Question of TV technology, if anyone can answer two questions

[[email protected]]

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

 

[[email protected]]

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.

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

I have two questions.

Why would anyone give 2 hoots about CRTs? The optics are crap. There
isn't enough light even from 3 mono CRTs. Convergence and geometry is
mediocre at best. Stability is poor and as you mentioned, reliability
is poor.

Question 2. Have you looked at the newer display types? Even the
'worst' one will blow CRTs out of the water.

We have a 4 year old Samsung DLP that has had one lamp fail and a
color wheel in 9100 hrs operation. Total parts cost < $200. Total
repair time < 2 hours. Its brighter and clearer than ANY CRT based
RPTV set ever. The 'convergence' is flawless and the geometry error is
unmeasurable. Plus it has a DVI input for 1:1 pixel mapping from a
modest PC that doubles as a high def video recorder. And it weighs
less. And it uses less power. And it has a smaller footprint. And it
has no fluids to leak. And it fits into a minivan. And it has no high
voltage to attract dust. And it makes less RFI.

OK, I didn't convince you. I, however, am done with CRTs and judging
by what is in the stores, I'm not alone.

GG

 

[William Sommerwerck]

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.

Using electrostatic deflection requires a CRT with deflection plates. Such a
tube would have (I believe) a thicker neck. Also, the output transistors
would have to swing at least a couple hundred volts to deflect the beam.

My guess is that Sony, et al, stick with magnetically deflected tubes
because they've been the standard for 60 years. That's the kind of tube they
build, and the kind of deflection circuits they design.

However "correct" your theories might or might not be, they run up against
industry practice. Electrostatic deflection is considered obsolete, at least
with respect to video displays. And pretty soon CRTs will be obsolete with
respect to video displays.

I have a Toshiba CZ-3299K HDTV that's about 12 years old. It has a 32"
magnetically deflected CRT, and runs at four times the normal scanning rate
(~ 63kHz) without problems.

Electrostatic deflection might be more affected by beam current
changes.

I don't think so. Try changing the screen brightness of a 'scope's CRT. Does
the deflection change?

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.

Again, I don't think so. If this were true, the image on any magnetically
deflected CRT would show severe geometric distortion that varied with image
brightness. It doesn't.

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.

Are you sure? How can you change beam current without changing brightness?

 

[Phil Hobbs]

Using electrostatic deflection requires a CRT with deflection plates. Such a
tube would have (I believe) a thicker neck. Also, the output transistors
would have to swing at least a couple hundred volts to deflect the beam.

My guess is that Sony, et al, stick with magnetically deflected tubes
because they've been the standard for 60 years. That's the kind of tube they
build, and the kind of deflection circuits they design.

However "correct" your theories might or might not be, they run up against
industry practice. Electrostatic deflection is considered obsolete, at least
with respect to video displays. And pretty soon CRTs will be obsolete with
respect to video displays.

I have a Toshiba CZ-3299K HDTV that's about 12 years old. It has a 32"
magnetically deflected CRT, and runs at four times the normal scanning rate
(~ 63kHz) without problems.



I don't think so. Try changing the screen brightness of a 'scope's CRT. Does
the deflection change?



Again, I don't think so. If this were true, the image on any magnetically
deflected CRT would show severe geometric distortion that varied with image
brightness. It doesn't.



Are you sure? How can you change beam current without changing brightness?

Electrostatic deflection can be much faster, but the focusing properties
are much worse. Electron microscopes all use magnetic focusing and
magnetic deflection for that reason--you can't get any sort of decent
spot size with electrostatic. For analogue oscilloscopes, the spot size
hardly matters--1mm is fine--but that's not true for TVs.

Cheers,

Phil Hobbs

 

[Clifford Heath]

For analogue oscilloscopes, the spot size
hardly matters--1mm is fine--but that's not true for TVs.

And they use narrow deflection angles, infeasible in a TV.
The problem would only be worse with wider angles.

