Quite often a spot is left on the tube face for about thirty
seconds after the set is switched off. First the picture
collapses in the form of a shrinking square, then the spot is
left at the centre.
Switch -off spot suppression is effected by returning the
brightness control to chassis via a v.d.r. – 2VDR1, type
E299/DD/P336. Check this, and the associated resistors
2R40 (330k2) and 2R39 (220k52). An ageing tube may give
a spot glow even if the suppression system is working.