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Zenith Pegs Future On Advanced Picture Tubes With PM-Zenith

April 22, 1991

MELROSE PARK, Ill. (AP) _ In a 1950s factory building that once turned out cookies, Zenith Electronics Corp. hopes to turn out high-tech television tubes that will allow it to leapfrog foreign competitors.

America’s last TV maker has invented a unique flat-screen tube that promises sharper, glare-free pictures.

Zenith’s ″flat tension mask″ tube is covered by more than 100 patents, said Charles J. Prazak, the company’s tube technology expert. He says there will be no competitors.

The tube’s origins trace to Zenith’s entry into the computer business in 1979.

When Zenith decided to start making color computer monitors, ″the Japanese had a seven- or eight-year lead on us,″ Prazak said during a tour of the suburban Chicago factory, which has produced conventional Zenith TV tubes for 26 years.

Rather than try to catch up - and hindered by limited funds - Zenith decided to reinvent the computer tube, eliminating the curved front. That allowed Zenith to improve the innards of tubes as well.

In all TV and computer tubes, tiny dots of red, blue and green phosphors are applied to the inside surface. When they are struck by electrons shot from a ″gun″ in the rear of the tube, they glow in the proper combination to form colors of the rainbow.

Behind the dots is a thin sheet of metal perforated with hundreds of thousands of holes. This screen, called a shadow mask, directs the electron beams to the dots.

In a conventional tube, the shadow mask must be curved to fit the tube’s face. Because of the curvature, it cannot be applied firmly to the screen, and instead is suspended by springs. That produces a drawback.

The electron beams cause the mask to heat up, allowing it to move slightly out of alignment because it is on springs. That causes a flickering of colors noticed, for example, when someone on TV is wearing a white shirt.

Zenith’s flat mask, by contrast, is stretched taut, preventing it from moving. That allows the tube to use a more powerful electron beam, producing a brighter picture with higher contrast.

Since 1987, Zenith has made flat-screen tubes for computers. By next year, it plans to have larger, 20-inch versions suitable for TVs.

But expanding the size has forced Zenith to reinvent the tube production process.

In tube manufacture, the phosphor materials are applied in a process that resembles photography. The phosphors, which are sensitive to light, are sprayed in a thin layer on the inside surface of the tube, then the mask is put on top like a negative. An ultraviolet light is shined through, exposing only the dots. The unexposed phosphor is washed away.

Due to conventional manufacturing methods, no two shadow masks are identical. That means each tube has its own unique mask that follows it around the tube plant as the three colors of phosphor are applied.

But with the flat tubes, the masks must be held in tension in a heavy frame as they move around the plant. Increasing the tube size to 20 inches would make the frames too unwieldy, Prazak said.

To fix this, Zenith has invented a computerized device that stretches the masks for flat tubes to an exact tolerance. That means one master mask can be used to apply the phosphors, eliminating the need to move the individual masks around the plant along with the tubes.

Soon, Zenith hopes to cut production costs further by using an offset printing process to apply the phosphors. That would eliminate the cumbersome process of exposing photosensitive phosphors through a mask.

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