 

[Tim Williams]

Using electrostatic deflection requires a CRT with deflection plates. Such a
tube would have (I believe) a thicker neck. Also, the output transistors
would have to swing at least a couple hundred volts to deflect the beam.

Actually, it's not too bad. At a paltry 1800V acceleration, the tube in my
Heathkit IO-103 gets pretty well 10V/cm, which is at least a 60V supply for
its window. Focus works just fine at this voltage, as sharp as my Tek.
It's beyond me why they designed the deflection circuits for 150 and 180V
supplies.

Tim

 

[Steve Kraus]

But that's not what he asked.

 

[[email protected]]

Haha William, nice try, I know there is more to this but for now ; the
main reason that CRT necks got smaller was to get the deflection yoke
coils closer to the beam, to enhance deflection sensitivity.

strat, I have a problem with any lightbulb based system. It does not
generate light, it only controls it. That light is being produced at
the maximum at all times, in fact bulb based projectors usually run
hotter when you turn down the brighness, such as for night viewing.

If the unit is showing you less light, you are using the same or more
energy.

Actualy I know I am not going to talk anyone out of it, but just what
kind of DLP is it ? I know they are more reliable than an LCD,
slightly. but I have seen some pretty new ones broken.

Also, they go to great lengths to compensate for beam current and
actual HV level. I mean there are sets that do not even use an active
HV regulator at all, but use a scheme by which they modify the
deflection drive and geometry circuits to maintain proper geometery.
Look up the ,,,,, it's either a TA8859 or a TDA8859, one or the other.
This chip can give an RPTV, CRT based, VERY stable geometry. More
stable than you think.

Granted, CRT based RPTVs have their own set of problems, believe me,
but the thing is, with plasmas and other types of units, very few are
repairable out of warranty.

In fact a buddy of mine calls me the other day. Computer geekish, not
completely but close. Bought a Samsung DLP in May of this year. Went
out around Thanksgiving, and parts are not available. The ASC has been
out there twice.

So it boils down to if you get one of these things, when it breaks,
throw it away.

Phil, you might be in error abiout that. Magnetic focusing is very
rarely done on modern CRT based RPTVs. Mitsubishi did it, but some
still had an eletrostatic electrode.

Plus most already have dynamic focus. All that circuitry added up
together, it gets to be nuts. The 8859 chip works on the standard
Phillips bus, and has lots of functions, integrates and monitors HV
and beam current, modifies not only the drive to the pincushion
circuit but the vertical drive as well.

So it is my belief that the level of complexity is already there.

Now, to address the question of electrostatic deflection, I realize it
is a pain in the ass. Instead of four parameters, two more which could
almost be called axes night be focus and astigmatism.

And I do not understand how it could be true that the same small spot
size cannot be achieved elecrostatically. That is bacause it is
already in almost any CRT based set. While deflection is magnetic,
focus is usually electrostatic.

The only reason I can thing of for this is that with current
technology, there is a problem with designing the deflection plates,
which do operate as an element of the tube. The only reason that makes
sense is that they need to have a certain surface area, as such, they
must be longer longitudinally, therefore there is some variant in the
focus of the particles. This is only a theory. If it were the most
important point of this whole thing I might go find out. I might, but
not tonight. It is after midnight.

JURB

 

[isw]

[email protected] wrote:

--snip--

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

 

[isw]

[email protected] wrote:

--snippety-snip--

Now, to address the question of electrostatic deflection, I realize it
is a pain in the ass. Instead of four parameters, two more which could
almost be called axes night be focus and astigmatism.

And I do not understand how it could be true that the same small spot
size cannot be achieved elecrostatically.

If not for the requirement to deflect the beam, it could be.

With electrostatic (but not magnetic) deflection, there is an increase
in the momentum of the electrons -- they are accelerated sideways while
traversing the deflection plates.

Recall that inside the tube, the "beam" is not a constant small diameter
"rod" the size of the spot; it's really two cones, base-to-base, with
the largest diameter located at the focus electrode. The spot on the
screen is really an image of the cathode.

