Why did not TV CRTs use deflection plates?

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spinnaker

Joined Oct 29, 2009
7,815
The TV CRT project thread g0t me thinking about old TV CRTs and scopes. And I may have learned this eons ago but I have forgotten. Why did not TV CRTs use electrostatic deflection / deflection plates as opposed to magnetic deflection with a yoke? Was it the cost of producing such a tube?

Never had to do it on a scope but did scopes need to have the beam aligned periodically, similar to convergence adjustment in a TV? In fact, I don't ever remember doing any kind of beam alignment on a black and white TV. I am guessing with only one gun, the alignment was not as critical as color with 3 guns. Was it the same with scopes?
 

bertus

Joined Apr 5, 2008
20,053
Hello,

As far as I know, the delfection plates have a smaller angle of deflection.
There are coil based deflection systems that can reach 110 °.
A plate deflection may reach about 60 °, wich would make the CRT housing much deeper.

From the wiki:

Electrostatic deflection is very useful for small deflection angles but is well known to be inferior to magnetic deflection for deflecting a charged particle beam into large angles - say over 10 degrees. The reason is that deflection aberrations become large as the deflection angle increases. This reduces the ability to finely focus the beam. Also in electrostatic deflection it has long been the practice to inject the beam midway between the charged deflection plates so as to avoid the fringe fields as much as possible. However it was found by computation methods that deflection aberrations would be significantly reduced if the beam were injected offset toward the attracting plate. That way the beam tends to follow equipotentials and the deflection force is normal to the beam direction. Thus offset, all the electrons in the beam are deflected into the same angle. There is an induced astigmatism that is correctable. This deflection idea has been tested and verified. Deflection angles of 50 degrees are reportedly possible without measurable deflection aberration. Optimal injection offset is approximately 1/3 of the plate gap toward the deflecting plate. The useful beam diameter is also approximately 1/3 of the gap. [2]

https://en.wikipedia.org/wiki/Electrostatic_deflection

Bertus
 

nsaspook

Joined Aug 27, 2009
6,280
On a TV the deflection frequencies are fixed making it much easier to design optimized linear scan magnetic systems by simply altering the magnetic fields externally. Generating the required magnetic fields generally required more power than electrostatic scanning for the same deflection.

I worked on several magnetically scanned ion implanters 30 years ago with acceleration energies up to 160keV. To lock in and control linear scans across even a 5" wafer required a massive amount of power for the magnets.

That first 19 inch rack was mainly for scan magnet control.
varian_160xp_788155.jpg
Those were old school heavy iron transformer supplies that weighed a ton.

A electrostatic scanning machine used a lot less power but was a hell of lot more dangerous as it used high voltage transmitter type tubes for the scanner plate driver to supply the deflection needed at 300keV ion acceleration.
varian_350de_120569.jpg
Scanner and beam control panel.
 
Last edited:

DickCappels

Joined Aug 21, 2008
5,890
Yes, color display CRT monitors need a lot more care in chassis design and manufacturing process design than monochrome displays (do they still make them?).

Many kinds of color image tubes have been invented. Here I am referring only to RGB shadow mask and aperture grille tubes.

Not only does the color monitor have to make a very nice, clean monochrome image, but it must make three images that and make the three images (R,G,B) precisely overlap -a property referred to as convergence. That makes for very complicated magnetic and gun design, and it also puts limitations on ways of getting nice straight raster edges and crisp text. This requirement also forced careful design of the chassis, the production alignment processes. The aperture grills and shadowmasks that served to assure that the three beams landed (mostly) only on their corosponding colored phosphors also added a lot of issues relating to color purity, evenness of raster linearity, luminance and allowable spot size.

They finally mastered all that and pulling off some stunning changes required by standards making organizations (often urged on by labor unions) and then...what do you know? In the space of just a couple of years around 2000 the color CRT makers cut back on their engineering staff, transferring many to vaguely related work and began shutting down CRT work while increasing investment in LCD production techniques.


