DIYbanter - View Single Post - Question of TV technology, if anyone can answer two questions

isw

In article
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wrote:

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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