The picture is normal when the set is first switched on, but
after ten minutes it begins to climb up from the bottom until
the bottom of the picture is an inch or so above the lower
edge of the tube face. The top and the rest of the picture are
not affected in any way. The field output transistors have
been replaced – that cured the trouble for a time – but six
weeks afterwards the trouble returned.
We suggest you check the field charging capacitors 5C30
and 5C31, and the field driver transistor 5VTIO (AC128).
More remote possibilities are 7C5 (400µF) on the
convergence panel – it forms the field scan current earth
return path via the convergence circuits – and the field
output stage bias stabilising thermistor 5TH1 (VA1034).
(Rank A823 chassis.