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    #1

    viewsonic vg720 no power, blown transistor on mainboard

    Hey guys heres another one. Its a viewsonic llcd monitor.

    no power on no led, dead..

    checked 120vac in, ok, checked output of rectifier, ok, check 5 volts to mainboard, ok. no 5V TO SWITCH PANEL.

    all caps look ok, found a small smd transistor fried. Theres a 0 ohm resistor that feeds this, that checks ok.

    How the heck do i figure out what kind of transistor to replace it with? its totally smoked. Im sure i have something here i could rob it from, just want to know whats the proper thing to go in there.
    Attached Files
    Last edited by cadillacman; 05-08-2011, 04:50 PM.
    Tags: None
    #2
    Re: viewsonic vg720 no power, blown transistor on mainboard

    Originally posted by cadillacman View Post
    checked 120vac in, ok, checked output of rectifier, ok, check 5 volts to mainboard, ok. no 5V TO SWITCH PANEL.
    Post a picture of the power board. It could be the small cap and/or SMPS controller that is bad.
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      #3
      Re: viewsonic vg720 no power, blown transistor on mainboard

      Okay, please supply the following info:

      1) the "transistor" that's burned is silkscreened U406? If yes, the "U" designator indicates that it's not a transistor.

      2) you're getting the 5V output. What about the 12V output? Usually, there are two diode packs located on the same heatsink. Measure the voltage output on the middle leg of each device. One should be 5V and the other 12-14V.

      3) does the power led light up at all on the front panel?

      Comment

        #4
        Re: viewsonic vg720 no power, blown transistor on mainboard

        #1: i dont know. Its burned pretty badly. I can see a "N H(? looks like half an h the other half is fried too bad to read) Blank and a 4
        so basically NH*4 its location is silkscreened as "u406" on the board.

        #2 i have 5.ov and 14.4v at the diode packs so yep theres my 12v.

        #3 no the indicator led does not light under any circumstance. No backlight flash, nothing. From the outside it appears 100% dead.


        checking power to the little smd thingy( please forgive me here smd stuff is new to me)

        i have 0v on the left pin, 5.0 on the center top pin, and 4.2v at the far right pin.

        school me: f is fuse.. c is capacitor. t is transistor. d is diode. what the heck is "u" ?
        Last edited by cadillacman; 05-08-2011, 07:20 PM.

        Comment

          #5
          Re: viewsonic vg720 no power, blown transistor on mainboard

          here wo go.. picture crazy. Sorry guys i thought i posted most of these the first time but apparently i did not.
          Attached Files

          Comment

            #6
            Re: viewsonic vg720 no power, blown transistor on mainboard

            U406 is probably a MAX810 reset controller that holds the scaler chip in a reset mode until the power supply comes up to proper voltage.

            1) verify that the filter caps around the two diode packs you just checked are not bulged. If yes, replace them.

            2) If you are game, piece everything back together. Connect the monitor to AC power. With a screwdriver, short the bottom two legs together. Press the on/off button on the front panel. See if you get any activity on the power led or screen. If not, try pressing the on/off button a few more times.

            3) Some people have reported that removing this chip allows the monitor to work. However, leaving the reset pin of the scaler chip "floating" is probably not the greatest idea. Best if you replace U406, if you have success with step 2.
            Last edited by jetadm123; 05-08-2011, 07:25 PM.

            Comment

              #7
              Re: viewsonic vg720 no power, blown transistor on mainboard

              ok will do..


              EDIT: you are the man!!!!!

              removed it, it powers up perfectly. powers down perfectly. Im kind of tempted to leave it the way it is.

              when i search for that part, i get this screen:
              http://search.digikey.com/scripts/Dk...X810&x=20&y=15

              too many what ifs there. Which one do you think i need?
              Last edited by cadillacman; 05-08-2011, 07:35 PM.

              Comment

                #8
                Re: viewsonic vg720 no power, blown transistor on mainboard

                Whoops. Misread the 1st post. I thought there was no 5V at all. Now I see 5V is there.
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                We respectfully ask that you make some time and effort to read some of the guides available for basic troubleshooting. After you have read through them, then ask clarification questions or report your findings.

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                Comment

                  #9
                  Re: viewsonic vg720 no power, blown transistor on mainboard

                  The original recommended digkey part was: MAX810MTRGOSCT-ND, which is out of stock.
                  If you type in max810m in the search window, you get several hits including the above, but made by a different company. Most look the same with the exception of different case styles.

                  Comment

                    #10
                    Re: viewsonic vg720 no power, blown transistor on mainboard

                    ok i will start hunting it down. If i cant find one mabye i will keep this as a personal monitor. I would hate to sell this to someone and have it set thier house on fire because i removed a part out of it.

