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Magnetic Fields on a CRT

Discussion in 'Electronics Videos' started by RODALCO, Jun 7, 2016.

  1. RODALCO

    RODALCO Well-Known Member

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    Was playing around with some old CRT's over the weekend, and trying to convert one as a simple oscilloscope.
    Well I got a bit side tracked as I had one TV smoking when I disconnected the horizontal coils. So ended up repairing the TV, one bulged capacitor and two resistors fried, which were in parallel with the hor. Coils.
    Coincidentally I held a magnet near the screen and the standard blue default colour produced some amazing magnetic lines.
    Here is a short video I made.
    Enjoy.
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I'm rather bemused about someone 'discovering' the effect a magnet has on the shadow mask on an old colour CRT :D
     
  3. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    I accidentally did this as a kid (held a magnet too close to the TV). There were purple and green splotches on the screen for the next couple of years :p
     
  4. dave

    Dave New Member

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  5. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Wow did they have magnets when you were a kid?

    Just joking
     
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  6. RODALCO

    RODALCO Well-Known Member

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    I did the same on a Philips TV and it took ages for the purple spot to disappear. Wonder if older TV's used degaussing coils.
    That was in the 1980's
     
  7. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    Mine certainly had a degaussing coil (I remember tearing it apart when it finally died) but it really didn't do much good :p
     
  8. RODALCO

    RODALCO Well-Known Member

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    I will do colds starts on the 2 TV's used in this demo and see how they come up. They are sitting in our garage.
     
  9. RODALCO

    RODALCO Well-Known Member

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    I had the same experience as DerStrom8 when I was a kid, and made a purple spot in our Philips TV, and got told off by my parents. Now I have lots of donated or roadside TV's which I often overvolt , strip for parts or blow up for fun.
    May as well have some fun with them and the effects are very cool.
    Some TV's keep working up to close to 500 Volts AC before they let go.
     
  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It should do, but they only work from absolutely stone cold - I've spent a LOT of time over the decades deguassing CRT sets :D

    As far as I'm aware all colour sets had deguassing coils/circuits, although in later years there was some talk of stopping fitting them.

    One of my memorable stories was a lady who kept complaining about 'funny colours' on her TV, it was a GEC valve set, and every time we went out we turned the set on, and it was perfect. Anyhow, one day we were driving past (and had a quiet moment), so thought we'd just pop in and see how she was doing. This was when it all got explained - catching her un-awares stopped her clearing everything away from the TV (to give us room), and right next to the TV was a huge great loudspeaker. Every time she called us out she moved it out of the way, and set deguassed itself when it came on :D
     
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  11. RODALCO

    RODALCO Well-Known Member

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    I love those historical stories from the field. Brilliant!!
    That are experiences never to be forgotten.
     
  12. atferrari

    atferrari Well-Known Member

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    Well, isn't Cristoforo reputed as "discovering" America...?
     
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  13. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    Fair point atferrari.
     
  14. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I have two deguassing coil winding machines in the top of the barn. Have not been turned on for 10 years. I though about making metal detector coils.

    Recently I re-purposed another machine. I doubt anyone will want coils 3 meters across. I have been in too many businesses that no longer exist.
     
  15. atferrari

    atferrari Well-Known Member

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    Ron,

    I recall, as a cadet, being on board of a vessel with her own degaussing system, used to reduce the magnetic "signature" thinking she could have to navigate an area planted with magnetic sensors fitted mines.

    In an old shipyard they even had a degaussing system to treat vessels under repair.

    Had you any experience on that area? Not even sure what vessels nowadays do on that regard.

    In the past it was a must for minesweepers.
     
  16. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    In WWII ships were commonly fitted with deguassing systems, to avoid magnetic mines.
     
  17. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    If I remember right, the process of welding will leave magnetic effects on the ship.
    It will take a great deal of energy to fix a ship in a timely manor.
     
  18. spec

    spec Well-Known Member Most Helpful Member

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    One of my huge transmission line speakers did a similar thing to our beautiful Sony 27" color TV, although it had built-in degausing. In the end I got a hand held degausing tool that fixed the TV for a couple of days before the rainbows started creeping back again. The degaussing procedure was a great entertainment for the kids, with the swirling colors on the screen and the loud hum from the degausing tool.

    The TV interference was an additional reason why my speakers got ejected from our front room. The chipboard finish, size and the four exposed speaker chassis and port filled with black wool didn't help much either.:p

    spec
     
    Last edited: Jun 25, 2016
  19. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    I remember doing that with a magnet on my color CRT monitor several years ago. To get rid of the effect i simply turned the magnet around so the opposite pole faced the spot, then moved it around a little.

    What always interested me was that magnets either attract other magnets or iron based objects or repel other magnets, so they move directly away from the first magnet or directly toword the first magnet, BUT if the object is a moving electron, the Lorentz force pushes it either up or down (assuming horizontal orientation of the magnet). It does not attract the moving electron nor repel it it the same direction as the magnet is facing like other objects, but instead pushes it at a RIGHT angle to the orientation of the magnet (and thus the field). If the electron was like any other object it would turn toward the magnet or at least move away (right or left not up or down). I did a video on this using the scope beam as the electron flow. The dot starts out at the center of the screen, then moves down when the magnet is brought near the side of the scope body.
    So there is something very different about the electron than other objects that interact with magnets (and thus their fields).
     
  20. JimB

    JimB Super Moderator Most Helpful Member

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    Electric charge?
    The electron has charge, and is moving, I am told that constitutes an electric current.
    Now think about the right hand and left hand rules for motors and generators, where the current the field and the force are in mutually orthogonal directions.

    JimB
     
  21. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Now that's something you don't read every day :D
     
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