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  Monochrome Picture Tube

Introduction to Monochrome Picture Tube

Modem monochrome picture rubes use electromagnetic deflection and electrostatic focusing. A classic black and white picture tube is displayed in diagram the deflection coils are mounted externally in a particularly designed yoke which is fixed close to the neck of the tube. The coils while feed concurrently with vertical and horizontal scanning currents deflect the beam at a fast rate to generate the raster. The composite video signal which is injected either at the cathode or grid of the tube modulates the electron beam to generate brightness differences on the screen. This effect in re-construction of the picture on the raster, bit by bit, likes a function of time. Though, the information thus acquired on the screen is perceived through the eye as a complete and continuous scene due to the fast rate of scanning.

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The several electrodes which comprise the electron gun are displayed in diagram. The cathode is indirectly heated and contains a cylinder of nickel which is coated at its end along with throated tungsten or barium and strontium oxides. These producing materials that have low work-function and while heated allow release of enough electrons to form the essential stream of electrons inside the tube. The control grid that Grid no. 1 is preserved at a negative potential regarding to cathode and con oils the flow of electrons from the cathode. Though, in place of wire mesh structure, like in a conventional amplifier tube, it is a cylinder along with a small circular opening to confine the electron stream to a small are. The grids which follow the control grid are the known as accelerating or screen grid that is Grid No, 2.and the focusing grid that is Grid No.3. These are retained at dissimilar positive potentials regarding to the cathode that change between +200 V to +600V. Every element of the electron gun are linked to the base pins and receive their rated voltages-and from the tube socket which is wired to the several sections of the receiver.

The focus anode is greater in diameter and it is operated at a greater potential than the first anode. The resultant field configuration among the two anodes is like that the electrons leaving the crossover point at several angles are subjected to both convergent and divergent forces like they move along the tube's axis. This in rum changes the path of the electrons in like a way that they meet at other point on the axis. The electrode voltages are so selected or the electric field is so changed that the second point in which all the electrons obtain focused is the screen of the picture tube. Electrostatic focusing is chosen over magnetic focusing since it is not affected much more through changes in the line voltage and requires no ion-spot correction.

To provide the electron stream enough velocity to arrive at the screen material with proper energy to cause it to fluoresce, a second anode is involved inside the tube. This is a conductive coating along with colloidal graphite on the within of the wide bell of the tube. This coating, termed as aquadag, generally extends from approximately half-way into the narrow neck to in 3 cm of the fluorescent screen as displayed in diagram. It is connected via a specially given pin at the top or side of the glass bell to an extremely high potential of over 15 KV. The accurate voltage relies on the tube size and is approximately 18 KV for a 48 an monochrome tube. The electrons which get accelerated under the affect of the high voltage anode area, reach extremely high velocities before they hit the screen. Several of these electrons go directly and are not collected through the positive coating since circular structure of it gives a symmetrical accelerating field around all sides of the beam.

Due to extremely high velocities of the electrons that hit the screen, secondary emission occurs. If these secondary emitted electrons are not collected, a negative space charge gets created near the screen that stops the primary beam from arriving at the screen. The conductive coating being at a extremely high positive potential collects the secondary emitted electrons and so serves the dial aim of increasing the beam velocity and-removing not wanted secondary electrons. So the way of the electron current flow is from cathode to screen, to the conductive coating by the Secondary emitted electrons and back to the cathode by the high voltage supply.

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