In article
,
Jeff Urban wrote:

OK, whether the field is retiring me or the other way around is
irrelevent. But I have been pretty much the foremost expert on three
tube PTVs in at least this state for some time. Of course you see that
is not exactly the hottest job in town. I am not saying that I am THE
expert on everything, but what makes a CRT RPTV work, as opposed to a
direct view, that has been my specialty. In fact if I went to work on
direct views I would probably have to work for ten bucks an hour.

But I get into theory and this thread is for folks like that. I know
what it takes to make those things work, I have worked damnear
miracles on them, but I never designed them. I have done some repairs
so serious that I guess I could claim I can build one, but I still
never designed one.

A couple months ago I ran into one of those Hitachis with the necked
down neck. I mean where the yoke goes the neck is narrower, which
means the yoke is manufactured on the tube and is TRULY a bonded yoke.
Not just glued like before.

They did this of course to get the coils closer to the beam, which
increases deflection efficiency.

Probably, they did it to get precisely the alignment they needed, to
make it the same between the tubes of a set, and to keep it that way.
For one thing, it makes replacing a CRT a whole lot less finicky.

Now MY question :

Why the hell didn't they just go with electrostatic deflection like in
a scope ?

Think about it, you people out there who know engineering, think about
it. Why not ? I understand about the CRT parameters and the variance
with beam current and I also know about beam density. I know these are
all problems, but using magnetic deflection solved none of them !

-- snip --

The insurmountably serious problem with electrostatic deflection is
astigmatism, and my understanding that there is in fact no way to solve
it.

Recall that the "beam" is really not a pencil like it's usually shown in
drawings; it's a pair of narrow cones, base to base, with one of the
points at the cathode and the other at the screen. Focusing the beam is
actually forming an image of the cathode on the screen. The Largest
diameter of the beam occurs wherever the focusing electrode (or coil)
is, but the beam still has a substantial diameter when it passes the
deflection plates.

When the beam is electrostatically deflected, the various electrons of
the beam are affected differently depending on where they are in the
beam, and the strength of the electric field between the plates (and
this changes during the deflection process). This causes the image to
become de-focused and elongated in one dimension. This distortion cannot
be pre-corrected by any sort of electron-optical mechanism. The same
thing happens when the beam passes through the second set of plates,
too, of course. The result is that the spot size and shape can be fine
in the center of the screen, but pretty poor at the edges, and
*terrible* in the corners.

O-scopes have long tubes to minimize the deflection angle, *not* because
high-angle deflection is especially difficult, but to minimize the
astigmatism problem, which gets worse very quickly as the deflection
angle increases beyond a few degrees.

Electrostatic deflection, unlike magnetic, involves a transfer of
momentum to the beam (put another way, electrostatic deflection works by
accelerating the electrons sideways as they pass between the plates, but
when an electron beam is bent by a magnet, the beam doesn't "notice"
that it's being bent, sort of like going around a properly canted curve
in a fast car).

The reason you can't have a CRT at 1080p is because of the inductance
of the yoke. You would have to stick a four thousand volt pulse to
it.

Actually, no. There have been many CRTs with that and higher resolution
used with computers and in high-quality imaging systems. Especially with
transistors, very high currents are no problem, and those systems can
incorporate very low inductance yokes; recall that it's amps times turns
that determines the field strength. Some years ago, I was involved with
a CRT-based system that got an honest 4096x4096 pixel resolution out of
a nearly standard 5" CRT. It used magnetic deflection; we never could
have done it with an electrostatic jug.

--

If you want to take a look at a truly *bizarre* video projection
technique, find out how an Eidophor works. Or worked; I really don't
know if they're in business these days. Think about vacuum pumps,
rotating oil-covered mirrors, Schlerin optical systems, ten-kilowatt
xenon short-arc lamps, and little air-locks for replacing the filaments
while in operation, all in one box. Times three, of course, for color.

Isaac