Magnetic Beam Focusing on a CRT

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This short video, the first for this year, explores magnetic electron beam focusing. It showcases an early method employed in electron guns to generate the necessary sharp dot on cathode ray tube screens. Two relatively large magnets are positioned on the tube neck and mechanically moved towards or away from each other to modify the electron path, bringing it into focus on the screen. The assembly is situated over the cylindrical final anode on the electron gun. Typically, the magnets are held in position within an aluminum housing, allowing movement only along the axis of the tube neck. In the presentation, they are displayed without this housing, influencing beam deflection, which would not normally occur.
Additionally, an alternative method is demonstrated using one permanent magnet and one electromagnet. This eliminates the need for mechanical adjustment, as varying the voltage applied to the coil will alter the field strength and achieve the same focusing technique. However, this approach is relatively energy-intensive compared to later electrostatic focus methods. While it adds complexity to the electron gun design, it eliminates the requirement for bulky external magnetic assemblies on the tube neck.
The CRT featured in this video is manufactured by Cinema Television Ltd (Cintel) and was designed for early admiralty radar use. The screen diameter is just 12cm.

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Опубликовано:

3 фев 2024

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Комментарии : 5

@rustymotor 8 месяцев назад

Excellent demonstration of the CRT, thanks for making this video!

@Qyonek 8 месяцев назад

Thanks!

@T2D.SteveArcs 8 месяцев назад

Hey mate, great demo 😁.. i was told that earlier radar system had rotating deflection coils? Sounds odd as you could do this electric ally and avoid the headache of slip rings or whatever they would use.. ps giz them focus magnets 🙏😂 ive got an electrostatic deflection tube (circular green long persistence) that doesnt seem to have a focus electrode😞, i assume it needs these magnets, i might be able to take a couple of large ferrite cores and wind something to do it electrically.. do you know the orientation of the poles? ... Steve

@tuopeeks 8 месяцев назад

Good question, I needed to set that up again just to verify it. Strange to have a tube with electrostatic deflection but magnetic focus, but anything is possible. I knew the two ring magnets are set as opposing from when I used to dismantle TV with these in them but didn’t know the poles. It looks like it works with either but for a small distance between magnets north poles opposing creates the correct focus. The magnet construction in these are uniform poles on each face, the same as the construction of the circular magnets used in oven magnetrons. However, the aperture in these is too small for the neck on these old tubes. These magnets are hard to find. Not only have the TV that used them been out of production for over 50 years. They smash into bit easily, just dropping one is enough. It will be possible to wind two coils to create the field electromagnetically. I think you might be right with very early radars using a mechanical system to synchronise the sweep with the aerial but never seen one, well before my time. 🙂

@T2D.SteveArcs 8 месяцев назад

@@tuopeeks okay thanks for that mate, old CRTs are really cool, its really quite surprising to me that they are already getting hard to find despite the colossal numbers produced and their relatively recent redundancy..