G.E.C. BT1743

For the first thirty minutes after switching on the picture on this set fills the screen correctly. After this period of time however, a dark margin, about 1″. wide, develops down each side of the
picture. I have checked the frame output valve by substitution but without improving the situation. -H. V. Powell (Lydbrook, Gloucestershire).
Since the width of the picture decreases, the
trouble lies either in the line or h.t. circuits, and
not in- the frame section. First check the line
time -base valves for emission, including the
booster diode, and replace if less than 75%
good.
If the valves are in order, check the h.t. rectifier and replace if the d.c. output voltage is less
than a.c. input voltage.