuse isn't there to protect the power supply, it's there to prevent damage to the inverter. Note that the component number for the fuse is in the same series as the rest of the inverter, not the power supply.

      To elaborate, let's say power supply components use '100' series numbers. Caps start at C101, resistors at R101, fuses at F101, etc. The inverter might use 800 series numbers (C801, R801, and F801, etc). It's not an output fuse for the power supply, it's an input fuse for the inverter.

      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.

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        #83
        Re: Problems with Samsung 740N monitors

        Originally posted by warragul
        ..... This is the first SMPS I've encountered with a fuse in the output. ....
        I see it as the fuse at the inverter's input, not at the PSU's output (I think of it as the fuse on the AC input of the PSU). If you had a monitor where all the three boards were separated (Video/signal, PSU & inverter boards), you won't find a fuse on the output of the PSU for the inverter : you will find a fuse on the input of the inverter board. Some PSUs I've seen have fuses (not the tube/glass ones, but the ones like small caps) right before the DC output to the signal board. That too can be considered either a fuse at the PSU's output or at the VBoard input ...
        There are 10 kind of people in this world: those that understand binary, and those who don't.
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          #84
          Re: Problems with Samsung 740N monitors

          i'm new to this and gonan give it a go, thanks guys!

          I'll let you know how it works out, just ordered the caps and fuse from digi-key, what a great place that is!

          Comment


            #85
            Re: Problems with Samsung 740N monitors

            This post was really help full in fixing my Samsung 930B. I had the same defective power/inverter board. I changed the 3x820uF by 2x1000uF + 1x470uF for approximately the same total amount of capacitance 2460uF. I also had to changed the C301 pico fuse. I only had in stock the SMT version so I removed the defective one and solder the SMT fuse between the 2 holes on the bottom side of the PCB.

            The Monitor now work perfectly. It was an 8$ Canadian fix and 1 hour of my time.

            Comment


              #86
              Re: Problems with Samsung 740N monitors

              IT WORKS!!!Awesome guys, thanks for the input. My first lcd repair, all went well! DIGI-KEY ftw.....

              Comment


                #87
                Re: Problems with Samsung 740N monitors

                Originally posted by mopar-mechanic
                IT WORKS!!!Awesome guys, thanks for the input. My first lcd repair, all went well! DIGI-KEY ftw.....
                Glad to know of a success. It feels good to repair something that you think is going to head right to the dumpster, right?
                There are 10 kind of people in this world: those that understand binary, and those who don't.
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                  #88
                  Re: Problems with Samsung 740N monitors

                  Hi guys. Greetings from Argentina. First post here.
                  I have a 740N monitor that stopped working. Blue power led on, no image, no inverter.
                  I've found the famous three 820 uF caps bulged and changed them to 1000 uF.
                  Now the blue led flashes very fast and PS makes noise as fast as the flash.
                  I try to measure the output voltages and they are way high. 8V at the 5.1V part and 18V at the 13V part.

                  Can you help me with this? Sorry for my english

                  Thanks a lot!

                  Comment


                    #89
                    Re: Problems with Samsung 740N monitors

                    Originally posted by DSpawnZ
                    Hi guys. Greetings from Argentina. First post here.
                    I have a 740N monitor that stopped working. Blue power led on, no image, no inverter.
                    I've found the famous three 820 uF caps bulged and changed them to 1000 uF.
                    Now the blue led flashes very fast and PS makes noise as fast as the flash.
                    I try to measure the output voltages and they are way high. 8V at the 5.1V part and 18V at the 13V part.

                    Can you help me with this? Sorry for my english

                    Thanks a lot!
                    See post #85 (up) - you changed 2460uf for 3000uf. Maybe there's where your problem lies.

                    And welcome to the forums.
                    There are 10 kind of people in this world: those that understand binary, and those who don't.
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                      #90
                      Re: Problems with Samsung 740N monitors

                      also check optocupler 817

                      Comment


                        #92
                        Re: Problems with Samsung 740N monitors

                        Greetings to all

                        I have changed 3 820uf capacitors and 3A fuse and now the CCFL lamps start for a second and they turn off again. After two seconds they startup again for a second and turn off again. I checked 13V line and it reads 18V and just as the lamps turn on for a second voltage drops to 13V and comes back to 18V when the lamps turn off. What can be the problem?

