Bipolar capacitor for vintage Apple CRT monitor

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  • tony359
    Badcaps Veteran
    • Oct 2018
    • 259
    • United Kingdom

    #1

    Bipolar capacitor for vintage Apple CRT monitor

    Good day all,

    I have a question about some old monochrome Apple monitors.

    Inside those monitors there is one bipolar capacitor. From the schematics I have it's part of the horizontal linearity circuit.

    A friend replaced a bipolar 4.7uF 25V capacitor with a Nichicon bipolar 4.7uF 25V. The cap popped after about one hour. The new capacitor was much smaller than the original one but that's not a surprise as the original one was from 1980. (I am not sure whether the original was was actually 4uF and was replaced with a 4.7uF?)

    The new capacitor: https://www.digikey.co.uk/en/product...4R7MDD/2539572

    The general consensus is now to use FILM capacitors - mostly because those Bipolar electrolytic are not easy to find anymore.

    My question is: why would a good quality BP cap pop in this scenario?

    Thank you!
    Click image for larger version

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  • stj
    Great Sage 齊天大聖
    • Dec 2009
    • 30997
    • Albion

    #2
    thats a high current circuit and modern bi-polar caps arent.
    use a film cap.
    it will be bigger so you may need to get creative, but all the widescreen tv's before CRT went out of fashion used film caps for this reason.

    Comment

    • tony359
      Badcaps Veteran
      • Oct 2018
      • 259
      • United Kingdom

      #3
      thanks, appreciated! I'm surprised my own Apple //e has survived all these years with a regular BP cap then. Yes, I am aware the film cap will need to be adapted a bit, no problem.

      How would current affect a capacitor? I am honestly asking, I do not know. I know that temp and voltage would affect a cap but what happens when current is involved? Where do I find that value in the datasheet? https://www.mouser.co.uk/datasheet/2...vp-3082211.pdf

      Comment

      • stj
        Great Sage 齊天大聖
        • Dec 2009
        • 30997
        • Albion

        #4
        a capacitor has a maximum current it can hold or pass through, if you try to exceed that it will get hot real fast and the liquid will boil and burst the vent open.
        if your unlucky and it doesnt have a vent then the can is going into orbit and you get bits of brown paper everywhere!

        Comment

        • stj
          Great Sage 齊天大聖
          • Dec 2009
          • 30997
          • Albion

          #5
          current is shown in that datasheet as "rated ripple"
          so 34mA is a joke

          Comment

          • tony359
            Badcaps Veteran
            • Oct 2018
            • 259
            • United Kingdom

            #6
            Thank you! I suspect the BP I installed in my monitor is getting hot them. It's rated 105C instead of 85C so that might be why it hasn't exploded yet. I'll check with a thermal camera and then replace it. Thank you again!

            Comment

            • R_J
              Badcaps Legend
              • Jun 2012
              • 9535
              • Canada

              #7
              Was the original capacitor bad? If it was not bad as in open, just reinstall the original

              Comment

              • tony359
                Badcaps Veteran
                • Oct 2018
                • 259
                • United Kingdom

                #8
                That is always an option of course - and don't worry, I am not a "let's recap for the sake of recapping" person

                Comment

                • harp
                  Badcaps Veteran
                  • Jun 2022
                  • 598
                  • Planet Earth

                  #9
                  Maybe to use two oposite electrolitic caps (in series - + + -) as one bipolar...
                  Pay attention for final capacitance and voltage.

                  Comment

                  • R_J
                    Badcaps Legend
                    • Jun 2012
                    • 9535
                    • Canada

                    #10
                    You could try NTE HD4.7M25 I still have one Daewoo 4.7µf/35v BP RHD series Ripple current is between 5 and 6 amps at 15.75 KHz
                    Attached Files
                    Last edited by R_J; 08-13-2024, 07:48 PM.

                    Comment

                    • tony359
                      Badcaps Veteran
                      • Oct 2018
                      • 259
                      • United Kingdom

                      #11
                      Thanks R_J

                      I see the massive current ripple for those indeed.

                      harp thanks - if the film cap is a suitable option, I'm happy to go with that!

                      But I will have to check the BP I installed in my //e monitor ages ago. I suspect it gets VERY hot and the only reason why it didn't blow up is because it's a 105C cap, so probably more resilient.

                      Comment

                      • stj
                        Great Sage 齊天大聖
                        • Dec 2009
                        • 30997
                        • Albion

                        #12
                        while your in it,
                        my experience with crt monitors is that the higher the voltage the sooner it fails.
                        so if you have any 160-250v small caps like 10-47uf in there you should check them.

                        Comment

                        • tony359
                          Badcaps Veteran
                          • Oct 2018
                          • 259
                          • United Kingdom

                          #13
                          Thanks - I have checked them all and none is bad, at least for my cheap MTester. The 100uF ones are the "worst" ones but still within specs.
                          Thanks for the hint!

                          Comment

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