NotTaR of Television Sets : Why projection TVs are not just normal T..  
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Why projection TVs are not just normal TVs in big boxes

In order to achieve the necessary brightness with a large display format, three separate monochrome CRTs are used with optics to combine the three images properly at the screen. This creates an entire set of additional problems in design.

(From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

The average projection TV has about twice as many parts as its direct-view counterpart. Some of the extra parts are essential for projection because CRT projection tubes require dynamic convergence. The other extra parts have to do with the fact that a more expensive TV also should have some extra features, like Dolby ProLogic sound, a satellite tuner and such.

Generally, the electronics are based on a standard chassis that is also used for direct-view CRT television. Even the deflection circuits require minor adaptations at most. The high-voltage circuit is different because the EHT, focus and G2 voltages must be distributed over 3 CRTs. So this requires a special high-voltage part, which also includes an EHT capacitor and bleeder.

There will be 3 CRT panels with video amplifiers. Because of the extremely high brightness, projection tubes will burn the phosphor screen immediately in fault conditions so a protection circuit is essential.

And last but certainly not least, there is the dynamic convergence panel. The heart is a waveform generator IC, often of a Japanese brand but nowadays there's also a digital variant by Philips. The old-fashioned way requires many many potentiometers to program the waveforms. Then there's 5 or 6 convergence amplifiers and a corresponding extra power supply. And usually this is where the single deflection circuits are distributed to the 3 CRTs. At the same time the deflection currents are sensed for the protection circuits.

Designing a PTV from a DVTV requires several man-years of work. In the factory, a special corner is devoted to the assembly. There you'll find specially educated people and the speed of the assembly line is a lot lower than usual. It requires many more adjustments, e.g. 3 optical and 3 electrical focus adjustments and then convergence.

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