Sounds like doming. More common on cheaper tubes & high brightess
& contrast. Cut this out for you. If you want to know everything
about CRT's click the link...........
What is Doming?
The shadow or slot mask inside the CRT is a thin sheet of steel or InVar positioned a half an inch or so behind the phosphor screen. The flatter the screen, the more susceptible it will be to thermal expansion effects: With individual phosphor dots spaced as as little as .13 mm apart (for a .22 mm dot pitch CRT), it doesn't take much inaccuracy in their position to result in a noticeable effect. (See the section: How to Compute Effective Dot Pitch.) As a result, high resolution CRTs tend to be more susceptible to doming problems.
(Portions from: Jac Jamar (
[email protected]).)
Doming is a deformation of the shadow mask or its support structure caused by heating and subsequent expansion in bright (high beam current) areas of the picture. This causes a shift in position of the finely spaced holes or slots in the mask. The result will be color purity problems - discoloration and brightness variations. For a .28 mm dot pitch CRT, a change of only .14 mm in the position of a hole or slot can totally shift the display from one of the primary colors to another.
InVar shadow masks can sustain a significantly higher current density than steel shadow masks (by as much as 3:1) without noticeable problems.
Trinitrons are more resistant to local doming effects as long as the wires are under enough tension. However, expansion of the suspension components can still result in doming with an overall bright picture.
The onset and disappearance of color purity problems will generally lag the cause due to the thermal mass of the affected components. For local heating resulting from picture highlights, this will be only a few seconds since the thermal mass of a small area of the mask is not that great. However, for effects having to do with expeansion of the suspension or support structure, it may take up to 30 minutes to reach equilibrium.
The orientation of the TV or monitor with respect to the earth's magnetic field and even whether the CRT was set up for the Northern or Southern hemispheres may affect the resulting color shift. Thus, the picture may tend toward yellow while the monitor is facing one way and blue when rotated 180 degrees on its base (even if degaussed at each position).
Reducing the brightness/contrast or setting the brightness limiter will prevent doming but may result in an unacceptably dark picture.
Shadow mask doming in itself is not something that becomes defective and has to be repaired. It is a characteristic of the CRT assembly. However, shifts in the position of purity adjustments can results in increased sensitivity to slight doming.
http://www.repairfaq.org/sam/crtfaq.htm
73 Zeno