I serviced calibrated some of those early Tektronix CRT scopes in the 1970's -no transistors on the mainframe but sometimes in the plug-ins. Pretty simple screen adjustments as I recall.
 

crutschow

Joined Mar 14, 2008
23,353
In summary, TV crt's required large deflection angles (to keep the tube neck a reasonable length) at a fixed deflection frequency, which is best done by magnetic deflection.

An oscilloscope tube requires electrostatic deflection to achieve linear response for the varied and high required deflection frequencies (especially vertically) and the large required tube length for the smaller deflection angles electrostatic deflection can provide can be tolerated, since oscilloscope tubes are much smaller than TV tubes.
 

ian field

Joined Oct 27, 2012
6,543
The TV CRT project thread g0t me thinking about old TV CRTs and scopes. And I may have learned this eons ago but I have forgotten. Why did not TV CRTs use electrostatic deflection / deflection plates as opposed to magnetic deflection with a yoke? Was it the cost of producing such a tube?

Never had to do it on a scope but did scopes need to have the beam aligned periodically, similar to convergence adjustment in a TV? In fact, I don't ever remember doing any kind of beam alignment on a black and white TV. I am guessing with only one gun, the alignment was not as critical as color with 3 guns. Was it the same with scopes?
Very early TVs DID use CRTs with electrostatic deflection, and post war amateurs were buying up war surplus H2S radar tubes to build their own TV. The move to magnetic deflection probably had something to do with retrace time, which was irrelevant in PPI radar displays and not all that important in oscilloscopes.

The inductive system makes fast retrace flyback fairly easy.

As Bertus mentioned: electrostatic deflection CRTs aren't great for wide angle deflection - a decent screen area could make the telly nearly as long as your living room.
 

MisterBill2

Joined Jan 23, 2018
3,883
Very early TVs DID use CRTs with electrostatic deflection, and post war amateurs were buying up war surplus H2S radar tubes to build their own TV. The move to magnetic deflection probably had something to do with retrace time, which was irrelevant in PPI radar displays and not all that important in oscilloscopes.

The inductive system makes fast retrace flyback fairly easy.

As Bertus mentioned: electrostatic deflection CRTs aren't great for wide angle deflection - a decent screen area could make the telly nearly as long as your living room.
Indeed, and there is a circuit of an electrostatic deflection TV with a 7 inch screen presently available at another website. AND, electrostatic deflection can be MUCH FASTER than magnetic deflection could ever be. The problem indeed was CRT length. So I.F. is correct, except about the retrace part. But the collapsing magnetic field did work well for generating the high voltage needed for the CRT.
 

crutschow

Joined Mar 14, 2008
23,353
But the collapsing magnetic field did work well for generating the high voltage needed for the CRT.
I remember when I first learned about that technique, thinking what a neat bit of engineering to use the high flyback voltage from the CRT horizontal deflection, which was already there, to generate the HV for the tube anode.
Of course it did put a lot of stress on the horizontal deflection tube, which was the highest power tube in the set.
I remember the first color TV I bought (a Sylvania 19"), the horizontal tube had to be replaced every couple years or so, usually due to low emission if I remember correctly.
 

Ramussons

Joined May 3, 2013
797
There is one more reason. Magnetic deflection retains the relative distances between the electrons in the beam, whereas an electrostatic field distorts the beam. This result is that the beam loses Focus increasingly as the deflection increases.

The reason is that, in an Electrostatic Field, the deflection is Inversely Proportional to the Distance from the Charged Plate; In a Magnetic Field, the Deflection is Proprtional to the Electron Velocity.
 

KL7AJ

Joined Nov 4, 2008
2,218
The TV CRT project thread g0t me thinking about old TV CRTs and scopes. And I may have learned this eons ago but I have forgotten. Why did not TV CRTs use electrostatic deflection / deflection plates as opposed to magnetic deflection with a yoke? Was it the cost of producing such a tube?

Never had to do it on a scope but did scopes need to have the beam aligned periodically, similar to convergence adjustment in a TV? In fact, I don't ever remember doing any kind of beam alignment on a black and white TV. I am guessing with only one gun, the alignment was not as critical as color with 3 guns. Was it the same with scopes?
Yep.....with electrostatic deflection plates, a decent sized TV CRT would have a neck about 15 feet long. :)
 
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