Re: Question of TV technology, if anyone can answer two questions

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So I pull him aside as he's walking by, because I saw the > pincushioning nagnets. I pointed at them and said "You recognize > these ?". He didn't remember, I told him "That's the pincushion > circuit, remember ?". Then he remembered. That is the old way. > > Of course we all know that you cannot use that method on a color CRT, > but projection TVs have monochrome CRTs. > > So why don't they stick with a tried and true method for this > application rather than overpushing the convergence circuit to the > point where it has become the most common RPTV fault ?

Permanent magnets do not provide the optimum field shape for pincushion correction, but a worse problem is that the pin correction in all three tubes must *match* for convergence. That requires electronic circuitry which can be adjusted.

That fact that the convergence circuit fails a lot is just a natural result of either incompetent design or overly aggressive cost-cutting. It does not need to be that way.

And further, the other question, why don't they use electrostatic > deflection ?

Electrostatic deflection causes uncorrectable astigmatism which defocuses the spot, especially in the corners. One solution (used in oscilloscopes) is a much longer tube and therefore much smaller deflection angles, but O-scopes also use a much smaller ultor voltage. The high accelerating voltage necessary for high brightness in TVs would necessitate ridiculously high deflection voltages on the plates, or else equally ridiculous tube length. High currents for deflection coils are far easier to achieve. A fundamental difference between electrostatic and magnetic deflection means that the latter causes essentially no spot astigmatism.

Isaac