Occasionally the brilliance of the screen decreases to such an extent that the picture disappears. At these times the voltage on the grid of the tube increases. Sometimes when this fault
occurs the EY51, which is new, glows blue. I have checked the brilliance control and all the
capacitors and resistors in its associated circuit. I have also tested all the valves.-G. F. Winter
If the tube grid voltage varies, even though the voltage applied to the grid from the brightness control is constant, then there is little doubt that the tube is at fault. However, check the capacitor connected to the grid for insulation resistance.