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Thread: Apple IIc Monitor Tube displaying strange behavior

  1. #21

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    Quote Originally Posted by VERAULT View Post
    Hugo if you have the schematic when you are home, please let me know. Here are some photos of the CRT. What is that strange outer coil? An inductor? Attachment 54717Attachment 54718Attachment 54719
    Most likely, by the look of it, that inductor has an adjustable core and is a width coil that will be wired in series with the horizontal coils, but to be sure I would have to look at the schematic again.

  2. #22
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    Quote Originally Posted by Hugo Holden View Post
    Most likely, by the look of it, that inductor has an adjustable core and is a width coil that will be wired in series with the horizontal coils, but to be sure I would have to look at the schematic again.
    i can see inside it has a hex bit slot so it looks adjustable.. What does it adjust exactly?

  3. #23

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    Quote Originally Posted by VERAULT View Post
    i can see inside it has a hex bit slot so it looks adjustable.. What does it adjust exactly?
    Horizontal scan width, most likely.

  4. #24

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    Yes, and this likely width coil is unrelated to your current problem which is likely with the vertical yoke coils.
    Also , do both the yokes and these coils look exactly the same on both CRT's ?

  5. #25
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    Quote Originally Posted by Hugo Holden View Post
    Yes, and this likely width coil is unrelated to your current problem which is likely with the vertical yoke coils.
    Also , do both the yokes and these coils look exactly the same on both CRT's ?
    Yes they are both identical on both Crt's.

  6. #26

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    Quote Originally Posted by SomeGuy View Post
    I think the idea was just to turn the yoke to see if the same pattern persists, but rotated. Although I am sure it would.

    No H/V plug swapping.
    Precisely.
    And I've seen bad CRTs in a couple of occasion (on two different oscilloscopes though, not monitors) that would produce a badly distorted image, more likely because of distorted/damaged anode grid.
    So if I'd need to remove the yoke, I would first try rotating it to exclude a bad CRT.
    Missing connection on a parallel coil arrangement is still my first suspicion though.

    Frank

  7. #27
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    Quote Originally Posted by iz8dwf View Post
    Precisely.
    And I've seen bad CRTs in a couple of occasion (on two different oscilloscopes though, not monitors) that would produce a badly distorted image, more likely because of distorted/damaged anode grid.
    So if I'd need to remove the yoke, I would first try rotating it to exclude a bad CRT.
    Missing connection on a parallel coil arrangement is still my first suspicion though.

    Frank
    So If I turn the yoke and the distorted image remains intact but turns as well you suspect a bad crt?

  8. #28

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    The abnormal deflection that you are seeing is coming about because of an abnormal magnetic deflection field produced by the deflection yoke's vertical coils.Also looking at the images posted, this kind of very distorted reduced amplitude vertical scan likely would only come about if the vertical output stage/amplifier was badly overloaded due to a large drop in inductance of the vertical yoke coils, meaning, likely shorted turns.

    In oscilloscope CRT's (quite different) the deflection field is electrostatic and is normally produced buy the CRT's gun deflection plates. These can get damaged if the CRT gets dropped etc. But a magnetic deflection CRT like you have does not have these, just a very simple gun structure to produce a focussed electron stream.

    When you rotate the yoke, if you do, the image you see on the screen will simply rotate too.

    Many scopes have round CRT's and to rotate the image you simply rotate the whole CRT which of course rotates the deflection plates. For square CRT scopes, it was necessary to include a rotation coil around the neck as you cannot rotate the tube.

    Often in TV/monitors etc, where the yoke is clamped to the neck, the plastic tongues (like a split sleeve)that project from the yoke under the metal clamp can stick to the neck, even after the metal clamp is loosened. Don't put too much force on it, try to free each of the plastic tongues off the neck first if you attempt rotation , but I cannot see there is any point in removing the yoke, from the CRT yet, unless you are replacing it. I think your CRT itself is probably fine.

    Have you compared the DC resistance of the vertical coils of the two yokes on the meter yet?
    Last edited by Hugo Holden; July 20th, 2019 at 08:08 AM.

  9. #29

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    Quote Originally Posted by VERAULT View Post
    So If I turn the yoke and the distorted image remains intact but turns as well you suspect a bad crt?
    It is the contrary: if the image turns with the yoke, it's a damaged yoke, if the image doesn't turn with the yoke, it's a bad CRT.

    Frank

  10. #30

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    I can't even imagine how a "modern" CRT could fail in this way. It must be the yoke.

    Regarding the OP, and Post 12 did you indeed swap yokes between CRTs? If so, did the problem follow the yoke? If so, the yoke is certainly bad. If not, then electronics in the monitor.
    Last edited by KC9UDX; July 21st, 2019 at 03:47 AM.

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