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    Main filter capacitor substitute.

    I'm repairing a power supply and the main filter cap is 400v 33uF.
    I have Rubycon YK 400V 47uF, can I substitute it or will it cause issues long term ?

    (Yes I know the PCB is crusty looking, it's had water damage.)
    Attached Files
    Last edited by diif; 12-28-2019, 05:46 PM.

    #2
    Re: Main filter capacitor substitute.

    ' 85c general purpose.
    How is that cap connected in the circuit? Is it connected to the bridge rectifier from the AC 50Hz (100 Hz ripple freq.)?
    Is the old one 105c?
    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


      #3
      Re: Main filter capacitor substitute.

      Should be absolutely fine.

      Going a notch or two up in capacitance for the main filter is never a problem.
      Heck, going even 2-3x the capacity may not be a problem... until we start talking big caps replacing big caps (i.e. over 500 uF).

      The worst that can happen if you use a main filter cap that is too big, is increase the inrush current to the point where it either blows the fuse or the thermistor.

      Comment


        #4
        Re: Main filter capacitor substitute.

        Front and back photos and one of the output caps. The small one is 470uF 25v.

        Ahh, hadn't noticed it's an 85c cap budm, unfortunately the one it's replacing has temporarily disappeared off my work bench.

        Thanks momaka, whilst I can generally diagnose and repair and have a rough idea of how they work I do not know the finer details and what thus what can be substituted.

        I have found a Rubycon WXA 68uF, perhaps that's a better replacement ?
        Attached Files

        Comment


          #5
          Re: Main filter capacitor substitute.

          Just a Small power supply filtering the main DC from rectifier, it should be fine for now, with bigger capacitance you just have to watch out for in-rush current that may blow that fuse..
          Last edited by budm; 12-28-2019, 09:58 PM.
          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


            #6
            Re: Main filter capacitor substitute.

            Thank you both. Yes, it is a 105c budm.

            What is the purpose of R1 and R2 ? They are 200k 5% with R1 connecting to the positive of the main capacitor, just visible bottom right in my first photo.

            Comment


              #7
              Re: Main filter capacitor substitute.

              Originally posted by diif View Post
              What is the purpose of R1 and R2 ? They are 200k 5% with R1 connecting to the positive of the main capacitor, just visible bottom right in my first photo.
              Looks like Gate biasing/supply for the main transistor (Q3). I don't see any ICs on the primary side, so this appears to be a 2-transistor self-oscillating power supply (even though this board appears to have 3 transistors on the primary total... but whatever. )

              Basically, when the adapter is plugged in the wall, R1 and R2 turn on Q3. This causes current to pass through the primary winding on the transformer. As this current raises, it also passes through that 0.68-Ohm resistor. Thus, a voltage drop appears across the 0.68-Ohm resistor, which is then used to turn On Q1 and/or Q2 (haven't analyzed the circuit completely there yet, so can't say which one does exactly what, but one of them does for sure.). With Q1/Q2 turned On, Q3's Gate is then shunted to ground and Q2 turned it Off. Since this is flyback topology, as soon as Q3 turns off, the magnetic field in the transformer collapses, causing a current pulse to appear on the secondary side winding and so you get voltage on the output. Once Q3 is turned Off, Q1 and Q2 will eventually turn off as well, causing Q3 to turn On again. And then the whole cycle repeats again - hence the term "self-oscillating".

              Note: all of that above is when the PSU is first plugged in and output voltage(s) still coming up. Once the output voltage(s) start going a little beyond what they are set to, this is where the optocoupler kicks in - i.e. it is used to turn On/Off transistors Q1/Q2 as needed in order to maintain the output voltage(s) at their desired levels.

              I hope any of that makes sense somewhat.
              Last edited by momaka; 12-29-2019, 10:51 PM.

              Comment


                #8
                Re: Main filter capacitor substitute.

                Thank you very much, yes it makes sense and I understand it a bit better. Q1 is an A1015 and Q2 is a C1815.
                Am I right in thinking the output voltage(s) are set by the 1% resistors connected to the transistor and opto ?

                The output voltages were both off by about the same multiplier and after cleaning and measuring most of the resistors, I found R1 reading very high.

                Proper main cap ordered along with the resistor.

                Comment


                  #9
                  Re: Main filter capacitor substitute.

                  Take note that spacing in the primary section is very bad. Pass some insulating laquer, it will also protect the exposed copper areas.

                  Comment


                    #10
                    Re: Main filter capacitor substitute.

                    very good sir

                    Comment


                      #11
                      Re: Main filter capacitor substitute.

                      Originally posted by Hitto View Post
                      Take note that spacing in the primary section is very bad. Pass some insulating laquer, it will also protect the exposed copper areas.
                      Thank you, yes, I was going to coat the bottom in transformer varnish once the repair was complete and tested.

                      Comment


                        #12
                        Re: Main filter capacitor substitute.

                        Originally posted by diif View Post
                        Am I right in thinking the output voltage(s) are set by the 1% resistors connected to the transistor and opto ?
                        You mean IC2 and the opto.
                        IC2 should be a 431 shunt regulator. This, and probably the resistors around it are what sets the output voltage. Generally, there is a voltage divider between the output and ground, and the voltage divider feeds into the 2.5V reference pin of the 431 shunt. By changing the resistance of the lower resistor in the divider (the one connected to ground), you can vary the output voltage. But don't go too high, as the PSU likely won't regulate properly with a load anymore.

                        Originally posted by diif View Post
                        The output voltages were both off by about the same multiplier and after cleaning and measuring most of the resistors, I found R1 reading very high.
                        Unfortunately, I don't have an explanation of why that happens to high-valuer resistors connected to the high voltage side... but I've seen it before: they sometimes go either very high resistance or completely open-circuit without seeming to have been overloaded.

                        If R1 and R2 are open, the PSU won't work. So good thing you found that.

                        Comment


                          #13
                          Re: Main filter capacitor substitute.

                          Thank you again momaka, yes I did mean IC2. I have no intention of changing any values to increase the output, I was just confirming my understanding of it's workings.

                          This thing has had a hard life, it's inside a humidifier and there was quite a bit of corrosion on the legs of most of the through hole components. I don't guess it takes much to seep inside ?

                          Comment


                            #14
                            Re: Main filter capacitor substitute.

                            The resistor and caps turned up this morning, the humidifier is now working. I coated the bottom with some thinned down transformer resin, I have a very happy friend and I'm a little more knowledgable.
                            Thank you all.

                            Comment


                              #15
                              Re: Main filter capacitor substitute.

                              Nice! Glad to hear everything is working out well.

                              Originally posted by diif
                              This thing has had a hard life, it's inside a humidifier and there was quite a bit of corrosion on the legs of most of the through hole components. I don't guess it takes much to seep inside ?
                              Yes, water will always find a way in through cracks and crevices.
                              I imagine the PSU is -not- enclosed in a tight water-proof box. If it was, I don't think there would have been any corrosion.

                              Comment


                                #16
                                Re: Main filter capacitor substitute.

                                No waterproof box, it's mounted on the underside of the plastic but right above the air vent for the fan.

                                Comment

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