On most TV's sub-brightness can be adjusted by an engineer via service mode within hidden menus on the handset, for which you would require the access code or button sequence.
There is a often a screen pot inside, usually situated on the flyback transformer (EHT). Sometimes these get a dead spot after several years, causing low brightness. However, the screen adjustment should be set to the level indicated in the service manual and not used as a sub-brightness control.
Cobweb, December 2004
