This has to do with why they do what they do. I consider myself fairly
knowedgable on that subject, but noone knows everytthing so here goes.

We had about a 1991 or 1992 Sony RPTV come through. This thing was
obviously low hours and I got that "tick" when I took the screws out
of the back. It may have never been serviced, and where it comes from,
I highly suspect that. This thing has strong CRTs and operated
perfectly except it needed a coolant job, actually being this kind of
Sony, more of a CRT face scraping job. As I had the CRTs out I called
the boss over. He is in his fifties and used to be a technician, but
once you start your own business that will keep you too busy to stay a
technician.

Actually he flunked a test in school he told me. He had the circuit
all designed and the teacher drew a candle on it. He had forgotten the
filaments ! I joked that he was ready for solid state.

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 ?

And further, the other question, why don't they use electrostatic
deflection ? At least for the horizontal. I am pretty sure that
today's transistors would have alot of trouble doing 1080i, if they
ever can at all, because the yoke is inductive. Start kicking H up to
67.2 Khz, it is no fun. But with a non inductive load, wouldn't the
scan rate changes be easier to manage ? They do it in spades in
scopes.

To maintain geometry, and even deal with convergence, the drive
circuitry to three sets of horizontal amps would be no more complex
that what drives today's digital convergence circuits.

Now there is one factor that might shoot this down other than cost.
Actually I don't think the cost would be all the great, and then all
they need to deal with is vertical.

If they did vertical electrostatically, there would be no convergence
circuit at all, it could all be done by the main sweep circuits. But
the one problem there might be with that is this.

Electrostatic deflection might be more affected by beam current
changes. I do not know enough CRT technology to know something of that
nature. However, they have already found out that steady deflection
along with precise HV regulation does not work. The raster will get
smaller because beam density affects deflection sensitivity. That's
why there are seperate resistors going to each CRT anode in a high
voltage splitter. That is also why they have abandoned extremely tight
HV regulation in favor of more precise and modulated control of the
deflection. They have integrated HV level with beam current, and also
use it to control the vertical drive now.

So why can't they use electrostatic deflection and deal with these
problems just like they do now. The only difference is that there
would be no great current flowing. But for the capacitance of the
deflection plates, which should be well easier to deal with than the
inductance of a yoke, why don't they do it ?

I don't think the deflection sensitivity issue is all that big. When I
was in my twenties I had a shop. Guy comes wants to work cheap and
learn. During one of the "lessons" I capacitively couple a video
signal to the Z axis input, and sync the horizontal and fed the
vertical waveform to the vertical. It did not seem to have a problem
with intensity modulation. Also, the scope circuits I have seen,
admittedly older ones, did not include anything elaborate to deal with
it. Therefore my assumption is that it is no more a problem than in a
magnetically deflected CRT.

What I am here for is to have holes shot in my theory. There must be a
reason, and money is no longer it. Many techs still recommend CRT
based RPTVs. There will still be a demand. But they could do 1080p !

JURB