Because of the momentum change in the electrostatic case, the position
of an electron within the beam as it transits the deflection plates
determines how much it is deflected (the electrons are not "running
parallel", they are converging), and so the spot becomes more oval the
more it is deflected (and that is why electrostatic tubes must be so
long). But it's even worse than that, because the ellipticity of the
beam caused by the first set of plates makes the distortion caused by
the second set very much worse (which is why things are so bad in the
corners).

The distortion of the beam (astigmatism) caused by deflection is not
correctable, even in theory. There is another source of astigmatism in
CRTs, caused by cathode asymmetry, which *is* correctable, by a properly
asymmetric lens.

Isaac

 

[clifto]

With electrostatic (but not magnetic) deflection, there is an increase
in the momentum of the electrons -- they are accelerated sideways while
traversing the deflection plates.

I'll show my ignorance: why doesn't magnetic deflection cause an increase
in momentum? Seems to me the electrons would be accelerated sideways while
traversing the magnetic field.

 

[Tim Williams]

I'll show my ignorance: why doesn't magnetic deflection cause an increase
in momentum? Seems to me the electrons would be accelerated sideways while
traversing the magnetic field.

Del cross B ;-)

The force is perpendicular to the beam, so it just arcs around.

Tim

 

[isw]

I'll show my ignorance: why doesn't magnetic deflection cause an increase
in momentum? Seems to me the electrons would be accelerated sideways while
traversing the magnetic field.

Fundamentally, magnetic fields are conservative; you can't get work out
of them. If an electron came out of the field moving faster than when it
went in, work must have been done on it. That's why those
geometry-correcting magnets stuck all over CRTs don't "run down".

Think of it this way:

The effect of magnetic deflection is a function of the electron's
velocity -- a stationary electron will not be deflected (accelerated) at
all, and high velocity ones are deflected faster than low velocity ones
(because a moving electron acts like a current-carrying conductor).
Assuming a uniform field (and in a well-designed yoke, it's pretty
close), the math works out such that while a slower electron spends more
time in the field than a fast one, they both wind up ultimately being
deflected by the same angle.

With electrostatic deflection, the lateral force an electron feels is
unrelated to its forward velocity -- a stationary one will still be
accelerated towards the positive plate. The result is that slower
electrons spend more time under the influence of the plates, and so are
deflected more than faster ones.

Even if all the electrons in the beam left the cathode with precisely
the same velocity (and despite best effort, they don't), they'd still be
moving in different directions -- remember that cone? It's the "forward
vector", not the absolute velocity, that determines how long an electron
is influenced by the deflection field, so all electrons not on the axis
of the cone spend more time being deflected.

Isaac

 

[MakeNoAttemptToAdjustYourSet]

Fundamentally, magnetic fields are conservative; you can't get work out
of them. If an electron came out of the field moving faster than when it
went in, work must have been done on it. That's why those
geometry-correcting magnets stuck all over CRTs don't "run down".

Think of it this way:

The effect of magnetic deflection is a function of the electron's
velocity -- a stationary electron will not be deflected (accelerated) at
all, and high velocity ones are deflected faster than low velocity ones
(because a moving electron acts like a current-carrying conductor).
Assuming a uniform field (and in a well-designed yoke, it's pretty
close), the math works out such that while a slower electron spends more
time in the field than a fast one, they both wind up ultimately being
deflected by the same angle.

With electrostatic deflection, the lateral force an electron feels is
unrelated to its forward velocity -- a stationary one will still be
accelerated towards the positive plate. The result is that slower
electrons spend more time under the influence of the plates, and so are
deflected more than faster ones.

Even if all the electrons in the beam left the cathode with precisely
the same velocity (and despite best effort, they don't), they'd still be
moving in different directions -- remember that cone? It's the "forward
vector", not the absolute velocity, that determines how long an electron
is influenced by the deflection field, so all electrons not on the axis
of the cone spend more time being deflected.

Isaac

Don't forget to mention the relativistic effects.