CRT... normally in the monitor setup accesible from the monitor itself has a special option... in mine Video Level... there're two 1.0V and 0.7V
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the 1.0V is dimmer than 0.7V.
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another way is change your contrast too other than brightness.
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if your CRT has a monitor software installed or you have special software for your graphic card then change the settings from within that software. choose default setting and try changing the settings there to suit you.
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if this wont work repair the monitor. no other reason possible.