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electron gun(s) and magnetic fields

Discussion in 'Electronic Repair' started by Gary DeWitt, Nov 12, 2004.

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  1. Gary DeWitt

    Gary DeWitt Guest

    I have a 54" RCA rear projection. I had a new hardwood floor installed
    with an electric heating grid underneath. The grid consists of heavy
    screen mesh, similar to window screen but 2-3 times heavier wire, laid
    in one continuous circuit back and forth across the room. There is a
    high voltage, low amperage current running through it. Here's the
    problem - when the heater is on, the guns in my tv misalign, looks
    like 3-d image viewed without the correct glasses. Soon as I turn off
    the heat, the tv returns to normal. How can I shield the tv from the
    floor induced field? Faraday cage? (I don't really want to view thru
    wire mesh!) Grounded sheet of steel under the set? Raise the set a
    foot or two off the floor? Both? Any suggestions, particularly from
    those who have solved this problem or engineers, would be greatly
    appreciated!

    Real E-mail: gpdewitt at earth link dot net (no space in earthlink)
     
  2. NSM

    NSM Guest

    | I have a 54" RCA rear projection. I had a new hardwood floor installed
    | with an electric heating grid underneath. The grid consists of heavy
    | screen mesh, similar to window screen but 2-3 times heavier wire, laid
    | in one continuous circuit back and forth across the room. There is a
    | high voltage, low amperage current running through it. Here's the
    | problem - when the heater is on, the guns in my tv misalign, looks
    | like 3-d image viewed without the correct glasses. Soon as I turn off
    | the heat, the tv returns to normal. How can I shield the tv from the
    | floor induced field? Faraday cage? (I don't really want to view thru
    | wire mesh!) Grounded sheet of steel under the set? Raise the set a
    | foot or two off the floor? Both? Any suggestions, particularly from
    | those who have solved this problem or engineers, would be greatly
    | appreciated!

    Interesting!

    1. The higher off the floor the TV is, the lesser the effect.

    2. Alternate layers of sheet steel and aluminium under the TV, insulated
    from each other by paper, might diminish the effect.

    3. Use a separate heater when you want to watch TV.

    N
     
  3. N Cook

    N Cook Guest

    2 ideas
    Find someone who has a heap of mu-metal sheets and lay under TV

    or trace with sniffer or pipe/metal detector the exact run of heater wires
    at a point away from TV. Excavate down and cut and bypass the few loops
    that go under the TV

    electronic hints and repair briefs , schematics/manuals list on
    http://homepages.tcp.co.uk/~diverse
     
  4. Any iron or steel (but not stainless steel) shield (such as sheet metal)
    that you can place between the heater and the TV will help. Iron has much
    lower reluctance than air, so the magnetic field from the heater will be
    concentrated in the shield. Stainless steel is weird in this regard and
    usually won't help even though it can be made up of mostly iron.
     
  5. A Faraday cage won't help. It's magnetic interference and magnetic
    fields are difficult to block.

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header is ignored.
    To contact me, please use the feedback form on the S.E.R FAQ Web sites.
     
  6. That won't help if the heating grid is powered from AC.

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header is ignored.
    To contact me, please use the feedback form on the S.E.R FAQ Web sites.
     
  7. Art

    Art Guest

    Also the Manufacturer will be absolutely no assistance regarding this since
    the symptom is being induced by it's environment. As noted, you may want to
    consider a DLP, MMD, or Plasma television reciever. Or, move the curent
    television to a room that is not heated in such a manner to resolve that
    particular problem.
     
  8. Jamie

    Jamie Guest

    sorry, i don't think you can.
    you would have to align the TV
    with the heat on and then align
    it again after.
    the only thing i can think of is
    that the electric heaters are
    not properly laid out over
    laying each other to cancel each
    other out which in this case
    can be hard to do..
    you could try elivating the
    tv on a stand or get your
    self a DLP projection.
     
  9. Graham

    Graham Guest


    Swap your rear-projection TV for a Plasma or LCD.
    Or use an alternative heat source.

    I am not trying to be flippant, if the convergence shift is as bad as you
    say, I don't think you will ever shield it.
     
  10. JURB6006

    JURB6006 Guest

    Here's an idea that'll blow you f______ mind :)

    An RCA doesn't need gravity to cool the CRTs, so, turn it upside down.
    Unfortunately you'll have to make a top for it and have it completely re-setup
    for this mode, but this will put the CRTs at the top, well away from the EMI.
    To get the desired screen height you might not really be able to put anything
    on top of it, the speakers will be at the top (and reversed, but you know what
    to do), and I would suggest a carefully designed cooling system. Yes a fan, but
    with it setup to draw air the right way through the chassis.

    In this case, this is the best I can do. If you're in NE Ohio I can do it for
    you, probably pretty reasonable. I'm not really that cheap, but it'll be well
    less than getting a DLP, and don't get a plasma until they perfect them.

    Just build whatever stand it takes to get the screen as high as you want it,
    and BTW, all the stuff that used to be on top could now be on shelves
    underneath.

    While this might be a fine example of lateral thinking, it may or may not be
    right for you, but it is an idea.

