TUBE REACTIVATION
The most common fault in picture-tubes is low emission, where the emissive coating on the cathode cannot release sufficient electrons to provide a bright sharp picture. Very often a reactivation process can be successfully applied by overrunning the heater for a few minutes and applying a positive potential to the grid with respect to the faulty cathode. The resulting heavy current disrupts the surface of the cathode, cleaning it and exposing a new surface. Sophisticated machines (see Chapter 23) are able to give controlled reactivation. Other possible gun faults are short-circuits between electrodes – which will turn the afflicted gun hard on, very often on a sporadic basis; and (rarely) an open-circuit heater or electrode, which is not curable except by rebuilding the tube with a new gun assembly. Inter- electrode shorts not involving any heater can often be blown clear by application of high current and voltage, typically from a charged capacitor.
HANDLING PICTURE-TUBES
The atmospheric pressure of the faceplate of a 51 cm tube is around 1600 kg, and over the total surface area is over 4000 kg. Sudden fracture of the glass results in very dangerous implosion, in which jagged fragments of glass can be hurled five metres or more if the tube is not fitted in its cabinet. Always wear protective goggles or a face mask and preferably gloves as well, when handling tubes. Hold the tube vertically screen down with both hands, one under the screen and one steadying the neck with small tubes, one hand on each side of large tubes. Place face down on a soft surface to avoid scratching, and always fit a shorting strap between anode connector, external graphite coating and rimband when removing or handling tubes: this avoids electric shock from the stored e.h.t. potential, particularly dangerous when carrying a tube.
TUBE PROJECTION DISPLAYS
The ‘home-cinema’ industry, and its enthusiastic customers, along with pubs, clubs and discos, have brought about a demand for larger screens than can be afforded with direct-view tubes. In a projection TV using thermionic tubes there are two choices: back-projection, in which the light sources and the (‘ground-glass’ type) screen are together in a large console cabinet; and front-projection, where the projector and the viewers are on the same side of large reflective screen, which – for CRT-type projectors – is concave to increase the reflective ‘gain’ of the screen at the expense of viewing area: seen from a large angle the screen is quite dim. Here the projector is either ceiling-or table-mounted.
For either configuration there are one each of red-, green- and blue- emitting tubes, typically 13 cm in diameter and running beam currents of several milliamps peak from their precision narrowbeam electron guns: this calls in some cases for cooling of their glass face- plates by either liquid or forced air. The tubes are mounted in line abreast and scanned in synchronism by identical deflection yokes, effectively connected in parallel to the common line and field output stages. The central tube (e.g. green) produces, via the optical system, a truly rectangular raster, but the images from the outer tubes – whose axes are necessarily neither coincident nor parallel to the centre tube – are distorted into a keystone shape on screen without correction. It is compensated for by modifying the deflection currents in the scan yokes of the outer pair: in modern sets these registration correction characteristics are held in EEPROM memory and implemented by bus-controlled scan-correction ICs and drivers working on the deflec- tion coils of the two outer tubes. Each tube has its own focus and first-anode voltage presets, plus video-drive adjustments for grey- scale tracking – in sophisticated sets they are automatic in opera- tion. More of this in later chapters of this book. In some ways, and especially in regard to fault-diagnosis and setting up, the three individual tubes correspond to the three guns of the shadowmask and Trinitron tubes already described in this chapter, and the rest of the projection set to the TV receivers which are the subject of the first half of this book.
In front of each of the projection-tube faceplates are optical systems to collect the divergent coloured light and concentrate it into a forward beam. It may consist of a lens or a Schmidt reflector, the latter (in expensive and sophisticated systems) built into the tube itself.