CATHODE RAY TUBE 



per ampere-turn in the winding. A deflector coil assembly is typically a 

 slotted annulus of magnetic material into which the windings are recessed, 

 which fits over the tube neck as shown in Figure 32.25. The arrangement of 

 windings may be easier to see in Plate 32.2, but here the annulus is not 

 slotted. For other and simpler forms, consult, e.g. Cocking^. Alternatively, 

 new slotted annuli in Ferroxcube may be obtained from Messrs Mullard Ltd. 

 (e.g., FX1154). 



Magnetic cathode ray tubes proper are singularly free from faults; most 

 of the display distortions are caused by imperfections in the deflector coil 

 assembly. An important tube fault is: 



Biaxial pin-cushioning (Figure 32.26). A given deflector coil current 

 produces a certain beam deflection angle {of. tangent of angle in electrostatic 

 tubes) and hence if the deflection distance is to be proportional to the 



(a) 



(b) 



Figure 32.27 



deflecting current the screen ought to lie wholly on the surface of an imagi- 

 nary sphere with centre to centre of the coil assembly. Such a curved screen 

 would be unsatisfactory, and practical screens are flatter than this, and in 

 consequence the length of the beam increases with deflection angle, to produce 

 greater sensitivity at the screen periphery, giving the grid of Figure 32.26. 

 The effect may be allowed for by designing the deflector coils so that the 

 fields are not uniform {Figure 32.27a) but are stronger near the tube axis 

 {Figure 32.27b). 



Deflection defocusing — This is a loss of sharpness of the spot as it is 

 deflected away from the tube axis. It is attributable partly to the change in 

 beam length due to the flat screen and partly to poor deflector coil design. 

 The fringes of the deflecting field contain components which act along the 

 axis of the tube to modify the focusing conditions {Figure 32.28). The 

 remedy is to have deflector coils which are long along the tube axis, so that 

 fringe effects are rendered as far as possible relatively insignificant ; also to 

 bend the ends of the deflector coils away from the tube {Figure 32.29). 



Lack of similarity between the two Y deflector coils can produce a grid of 

 the type shown in Figure 32.30a, and between the X coils. Figure 32.30b. 

 Insufficient perpendicularity between the X and Y fields can produce 

 orthogonality errors {Figure 32.31) though this is unlikely to occur if the 

 coils are accurately located by fitting tightly the slots in the moulded core. 



463 



