CRT question, partly curiousity, engineering wise

Magnetic deflection can bend the beam at a sharper angle than the electrostatic deflection.

Look inside an old Tektronix scope, which uses electrostatic deflection. That neck is LONG because the angle of deflection for physically realizable voltages is so small.

Forgot to mention the obvious: The required Electrostatic deflection voltage goes UP as the HV goes up, because the electron spends less time in the gradient between the plates

Anybody out there confirm the following statement? In contrast, the required Magnetic deflection goes DOWN as the HV goes up because the electron is moving faster through the field and gets 'bent' more.

Nope. :)

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snipped-for-privacy@repairfaq.org (Samuel M. Goldwasser) wrote in news: snipped-for-privacy@repairfaq.org:

and with a long neck and electrostatic deflection,the CRT was amazingly sensitive to Earth's magnetic field,thus requiring shielding. Telequipment scopes were infamous for their poor trace rotation calibration,they would never hold the setting.Move the scope,and you had to readjust the TR.

it depends on the electron gun structure. the TEK 2465 tube had specially constructed deflection plates and a focusing "lens" instead of the mesh lens typically used. It was designed to use low deflection voltages that could be provided by hybrid ICs instead of discrete defl.amps with HV transistors. the 2465 tube was fairly short,too. I used to have a booklet that described the CRT tech for the 2465 tube,lost it when the Orlando Service center was shut down.

Alas,all that technology is gone now.TEK no longer makes it's own CRTs.

I fail to see how a "faster" E-beam will bend more under magnetic fields,but not under electrostatic fields.

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Jim Yanik
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Electrostatic deflection is fast, which is why it was used in CRT scopes. However, the spot size is the pits by comparison--magnetic deflection and magnetic focusing gives a much, much sharper spot.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

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.

-- 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).

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

Actually, I think Eidophor color used a rotating wheel.

The Eidophor system is more than 50 years old. GE glommed onto it back in the 70s (I believe), and produced a very compact projection system (far smaller than the original), though I don't know if it was commercially successful. GE also designed a video recording system -- "photoplastic recording" -- that froze the Eidophor ripples on a moving strip of plastic. It was never commercialized.

You're right. The beam, doesn't get bent more it gets bent less. That's why CRT TVs bloom when the APL goes up. More beam current, drooping EHT, beam slows down and spends more time in the deflection field, bends more and the picture gets bigger. Down with CRTs. May they rot in the landfill. May we have something with as good color though?

G=B2

I think most of the explanations given here are dead wrong, but it's been a while since I've studied this stuff, and I don't want to get into it too deeply.

GE, just before they closed their own television factories in the mid

80s, planned to bring out a home rear projection television using the oil technology. The techs, at a training seminar for one of their other chassises, were told that we should sign up for a training session 6 months hence. The Roanoke factory was closed before the seminar was scheduled. Chuck-

There were both kinds: a monochrome one with a wheel (mostly for computer output purposes), and a three-beam one -- R, G, B -- which worked better for "regular" TV.

The three-color ones were capable of producing a 65-foot-diagonal image in which you could see the NTSC color subcarrier crawling up the screen (demonstrating really good bandwidth and focus). They were also the only imaging devices of that time that could properly display the "I" and "Q" bars of a composite color bar test pattern. That was because the light source was a full-spectrum arc, and dichroic filters were used to separate the primaries, so it was possible to select the three primary colors with great precision (you can't do that if you start with phosphors).

Isaac (who once made an emergency air trip from LA to Houston because somebody had let one of the dumpster-sized things swing 30' into a concrete wall while it was being hoisted into place)

You need quite a lot of voltage to swing the beam. Vertical is not so tough, but a linear amp to swing maybe 500+ Volts at horizontal frequency and have high enough bandpass for the horizontal retrace is ALSO going to burn a good deal of power. The magnetic sweep system is mostly a switching power supply, and so gets a good reduction in power dissipation that way.

Jon

Right, in the high-end Tek scopes, they had a tapped delay line inside the CRT, and so the deflection voltage followed the beam as it shot toward the screen. That should tell you the time spent between the deflection plates is in the tens of ns!

Umm, no, I don't think so. But, I'm not a whiz at electrodynamics.

Jon

Early TVs did use electrostatic deflection, but the beam quality (and hence the spot size) is far better with magnetic.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

The transverse momentum you get from electrostatic deflection is proportional to deflection plate voltage. The longitudinal momentum is proportional to accelerating voltage (for color phosphors, 25 kV typical). So, tangent of deflection angle is at most Vdeflection/Vacceleration. Even infinite deflection voltage only gets 90 degrees deflection.

For magnetic deflection, the beam acquires a curvature proportional to B fi= eld;=20 the deflection angle can be anything you want. When it gets to 360 degrees= , your tube becomes a 'cyclotron'. I've seen multigigavolt beams doing loops, at near the speed of light, for hours. You can't do that with elec= trostatic deflection. you can get any deflection you want, no limit

electrostatic

Also, electrostatic deflection defocuses and aberrates the beam, whereas magnetic doesn't.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net