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The raster on this set flashes white two or three times then
stays white, with the picture in the background and
prominent flyback lines. The brightness control has no
effect on the white screen.
The trouble is often due to heater -cathode leakage in the
tube in these sets. To check this, measure the three cathode
voltages on the tube base. If these are low, remove the tube
base and see whether the voltages on the base return to
normal. If so, the tube is almost certainly at fault, and fitting
a 6.3V heater isolating transformer will overcome the
problem. If the tube is not at fault, check the two transistors
(luminance output) on the right side of the tube base and the
associated voltages.

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