There is a Screen Adjustment control on the High-Voltage transformer
(flyback) that controls the background brightness. Just take the cover
off, turn the set on and display your normally dim picture. Turn the
bottom control knob on the transformer and watch the screen. Adjust it
so that you can just barely see the grey around the sides of the
raster.
Note: the flyback is the black transformer with the big red wire going
to the top of the picture tube.
Hope this helps...
John