                    Comment

                      #11
                      Re: viewsonic vg720 no power, blown transistor on mainboard

                      digikey came up with this:

                      http://search.digikey.com/scripts/Dk...10MVNB713CT-ND


                      what do ya think?

                      Comment

                        #12
                        Re: viewsonic vg720 no power, blown transistor on mainboard

                        Did a little research and the original viewsonic part was:

                        ASM810MEURF-T where T is a reset threshold voltage of 3.08V

                        Maybe digikey TCM810TVNB713CT-ND ?

                        Comment

                          #13
                          Re: viewsonic vg720 no power, blown transistor on mainboard

                          honestly i think im going to leave it the way it is now.

                          Theres a good chance i could get the wrong part and make things much worse. Its working fine now, i figure if it isnt broken dont fix it right? Its been running on my pc for 2 days now and all is happy from power up/power down and even going to sleep more and waking back up.. working fine without that piece in there. so yeah i think im happy with it the way it is.

                          Comment

                            #14
                            Re: viewsonic vg720 no power, blown transistor on mainboard

                            Another failure of those crappy reset thingies. It seems they really are designed to kill stuff rather than protect it. If it works, don't fix it.
                            Originally posted by PeteS in CA
                            Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                            A working TV? How boring!

                            Comment

                              #15
                              Re: viewsonic vg720 no power, blown transistor on mainboard

                              Originally posted by Th3_uN1Qu3 View Post
                              Another failure of those crappy reset thingies. It seems they really are designed to kill stuff rather than protect it. If it works, don't fix it.
                              I think you are mistaken. This is most likely an engineering problem. While a full datasheet is not available, in some cases the processor has an internal reset controller. The reset controller is unnecessary, and we are seeing the result of two outputs trying to drive one another. The external reset controller is being destroyed by the one in the processor!!

                              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

                                #16
                                Re: viewsonic vg720 no power, blown transistor on mainboard

                                It's been enough fails already for it to simply be an "engineering problem". But, in the age where everyone copies everyone else without even bothering to take a good look at the design they are copying, it is still plausible. But i don't believe it - smells too much of planned obsolescence.
                                Originally posted by PeteS in CA
                                Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                                A working TV? How boring!

                                Comment

                                  #17
                                  Re: viewsonic vg720 no power, blown transistor on mainboard

                                  Originally posted by Th3_uN1Qu3 View Post
                                  It's been enough fails already for it to simply be an "engineering problem". But, in the age where everyone copies everyone else without even bothering to take a good look at the design they are copying, it is still plausible. But i don't believe it - smells too much of planned obsolescence.
                                  Don't forget that if 'generation 3' required a reset controller, an engineer may assume that 'generation 4' of the IC would also require it. Problems that don't occur in the first weeks of design are much more likely to make it into production. Design errors that take several years to result in a failure will NEVER be caught, they are now using 'Generation 6 or 7' in their products.

                                  Heinlein's Razor. "Never attribute to malice that which can be adequately explained by stupidity, but don't rule out malice."

                                  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

                                    #18
                                    Re: viewsonic vg720 no power, blown transistor on mainboard

                                    I suspect there was never any need for a reset controller at all.
                                    Originally posted by PeteS in CA
                                    Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                                    A working TV? How boring!

                                    Comment

                                      #19
                                      Re: viewsonic vg720 no power, blown transistor on mainboard

                                      Originally posted by Th3_uN1Qu3 View Post
                                      I suspect there was never any need for a reset controller at all.

                                      That's a pretty bold statement. Are you suggesting some IC designers said "Hey, let's design an IC nobody needs. We will price it at $.73 each. Since nobody will ever need one, we don't have to make sure it works properly."?

                                      In another thread, I've pointed out that a RC reset circuit has a number of problems that a reset controller solves. For example, if the power supply drops slightly, then recovers the RC circuit will not reset the processor. The RC circuit requires either a relatively large capacitor or high value resistor. By it's very nature, the RC reset circuit takes the reset input through an undefined logic state rather slowly. All of these can lead to problems.

                                      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

                                        #20
                                        Re: viewsonic vg720 no power, blown transistor on mainboard

                                        Originally posted by Th3_uN1Qu3 View Post
                                        I suspect there was never any need for a reset controller at all.
                                        Or it's blind luck that the cpu starts without it.
                                        36 Monitors, 3 TVs, 4 Laptops, 1 motherboard, 1 Printer, 1 iMac, 2 hard drive docks and one IP Phone repaired so far....

                                        Comment

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