                        Comment


                          #93
                          Re: Problems with Samsung 740N monitors

                          Originally posted by ethernet
                          Greetings to all

                          I have changed 3 820uf capacitors and 3A fuse and now the CCFL lamps start for a second and they turn off again. After two seconds they startup again for a second and turn off again. I checked 13V line and it reads 18V and just as the lamps turn on for a second voltage drops to 13V and comes back to 18V when the lamps turn off. What can be the problem?
                          First of all, welcome to Badcaps.Net
                          Post a picture of your board to see if we can see something suspicious. You might have more bad components in there. Post also brand/series/ V rating and capacitance rating of remaining caps. That would help you to get the info you need.
                          There are 10 kind of people in this world: those that understand binary, and those who don't.
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                            #94
                            Re: Problems with Samsung 740N monitors

                            740N use 13V supply. your feedback regulation is screwed up. Change more capacitors in the rest of PSU and see.

                            Cheers, Wizard

                            Comment


                              #95
                              Re: Problems with Samsung 740N monitors

                              Originally posted by Wizard
                              740N use 13V supply. your feedback regulation is screwed up. Change more capacitors in the rest of PSU and see.

                              Cheers, Wizard
                              Yep. Checking my notes I found that all the 17/19" LCD units from Samsung that I have checked used 13v. Again, post cap info and/or pics.
                              There are 10 kind of people in this world: those that understand binary, and those who don't.
                              • ASUS ROG Maximus IX Code
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                                #96
                                Re: Problems with Samsung 740N monitors

                                You guys here are quick. Didn't expect a reply so soon.

                                I replaced all caps again and no luck. I put in 3 CapXon 25v 1000uf. I did that on my friends monitor and his is working like a charm. Mine just wont stay on. Itried 2 1000uf + 1 470uf and the problem remains.

                                1. I turn the monitor on,
                                2. Big green transformer starts to make hissing sound and 13V line reads 18,3V
                                3. Lamps go on for a split second, 13V line reads 10-13V
                                4. Big green transformer stops making noise and lamps turn off 13V line reads 18,3V again
                                5. After two seconds transformer starts making hissing noise and we are back at step 2. and so it goes round and round.

                                And i tried to messure voltage on the big capacitor (450V) and it reads 750V!! If the capacitor is rated at Max 450V how can it read 750V?
                                Can this be a faulty transformer or is it something else?

                                Here is the pic you requested
                                Attached Files

                                Comment


                                  #97
                                  Re: Problems with Samsung 740N monitors

                                  Do not power up again, you will kill the HV section on the inverter! Time to disconnect the link to the lamp inverter so you can keep troubleshooting the feedback section.

                                  This feedback usually tap the important voltage which is 5V on this board, I do see a microamp regulator feedback (is that a TL431 in there above the optisolator IC?). Measure that 3 legged device and one of pin should be 2.5V if not, either resistor network or that device is bad. Ohm all the resistors.

                                  Are you using low ESR with high current ripple capacitors?

                                  Remember, always measure: cold side voltages with cold ground or hot side voltages with hot ground, not cross each other when where to put DMM probe on the particular ground plane in this SMPS.

                                  Cheers, Wizard

                                  Comment


                                    #98
                                    Re: Problems with Samsung 740N monitors

                                    Still no luck,
                                    i detached the 3A fuse so the inverter wouldnot startup a did everything you said Wizard. All resistors check out, and yes, on tl431 there is 2.49V. I am using low ESR capacitors and i still dont have a clue what is wrong. Do u have some other advice i could try? I would hate to throw away this monitor for a problem that maybe cost a few bucks.

                                    I noticed that voltage dropped to 16.30V when i removed the 3A fuse. Is this information helpful?

                                    Comment


                                      #99
                                      Re: Problems with Samsung 740N monitors

                                      Still not good. What is your reading on 5V while other 13V is at 16V?

                                      Cheers, Wizard

                                      Comment


                                        Re: Problems with Samsung 740N monitors

                                        I was busy eating, now I can think and talk,
                                        Keep fuse disconnected for now.

                                        while checking the 5V as I asked, ohm the rest of resistors have you? Some of the circuit better unsolder one end of resistor and recheck. I know some places is SMD resistors makes it bit tricky. What about the small capacitor in the hot side of SMPS?
                                        This filters the hot side DC supply for the SMPS IC.

                                        Also I check stuff in the hot side resistors as well.

                                        Remember to discharge the main capacitor with piece of 100K resistor after power cord is unplugged for your safety if working/troubleshooting this.

                                        Cheers, Wizard

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