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Magnavox DRV power supply SMPS

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    #21
    Re: Magnavox DRV power supply SMPS

    Must be really bad lamp for it to blow the filament that is made to handle 120V 60W.
    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


      #22
      Re: Magnavox DRV power supply SMPS

      Originally posted by johnfin View Post
      in that older schematic attached in this thread under the problem flow chart it says if the fuse is blown check the "primary component". Like what would that be?
      Rectifiers, MOSFET, Transistor, Diodes, caps, etc.
      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


        #23
        Re: Magnavox DRV power supply SMPS

        So the light bulb is on. Will this allow me to check voltages?

        Comment


          #24
          Re: Magnavox DRV power supply SMPS

          Is it on full bright or dimly lit? If it is on full bright then it means you still have shorts circuit.
          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


            #25
            Re: Magnavox DRV power supply SMPS

            I started out looking for voltage drops while plugged in and the light is on but I guess that's not the best way to do it. Can someone explain to me (limited electronics tech experience) the best way to look for a short in a circuit like I have. Thanks

            Comment


              #26
              Re: Magnavox DRV power supply SMPS

              Originally posted by johnfin View Post
              Can someone explain to me (limited electronics tech experience) the best way to look for a short in a circuit like I have. Thanks
              Take your multimeter and set it to the lowest resistance scale (if it's a manual-ranging multimeter). Then start testing components.

              Diodes and MOSFETs tend to go short-circuit / low resistance when they go bad. For MOSFETs, first look for a short-circuit / low resistance between Gate and Drain or Gate and Source. If that passes, look for short-circuit between Drain and Source.

              Bipolar transistors (BJTs) usually tend to go short-circuit / low resistance between Base-Emitter or all three pins, but can sometimes also go open-circuit. Use diode-test on your multimeter to test the transistor.
              For NPN -type transistors, you should get a diode reading (400-800 mV or 0.4-0.8V, depending on your meter) between Base-Emitter and Base-Collector pins, with the red multimeter probe connected to Base, and black multimeter probe connected to either Emitter or Collector pins.
              For PNP -type transistors, you should get a diode reading (400-800 mV or 0.4-0.8V, depending on your meter) between Base-Emitter and Base-Collector pins, with the *black* multimeter probe connected to Base, and *red* multimeter probe connected to either Emitter or Collector pins.

              Resistors like to go open-circuit or high-resistance when they go bad. If a resistor is showing lower resistance in circuit than what it actually is, then it's *probably* okay.

              Inductors and transformers, especially ones used in SMPS, will show short-circuit / low resistance when measured both in and out of circuit. This is *normal*. That said, inductors and transformers *almost* never go bad (save for inverter transformers on LCDs and flybacks in CRTs). Thus, don't waste your time removing any transformers and inductors to check them. 99.99% of the time, they won't be the issue.

              Electrolytic capacitors you cannot test in circuit, but if there is a short-circuit or low resistance across one (very rare), check if there are any low-resistance resistor connected it accross it. If none, you should remove the electrolytic cap from the circuit board and test it for short-circuit with the multimeter. If there is no short-circuit, also check it with an ESR meter (if you have one) to verify ESR and capacitance.

              That said, I typically define short-circuit as a resistance of 10 Ohms or less. A *possible* short-circuit could be up to 30 Ohms and in rare cases, up to 50 Ohms.

              Before starting to measure resistance with your multimeter, first touch both probes together and see what resistance you get. It should be less than 1 Ohms ideally, though cheap multimeter can sometimes show up to 3 Ohms with probes touching. This test shows you the lowest resistance your multimeter can measure. When you get that resistance across some component, that means you probably have a short-circuit. But again, for inductors and transformers, this is normal.

              Another suggestion: DO NOT try testing the power supply with the transformer removed, as it will tell you nothing. You need everything to be connected when you apply power.

              And continue using the incandescent bulb in place of the fuse until you get the power supply fixed. Otherwise, you will just keep blowing up parts every time and not certainly knowing what blew up.

              Comment


                #27
                Re: Magnavox DRV power supply SMPS

                ^ Or blowing parts that were otherwise OK. Say, gate drivers ICs.
                Things I've fixed: anything from semis to crappy Chinese $2 radios, and now an IoT Dildo....

                "Dude, this is Wyoming, i hopped on and sent 'er. No fucking around." -- Me

                Excuse me while i do something dangerous


                You must have a sad, sad boring life if you hate on people harmlessly enjoying life with an animal costume.

                Sometimes you need to break shit to fix it.... Thats why my lawnmower doesn't have a deadman switch or engine brake anymore

                Follow the white rabbit.

                Comment


                  #28
                  Re: Magnavox DRV power supply SMPS

                  Should i check all of those tiny parts on the bottom of the board?