    JURB
     
  11. Asimov

    Asimov Guest

    "Sam Goldwasser" bravely wrote to "All" (12 Nov 04 16:04:32)
    --- on the heady topic of "Re: electron gun(s) and magnetic fields"

    SG> From: Sam Goldwasser <>

    SG> A Faraday cage won't help. It's magnetic interference and magnetic
    SG> fields are difficult to block.

    A magnetic field is impossible to block, however it can be readily
    redirected. If the tv were completely enclosed in an iron box the
    interfering AC field would flow around the tv and through the iron
    instead. This may not be very practical however, after all the screen
    needs to be seen to be of use. Perhaps a metal shelf could work but
    this is often a cause of colour purity problems in itself.

    There are other strategies, for example we all know magnetic field
    intensity decreases as the square of the distance. Therefore raising
    the tv away from the floor enough may be all that is needed. Perhaps
    leaving a sheet of iron on the floor below the tv's stand would
    further decrease the field even more in the vicinity of the tv.

    Another idea would be to induce a small field under the tv with the
    opposite phase in the hope that the second field would cancel the
    first enough to allow the tv to operate properly. Maybe just a few
    loops of wire wound under the base of the tv would be enough for the
    same effect. Of course phasing the small cancellation field would be
    trial and error or how for example it should be oriented.

    However, as a side issue, one concern I might have would be of living
    in a space with such a significant magnetic environment that it
    noticably affects the tv. I've heard of people living over transformer
    rooms having a myriad of problems from infertility on through to
    childhood leukemia. Then again people have been using electric
    blankets for ages and I don't think any co-relation has been shown
    there. It's a concern I really don't feel qualified to comment about.

    A*s*i*m*o*v

    .... Inductor designers do it in the gap.
     
  12. Gary DeWitt

    Gary DeWitt Guest

    2 ideas
    What is mu-metal, and is there a source where it can be salvaged from?
    I saw something like this on the web, poorly described and priced at
    hundreds per square foot!
     
  13. Mumetal is a highly magnetically permiable material. However, it's expensive
    as you note, and difficult to work (you can't bend it without reannealing for
    maximum effectiveness).

    And, since you can't see through it, at least the front of your TV
    will be exposed. If it is simply put under the TV but the magnetic field
    will still come in from the beyond.

    You will end up spending much more on bandaids than on an LCD TV.

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header is ignored.
    To contact me, please use the feedback form on the S.E.R FAQ Web sites.
     
  14. Additional information:

    "So, in our example of placing a steel plate between the magnet and the area
    we desire to shield from the magnetic field, the ends of the steel plate
    will have all the flux lines possible trying to get into it, traveling down
    to where they must exit to get back to the magnet (the flux channel) and
    exiting in a big bunch. Remembering that the more flux lines, the stronger
    the field, what we have accomplished with our actions is to make the
    magnetic field near the ends of the steel plate stronger (collected flux
    lines) and more inhomogeneous (bent more). Both of these points make the
    stray magnetic field more of a problem near the ends of the steel plate. One
    way to overcome this problem is to make the steel plate much larger that the
    desired area to shield. As the dimensions of the steel plate get larger, the
    larger the area of the lowest magnetic field behind the steel plate."

    It would seem that if the steel plate was large enough, the flux lines at
    its edges should be far enough away from the electron guns and the problem
    will be solved or at least greatly reduced.

    Source: http://www.acornnmr.com/appnotes/shielding.htm
     
  15. NSM

    NSM Guest

    | Mumetal is a highly magnetically permiable material. However, it's
    expensive
    | as you note, and difficult to work (you can't bend it without reannealing
    for
    | maximum effectiveness).

    And a cheaper alternative is alternate layers of steel and aluminium,
    insulated with paper.

    NM
     
  16. James Sweet

    James Sweet Guest


    I would be concerned about coolant leaking and airflow for cooling, not to
    mention this would look pretty ugly. I think it'd be easier to set the TV up
    on something, a shelf a foot off the floor would probably be plenty, it's
    worth a try, cinder blocks and a piece of plywood will do the trick,
    something nicer looking can always be built once the problem is fixed.
     
  17. Jim Adney

    Jim Adney Guest

    I agree with this. I don't know if the makers of this kind of heater
    have thought of this problem, but they should have. It should be
    possible to lay the heating material in such a way that the external
    field is minimized, but it sounds like this was not done in your case.

    It might be worthwhile to check with the maker of your heater to see
    if they have guidelines in installing it. I have no idea how hard it
    might be to get it corrected.

    I agree with Sam, that trying to shield it is likely to be futile.

    -
     
  18. Jim Adney

    Jim Adney Guest

    Very interesting, and creative, suggestion. It's a lot of work, but
    probably a lot less than digging up the floor.

    It's also possible that it would not help significantly. It all
    depends on the nature of the mag field set up by the heating coils.

    -
     
  19. Graham

    Graham Guest


    I see, you mean like a bifilar coil?

    Is that standard practise with underfloor heating?

    We don't see much of it now here in the UK.

    Was put in to apartments in the '60's but became notoriously expensive to
    run.
     
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