Aviation Electronics Technician 1 - Historic Naval Ships Association

The SEC tube has applications in extremelylow-light, nighttime military TV systems where ahigh internal amplification and a fast speed orresponse to moving images are important.Picture TubesFigure 5-32.-SEC camera tube target operation.electron striking the target are emitted and stored(fig. 5-32).The photoelectrons possess sufficient energy topenetrate the thin, metallic signal and support plate.As they travel through the porous potassium-chloridelayer, many secondary electrons are emitted as thebeam strikes the interlined particles. Thesesecondary electrons either escape to the positivecollector screen or they travel through the spaces ofthe porous layer to the positive collector plate. Thisloss of electrons produces a positive charge on thescanned side of the target. Several hundred secondaryelectrons are emitted for each incident electron,producing a substantial gain at the target.The video signal is developed from the target bythe scanning beam discharging the positively chargedareas of the target in the same manner as in a vidicontube. This charging current, flowing out of the signalplate connection, is then amplified by an externalamplifier.A monochrome picture tube is a specialized formof the cathode-ray tube. An electron gun in the tubedirects a beam of electrons toward a fluorescentmaterial on the screen. The screen glows when struckby the electrons. Between the gun and the screen aredeflection coils that deflect the beam horizontally andvertically to form a raster. The brightness of thepicture is controlled by varying the grid-bias voltagewith respect to the cathode voltage. This bias can bechanged by varying either the cathode voltage or thegrid voltage.Color picture tubes operate on the same basicprinciple as monochrome picture tubes. Thedifference between the two systems is the types ofphosphors that coat the screen. The different types ofphosphors produce colors when struck with theelectron beam. Three basic or primary colors are usedin combination to produce all the other desired colors.These primary colors are red, green, and blue. In athree-gun color picture tube (fig. 5-33), there is aseparate gun for each of the color phosphors. Thetube’s screen consists of small, closely spacedphosphor dots of red, green, and blue. The dots arearranged so a red, green, and blue dot form a smalltriangle. The shadow mask provides a centering holein the middle of the triangle of dots. The convergenceelectrode causes the three separate electron beams tomeet and cross at the hole in the shadow mask.Figure 5-33.-Color picture tube.Figure 5-34.-Color picture tube. A. Side view. B. Front view.5-26