Color CRTs, Shadowmasks, Magnetic Fields, and Degauss.
All color CRTs utilize a shadowmask or aperture grill a fraction of an inch(1/2"
typical) behind the phosphor screen to direct the electron beamsfor the red, green, and
blue video signals to the proper phosphor dots. Since the electron beams for the R, G, and
B phosphors originate from slightly different positions (individual electron guns for
each) and thus arrive at slightly different angles, only the proper phosphors are excited
when the purity is properly adjusted and the necessary magnetic field free region is
maintained inside the CRT.
Note that purity determines that the correct video signal excites theproper color while
convergence determines the geometric alignment of the 3 colors. Both are affected by
magnetic fields. Bad purity results in mottled or incorrect colors. Bad
convergence results in color fringing at edges of characters or graphics.
The shadowmask consists of a thin steel or InVar (a ferrous alloy)with a fine array of
holes - one for each trio of phosphor dots - positioned about 1/2 inch behind the surface
of the phosphor screen.
With most CRTs, the phosphors are arranged in triangular formations called triads with
each of the color dots at the apexof the triangle. With many TVs and some monitors,
they are arranged as vertical slots with the phosphors for the 3 colors next to one
another.
An aperture grille, used exclusively in Sony Trinitrons (and now their clones as well),
replaces the shadowmask with an array of finely tensioned vertical wires. Along with
other characteristics of the aperture grille approach, this permits a somewhat higher
possible brightness to be achieved and is more immune to other problems like line induced
moire and
purity changes due to local heating causing distortion of the shadowmask. However,
there are some disadvantages of the aperture grille design:
Apparently, there is no known way around the need to keep the fine wires from vibrating or changing position due to mechanical shock in high resolution tubes and thus all Trinitron monitors require1, 2, or 3 stabilizing wires (depending on tube size) across thescreen which can be see as very fine lines on bright images. Somepeople find these wires to be objectionable and for some critical applications, they may be unacceptable (e.g., medical diagnosis). Why is the Shadowmask or Aperture Grille made of a magnetic material?--The question often arises: Well, if magnetization and the need for degauss is a problem, why not make the shadowmask or aperture grille from something that is non-magnetic? The shadowmask *must* be made of magnetic material! This may seem to be undesirable or counter intuitive but read on: Together with the internal shielding hood it forms sort of a closed space in which it is attempted to achieve a field-free space. The purpose of degaussing is *not* to demagnetize the metal, but to create a magnetization that compensates for the earth's magnetic field. The *sum* of the two fields must be near zero! Degaussing coils create a strong alternating magnetic field that gradually decays to zero. The effect is that the present earth magnetic field is "frozen" into the magnetic shielding and the field inside the shielding will be (almost) zero. Non-zero field will cause colour purity errors. Now you will understand why a CRT must be degaussed again after it has been moved relative to the earth's magnetic field. This will also explain why expensive computer monitors on a swivel pedestal have a manual degaussing button, you must press it every time after you have rotated the monitor. The axial component of the magnetic field is harder to compensateby means of degaussing. Better compensation may be achieved bymeans of a "rotation coil" (around the neck or around the screen), this requires an adjustment that depends on local magnetic field. CRT's for moving vehicles (like military airplanes) may be equipped with 6 coils to achieve zero magnetic field in all directions. They use magnetic field sensors and active compensation, thus they don't need any degaussing function. This is too expensive for consumer equipment. Tubes for all Nations: CRT Manufacturers actually make different versions of their tubes forTV's for the northern and southern hemisphere, and sometimes a 3rd neutral type. These are so-to-say precorrected for the uncompensated field. (Note that the term 'tube' here includes much of the convergence hardware as well - not just what is inside the glass.) I remember when we exported projection televisions from Belgium to Australia, a couple of years ago. They all had to be opened on arrival to re-adjust the rotation settings on the convergence panel, due to the different magnetic field in Australia. Projection TV's don't have degaussing (there is nothing to degauss), and the customer can only adjust red and blue shift, not rotation. Our CRT application group has a "magnetic cage". This is a wooden cube (approx. 