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Yoke Replacement

Discussion in 'Electronic Repair' started by Chris F., Jun 4, 2005.

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  1. Chris F.

    Chris F. Guest

    This must be one of those "really desperate to fix it" projects. A little
    9" color TV/radio combo I picked up in the trash, had a badly burned yoke. I
    was able to isolate and insulate the shorted windings, but disturbing the
    windings threw the convergence way off and the yoke was still as useless as
    ever. Now I just happened to have a good yoke from an identical CRT
    (27GDC85X), however it was designed for a completely different set. I
    installed it anyway, the vertical windings are a perfect match but the
    horizontal ones have too low an impedence - causes high-voltage shutdown
    unless powered with a Variac. I determined that the original yoke H winding
    was about 13.2 ohms, and the replacement yoke H winding is about 3.9. A bold
    idea came to mind; wind an impedence-matching transformer from an old
    flyback core and some magnet wire. I have several pounds of AWG 27 and 38
    magnet wire that I could do this with. The question is: do I need a 1:1
    turns ratio? I suspect I do, this means that the two windings would have the
    same number of turns but use different wire gauges.
    Think this would work? I know it's a lot more trouble than the old set is
    worth, but I'm not exactly busy these days and I hate to junk such a nice
    set. 9"-ers are not abundant in my supply, so it's certainly worth an hour
    or twos work to get this going.
    Thanks for any advice.
     
  2. James Sweet

    James Sweet Guest

    More the the impedance is the inductance. Try winding an inductor and
    placing it in series with the yoke, that may well work.
     
  3. Chris F.

    Chris F. Guest

    I was just thinking the same thing, I will give that a try and see what
    happens.

    Chris
     
  4. Art

    Art Guest

    May consider the value of the yoke ringing caps in the input and return
    circuits??
     
  5. The turns ratio would need to be the square root of the impedance ratio.
    But bear in mind that the DC resistance isn't the same
    as the impedance at the operating frequency. I'd guess 2:1 turns ratio
    would be close (the 2 on the driving side and the 1 on the yoke side.)
    Wire gauge is not critical, use the thickest that fits mechanically.
    However this may not work well, due to losses and disturbing the
    expectations of the driving circuitry.

    However, I suspect there is a _much_ easier solution. Yokes usually have
    two separate coils, connected in either series or parallel. There's a
    good chance you'll find yours is in parallel, in which case if you
    convert it to series you'll end up with a 4x higher impedance and you'll
    be quite close to the original yoke impedance, probably close enough to
    work with minor adjustments.

    Good luck!

    Mike.
     
  6. Doug

    Doug Guest

    What brand and model is this set?
    Doug
     
  7. Chris F.

    Chris F. Guest

    Set fixed! I simply connected the H winding from another yoke in series, and
    everything seems to work great. Now I just have to find a remote that will
    adjust the color/brightness/etc, and the set is ready to use.
    Thanks for your help.
     
  8. Art

    Art Guest

    Congrats Gov.
     
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