Problems with Samsung 740N monitors

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  • selldoor
    Slow Learner
    • Dec 2010
    • 7870

    #421
    Re: Problems with Samsung 740N monitors

    You found the little green fuse under the the left hand side halfway down under the glue between the heatsink and the capacitor.? If it is back in the set what is the voltage on each side with respect to a ground screw. If it is out of the set can we have a picture of the back. Also can you test the secondary windings on the inverter transformers.
    Meter on Ohms 2000 give answer in Ohms
    Please upload pictures using attachment function when ask for help on the repair
    http://www.badcaps.net/forum/showthread.php?t=39740

    Comment

    • mawais15
      New Member
      • Aug 2014
      • 7
      • Pakistan

      #422
      Re: Problems with Samsung 740N monitors

      Yes I have checked the fuses, both near the heats ink and near the power plug (cylindrical one) both are short. I could not understand where to measure the voltage on board. Cantilever please highlight it
      Attached Files

      Comment

      • selldoor
        Slow Learner
        • Dec 2010
        • 7870

        #423
        Re: Problems with Samsung 740N monitors

        OK I have marked for testing S1 to S2 and S3 to S4 But as you are getting NO flash I am more concerned with other items.
        In the pink Square that looks a bit burnt on the picture - is it ok
        In the Yellow squares the items are not soldered???
        In the Red Squares check that these are not bridged
        Attached Files
        Please upload pictures using attachment function when ask for help on the repair
        http://www.badcaps.net/forum/showthread.php?t=39740

        Comment

        • mawais15
          New Member
          • Aug 2014
          • 7
          • Pakistan

          #424
          Re: Problems with Samsung 740N monitors

          Originally posted by selldoor
          OK I have marked for testing S1 to S2 and S3 to S4 But as you are getting NO flash I am more concerned with other items.
          In the pink Square that looks a bit burnt on the picture - is it ok
          In the Yellow squares the items are not soldered???
          In the Red Squares check that these are not bridged
          The voltages between s1 and s2 as well s3 and s4 are zero when testes the board alone. The pink 8 pin component is 4502C and it looks in bad condition, I'm not sure if it's fine ( how do I check it). Those capacitors in yellows squares are soldered. In one of the red square that seems bridged is the fuse. And the 2nd red square is not bridge.

          Comment

          • selldoor
            Slow Learner
            • Dec 2010
            • 7870

            #425
            Re: Problems with Samsung 740N monitors

            OK you were not checking VOLTAGE --it is 2000 Volt
            s on those joints!!!!!
            NO POWER SWITCHED ON
            You are checking resistance - Meter on OHMS 2000.
            Please upload pictures using attachment function when ask for help on the repair
            http://www.badcaps.net/forum/showthread.php?t=39740

            Comment

            • mawais15
              New Member
              • Aug 2014
              • 7
              • Pakistan

              #426
              Re: Problems with Samsung 740N monitors

              Sorry I should have measured resistance across secondary windings, the values are 1335 and 1360 ohms across secondary windings

              Comment

              • selldoor
                Slow Learner
                • Dec 2010
                • 7870

                #427
                Re: Problems with Samsung 740N monitors

                They are ok then
                4502C




                If any reading is less than 30 ohms you might have shorted mosfet. Remove from circuit and repeat the tests to verify.

                Some mosfets are more than 3 pins. To test those, identify the part number and search for its datasheet. Once you find the datasheet, the pins will be designated source (S), gate (G), and drain (D). It will probably be documented as S1, S2, G1, G2, D1, D2.

                Simply test

                a) black on pin S1- red on pin G1 - record ohms
                b) black on pin S1- red on pin D1 - record ohms
                c) black on pin G1- red on pin D1 - record ohms

                Repeat for the "2" pins. That is S2-G2, S2-D2, G2-D2.

                Note: a shorted mosfet would likely cause a very brief flash of the backlight or no backlight.
                Please upload pictures using attachment function when ask for help on the repair
                http://www.badcaps.net/forum/showthread.php?t=39740

                Comment

                • mawais15
                  New Member
                  • Aug 2014
                  • 7
                  • Pakistan

                  #428
                  Re: Problems with Samsung 740N monitors

                  Originally posted by selldoor
                  They are ok then
                  4502C




                  If any reading is less than 30 ohms you might have shorted mosfet. Remove from circuit and repeat the tests to verify.

                  Some mosfets are more than 3 pins. To test those, identify the part number and search for its datasheet. Once you find the datasheet, the pins will be designated source (S), gate (G), and drain (D). It will probably be documented as S1, S2, G1, G2, D1, D2.

                  Simply test

                  a) black on pin S1- red on pin G1 - record ohms
                  b) black on pin S1- red on pin D1 - record ohms
                  c) black on pin G1- red on pin D1 - record ohms

                  Repeat for the "2" pins. That is S2-G2, S2-D2, G2-D2.

                  Note: a shorted mosfet would likely cause a very brief flash of the backlight or no backlight.
                  Thanks for your help :-) . I will check the mosfets once I find the datasheet.
                  One more thing I've noticed is the voltage across the DC pins of bridge rectifier is above 300V. Is this OK?

