PURITY ADJUSTMENT
When the colour tube is in operation the origin of the beams (so far as the shadowmask is concerned) is the deflection centre, a point in the tube neck at the centre of the deflection coil assembly. The electron beams have to be manipulated to bring them to the correct deflection centre, and this is achieved by a pair of ring magnets (on the neck of the tube) whose two-pole field can move all three beams together sideways to correctly align the trajectories of the beams through the deflection centre. Adjustment of these purity ring magnets will give correct purity at screen centre, but at large deflec- tion angles (i.e. towards screen edges) some impurity may be present. To eliminate this the deflection centre itself must be aligned axially by sliding the entire deflection yoke assembly along the tube neck, fixing it in position where full-screen purity is obtained. In practice there is a small ‘sliding range’ over which good purity obtains – for
best tolerance and to provide for various drift influences the yoke should be fixed in the middle of this range.