2 meter long sides) with copper coils around each of the 6 surfaces. With this they can simulate the earth magnetic field forevery place on earth (as indicated on a map on the wall). Degaussing (demagnetizing) a CRT: Degaussing may be required if there are color purity problems with thedisplay. On rare occasions, there may be geometric distortion caused by magnetic fields as well without color problems. The CRT can get magnetized:* if the TV or monitor is moved or even just rotated.* if there has been a lightening strike nearby. A friend of mine had a lightening strike near his house which produced all of the effects of the EMP from a nuclear bomb.* If a permanent magnet was brought near the screen (e.g., kid's magnet or megawatt stereo speakers).* If some piece of electrical or electronic equipment with unshielded magnetic fields is in the vicinity of the monitor. Degaussing should be the first thing attempted whenever color purity problems are detected. As noted below, first try the internal degauss circuits of the monitor by power cycling a few times (on for a minute, off for 30 minutes, on for a minute, etc.) If this does not help or does not completely cure the problem, then you can try manually degaussing. Commercial CRT Degaussers are available from parts distributorslike MCM Electronics and consist of a hundred or so turns of magnet wire in a 6-12 inch coil. They include a line cord and momentary switch. You flip on the switch, and bring the coil to within several inches of the screen face. Then you slowly draw the center of the coil toward one edgeof the screen and trace the perimeter of the screen face. Then return to the original position of the coil being flat against the center of thescreen. Next, slowly decrease the field to zero by backing straight up across the room as you hold the coil. When you are farther than 5 feet away you can release the line switch.The key word here is ** slow **. Go too fast and you will freeze the instantaneous intensity of the 50/60 Hz AC magnetic field variation into the ferrous components of the CRT and may make the problem worse. It looks really cool to do this while the CRT is powered. The kids will love the color effects. Bulk tape erasers, tape head degaussers, open frame transformers, and the"ass-end" of a weller soldering gun can be used as CRT demagnetizers but it just takes a little longer. (Be careful not to scratch the screenface with anything sharp.) It is imperative to have the CRT running when using these wimpier approaches, so that you can see where there are still impurities. Never release the power switch until you're 4 or 5 feet away from the screen or you'll have to start over. I've never known of anything being damaged by excess manual degaussing though I would recommend keeping really powerful bulk tape erasers turned degaussers a couple of inches from the CRT. If an AC degaussing coil or substitute is unavailable, I have even degaussed with a permanent magnet but this is not recommended since it is more likely to make the problem worse than better. However, if the display is unusable as is, then using a small magnet can do no harm. (Don't usea 20 pound speaker or magnetron magnet as you may rip the shadowmask right out of the CRT - well at least distort it beyond repair. What I have in mind is something about as powerful as a refrigerator magnet.) Keep degaussing fields away from magnetic media. It is a good idea to avoid degaussing in a room with floppies or back-up tapes. When removing media from a room remember to check desk drawers and manuals for stray floppies, too. It is unlikely that you could actually affect magnetic media but better safe than sorry. Of the devices mentioned above, only a bulk eraser or strong permanent magnet are likely to have any effect - and then only when at extremely close range (direct contact with media container). All color CRTs include a built-in degaussing coil wrapped around the perimeter of the CRT face. These are activated each time the CRT is powered up cold by a 3 terminal thermister device or other control circuitry. This is why it is often suggested that color purity problems may go away "in a few days". It isn't a matter of time; it's the number of cold power ups that causes it. It takes about 15 minutes of the power being off for each cool down cycle. These built-in coils with thermal control are never as effective as external coils. An exception is the type in the better workstation CRTs that include a manual degauss button. Note that some manufacturers warn of excess use of these buttons due to their designs (read: inferior) where certain components like the coil or control circuits may overheat. It has nothing to do with excess degaussing - just excess use of their degauss circuitry.