                  Comment


                    #29
                    Re: Magnavox DRV power supply SMPS

                    Originally posted by johnfin View Post
                    Should i check all of those tiny parts on the bottom of the board?
                    It wouldn't be a bad idea.

                    But I suggest starting with the main switching transistor on the primary side first (the one attached to the heatsink) and also everything directly connected to it. The fact that your light bulb stays lit means either that transistor is faulty or whatever is driving it is faulty, making it stuck fully turned on all the time when it shouldn't be (it should be switching at a high frequency.... hence the name switch mode power supply or SMPS for short).

                    If you provide bigger pictures (ones where we can actually see the board designators for all of the parts), we may actually be able to give you more detailed instructions what to check and how to check it.
                    Last edited by momaka; 10-26-2017, 02:33 PM.

                    Comment


                      #30
                      Re: Magnavox DRV power supply SMPS

                      MOSFET is fine. Must be a tiny part on bottom or tranny because I have checked all of the other parts.
                      Attached Files

                      Comment


                        #31
                        Re: Magnavox DRV power supply SMPS

                        Originally posted by johnfin View Post
                        MOSFET is fine. Must be a tiny part on bottom or tranny because I have checked all of the other parts.
                        If the transformer was blown, that board of yours would have looked like a Kingsford briquet.
                        No seriously, it takes a lot of current to blow a transformer. The only time you can blow it is if you either incorrectly replace the fuse with one of much higher current rating or install a jumper in its place. And even then, you're more likely to see the input choke burn up or your house's breaker trip.

                        So you have a fault somewhere else, that is for sure. Now the pictures you provided are still way too small for me to be able to trace anything, so I can't tell you if that 2-transistor oscillator circuit actually has self-limiting circuit to prevent the main transistor from getting stuck ON. Thus, I suggest replacing the optocoupler and 431 shunt regulator on the secondary side if you have some spare ones.

                        Comment


                          #32
                          Re: Magnavox DRV power supply SMPS

                          Funny you should mention that, did it, opti and all 431's. Pulled all caps and put them on an esr and sprague tel-ohm. Both the mosfet and NPN are fine. Next to the diodes again. Been down this road once and again the last part left is the tranny.

                          Comment


                            #33
                            Good news

                            Possible good news. I think I found the bad part. Fingers crossed. It was one of the last transistors in the primary side. to-92 case. C3032. It tested ok before with a b&k transistor checker but I dont trust the machine now. I used the multimeter and he transistor is no good.

                            Comment


                              #34
                              Re: Magnavox DRV power supply SMPS

                              Well, no luck, the new transistor did not work. Bulb still bright. It either the tranny or the small parts on the bottom.

                              Comment


                                #35
                                Re: Magnavox DRV power supply SMPS

                                Originally posted by johnfin View Post
                                Well, no luck, the new transistor did not work. Bulb still bright. It either the tranny or the small parts on the bottom.
                                Well, you only have like 15 SMD parts on the bottom side, and most of them appear to be resistors with a few diodes and ceramic caps, so that shouldn't be too hard to test. Ceramic caps and diodes you test for short-circuit (bad) and resistor test for open-circuit or abnormally high resistance (may have to remove some of them for the measurement, though).

                                Again, I don't think it'd be the transformer. But hey, at this point, you might as well take it out again and actually disassemble it. Just peel the tape around it first. Then separate the two core pieces. Should be either two "E" core pieces glued together or an "E" and an "I" core piece. After that, it's just a matter or removing more tape to expose the wire in the windings.

                                Comment


                                  #36
                                  Re: Magnavox DRV power supply SMPS

                                  Definitely not the transformer at this point. Check the power semiconductors and the PWM controller again.
                                  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


                                    #37
                                    Re: Magnavox DRV power supply SMPS

                                    ^ No PWM controller on this one. It's an oldschool 2-transistor self-oscillator.

                                    Comment


                                      #38
                                      Re: Magnavox DRV power supply SMPS

                                      See post 12 and 15 diagram for the topology being used in this power supply, if the switching control transistor is stuck in the OFF mode then the main power MOSFET will be stuck on.
                                      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


                                        #39
                                        Re: Magnavox DRV power supply SMPS

                                        Its closer to the mdr515 schematic, not exact though. I need some help diagnosing this thing. Can I put a signal tracer on the transistors, and mosfet to see if it is switching? I would like to check it out when the power is on and bulb on. As far as the small parts on the back of the board, some are micro transistors, not just resistors and diodes. I was told not to remove the transformer yet when I did the short went away so how about this, cut some of the output lines on the transformer that dont return to the primary side so I can try to isolate the problem.

                                        Comment


                                          #40
                                          Re: Magnavox DRV power supply SMPS

                                          What resistance do you get by measuring between the two legs of the main filter cap?
                                          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

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