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Thread: Non polarity cap questions on CRT monitor repair.

  1. #1
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    Default Non polarity cap questions on CRT monitor repair.

    So I was given an apple II monitor model A2m2010. IT was in pretty rough shape. After a preliminary cleaning and inspection I decided to power the unit up. I did get raster but there was a very high pitch sound. I discharged the CRT and figured it might be a faulty capacitor as I have come across that before. Once I got the board out i noticed in fact a lot of caps had leaked... probably along time ago as there was no "wet" electrolytic instead solid "crusteez" under the caps. I replace the main culprits and the high pitch is almost gone although the image isn't correct yet; I can see images across the screen when I type but they are stretched and illegible.

    Looking on some schematics online i found that one of the caps Id like to replace is a non-polarized electrolytic cap. C420 is a 16uf 25v c420.jpg The strange thing is the cap is huge for such a small increment. I'm just uncertain about which cap to buy to replace this thing with. Could someone shed some info on these non-polarized electrolytic caps?

    Thanks,
    Mick

  2. #2

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    Although, I've never done so, you can make a non-polar capacitor from two placed in series of opposite polarity. I'd guess you'd need to use values of 2X capacitance but of equal voltage. In this case 25V each.
    Dwight

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    Hey Dwight. So if I am understanding you. I can take two 16UF 25V (or are you saying 32uf) caps and connect them together positive to negative leg. Then connect the remaining positive and negative [unconnected] legs to the appropriate through holes and that would work?

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    Since the caps are in series, you need 32uF, and the hookup is - to -, leaving the two + to go into your circuit.

    Personallly, I don't like the arrangement. I'd rather use a single film capacitor (which is nonpolar).

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    I find that sometimes this works. In theory, it always works. But in theory, capacitors are perfect insulators. Like Chuck, I'd opt for a non-polarised capacitor in this (and most every such) case.

    If you do series them, they must be each twice (32uF) the capacitance of the original. This nets a single capacitor with half (16uF) the value of the individual capacitors.

  6. #6

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    What Chuck and Space Commander said.
    Dwight

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    I also use film caps. Nonpolar/bipolar electrolytics don't seem to age well. I keep a variety of largeish film cap values on hand and just parallel them to make whatever I need.

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    Also note that in this arrangement, the ESR is doubled. Not so good.

    A polyester non-polar is inexpensive and will do the job.

    On an aside, in the traditional back-to-back electro setup, the capacitance is 0.5C. I've seen discussions about polar caps with paralleled diodes to give an effective capacitance of 1.0C:



    I suspect that there may be linearity issues with this setup, but I don't see where it would be awful.

  9. #9
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    Thanks Guys this is all very insightful. I am sensing the caps-in-series solution may be best as a last ditch. I suppose I will look for a polyester film alternative. Just to ask, why was the electrolytic cap 5 times the normal size of what a polarized electrolytic cap would be?

  10. #10
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    Just the construction and materials. Imagine what size a 16 uF silver mica cap would be...

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