Doming

The discoloration in this picture illustrated in the woman white shirt sleeve. In a previous picture the television was tuned to to a closed circuit video source where a security camera was trained on a lobby for a security apartment complex. On the left side of the picture was a tall window that allowed daylight to enter the room and the iris of the camera. This intense light caused the iron shadow mask to expand in area of that window. The expansion caused a blistering or warping of the Mask material. In turn the electron beam hits the wrong phosphor. Once the video source was changed the discoloration disappeared after 10 minutes.

The doming effect or blistering that occurs when the shadow mask heats up causes the mask to expand effecting the "Q" distance. {The "Q" distance is the distance between front surface of the shadow mask to the face plate of the CRT.} The distance is critical to insure correct beam landing. Each electron beam must hit it's appropriate color target. If not a purity defect will appear.

We have seen a rise this problem because of the demand for higher brightness and contrast levels.  Conventional iron shadow masks expand in areas where they are heated to high temperatures, but they have poor thermal conductivity.  In other words they cannot dissipate the heat throughout the shadow mask fast enough to keep the area from expanding.

A solution is a Invar shadow mask. It is a Nickel / Iron alloy that has very good thermal conductivity. If the same area heats up again the heat conducts across the entire surface of the shadow mask. This reduces the the expansion to an acceptable level. Of course nothing has been developed to completely eliminate this effect entirely

Unfortunately the Invar tubes are not available for all televisions. They are still quite expensive and in limited supply. This phenomena cannot be corrected by changing the CRT. It can usually be controlled by reducing bean current by reducing brightness, color, or contrast.

Dennis Viereck

wa6ati@mindspring.com