                  Comment

                  • selldoor
                    Slow Learner
                    • Dec 2010
                    • 7870

                    #429
                    Re: Problems with Samsung 740N monitors

                    Datasheet is at top of my post
                    DC voltage at Bridge should be 1.41 x your mains AC voltage - you can also check that at the BIG CAP - carefully test to see if it holds steady for 7-10 seconds.
                    Please upload pictures using attachment function when ask for help on the repair
                    http://www.badcaps.net/forum/showthread.php?t=39740

                    Comment

                    • mawais15
                      New Member
                      • Aug 2014
                      • 7
                      • Pakistan

                      #430
                      Re: Problems with Samsung 740N monitors

                      I just read the part number on mosfet and it's 4502C AA5NP if my eyes caught it right. My main AC is 240V that means bridge rectifier is ok same voltage is across big Capacitors around 328V. NOW I'll be checking mosfet .

                      Comment

                      • RJARRRPCGP
                        Badcaps Legend
                        • Jul 2004
                        • 6301
                        • USA

                        #431
                        Re: Problems with Samsung 740N monitors

                        Originally posted by kc8adu
                        those are samxon gl.
                        known junk.
                        they are a replace on sight part.
                        I dunno if even GLs are that bad. (And possibly the same for GF)

                        But, definitely real worried of the KMs!

                        There was a report of KMs being real bad, even without bulging. At least many, if not most caps, usually bulge or leak at the bottom when bad.
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                        Comment

                        • Pentium4
                          CapXon Be Gone
                          • Sep 2011
                          • 3741
                          • USA

                          #432
                          Re: Problems with Samsung 740N monitors

                          GF are bad, KM are mediocre

                          Comment

                          • Crauler
                            New Member
                            • Aug 2011
                            • 7

                            #433
                            Re: Problems with Samsung 740N monitors

                            I just replaced all these caps, even those that are not inflated and my monitor "Samsung 740N D" works!
                            Attached Files

                            Comment

                            • Behemot
                              Badcaps Legend
                              • Dec 2009
                              • 4845
                              • CZ

                              #434
                              Re: Problems with Samsung 740N monitors

                              Are those Panasonic or Hermei?
                              Less jewellery, more gold into electrotech industry! Half of the computer problems is caused by bad contacts

                              Exclusive caps, meters and more!
                              Hardware Insights - power supply reviews and more!

                              Comment

                              • me.just88
                                Member
                                • Mar 2013
                                • 11
                                • Romania

                                #435
                                Re: Problems with Samsung 740N monitors

                                hi ,i'm new to this forum.

                                I also have the same problem like "ethernet","lit","selldor"
                                I chenged all the caps and fuse,after that the monitor is restarting.
                                has anyone solve that problem?

                                Comment

                                • Behemot
                                  Badcaps Legend
                                  • Dec 2009
                                  • 4845
                                  • CZ

                                  #436
                                  Re: Problems with Samsung 740N monitors

                                  Have you checked inverter transistors and transformers?
                                  Less jewellery, more gold into electrotech industry! Half of the computer problems is caused by bad contacts

                                  Exclusive caps, meters and more!
                                  Hardware Insights - power supply reviews and more!

                                  Comment

                                  • Gemat
                                    Member
                                    • Nov 2014
                                    • 23
                                    • Ελλάδα

                                    #437
                                    Re: Problems with Samsung 740N monitors

                                    Hi,

                                    I finaly managed to fix my 740 Samsung with the PSU like the one pictured in the post above!

                                    I had to replace the three 820uF capacitors, the 3amp micro fuse and one of the 8 pin ICs on the solder side of the board.

                                    Thanks for all the input in the above posts...

                                    Comment

                                    • Behemot
                                      Badcaps Legend
                                      • Dec 2009
                                      • 4845
                                      • CZ

                                      #438
                                      Re: Problems with Samsung 740N monitors

                                      That's one of those transistor IIRC.
                                      Less jewellery, more gold into electrotech industry! Half of the computer problems is caused by bad contacts

                                      Exclusive caps, meters and more!
                                      Hardware Insights - power supply reviews and more!

                                      Comment

                                      • Gemat
                                        Member
                                        • Nov 2014
                                        • 23
                                        • Ελλάδα

                                        #439
                                        Re: Problems with Samsung 740N monitors

                                        Originally posted by Behemot
                                        That's one of those transistor IIRC.
                                        Its a surface mount mosfet (AF4502C). I replaced it with a SP8M3.

                                        It also works with 4620 (I temporarily stole one from another working PSU)
                                        Last edited by Gemat; 11-06-2014, 12:37 PM.

                                        Comment

                                        • selldoor
                                          Slow Learner
                                          • Dec 2010
                                          • 7870

                                          #440
                                          Re: Problems with Samsung 740N monitors

                                          I think we usually suggest replacing both mosfets as a pair
                                          Please upload pictures using attachment function when ask for help on the repair
                                          http://www.badcaps.net/forum/showthread.php?t=39740

                                          Comment

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