This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
A monitor which has a picture that is very dark and cannot be adequately set with the user brightness and contrast controls may need internal adjustment of the screen (the term, screen, here refers to a particular electrode inside the CRT, not really the brightness of the screen you see, though it applies here), master brightness, or background level controls. As components age, including the CRT, the brightness will change, usually decrease. The following procedure will not rejuvenate an old CRT but may get just enough brightness back to provide useful functionality for a few months or longer. If the problem is not with the age of the CRT, then it may return the monitor to full brightness. The assumption here is that there is a picture but the dark areas are totally black and the light areas are not bright enough even with the user brightness control turned all the way up.
In most cases, the cover will need to be removed. The controls we are looking for may be located in various places. Rarely, there will be access holes on the back or side.
The controls may be located on the:
* flyback transformer. Usually there is a master screen control along with a focus control on the flyback transformer. The flyback (or L.O.P.T. for non-U.S. readers) is the component that generates the high voltage for the CRT - it has the fat red wire attached to the CRT with a thing that looks like a suction cup.
* a little board on the neck of the CRT. There may be a master screen control. a master brightness control, a master background level control, or individual controls for red, green, and blue background level. Other variations are possible. There may also be individual gain/contrast controls.
* main video board is less common, but the background level controls may be located here.
Display a picture at the video resolution you consider most important which includes both totally black and full white areas which also includes sharp vertical edges.
Set the user brightness control to its midpoint and the user contrast control as low as it will go - counterclockwise.
Let the monitor warm up for at least 15 minutes so that components can stabilize.
If there is a master brightness or background level control, use this to make the black areas of the picture just barely disappear. Them, increase it until the raster lines just appear. (They should be a neutral gray. If there is a color tint, then the individual color background controls will need to be adjusted to obtain a neutral gray.) If there is no such control, use the master screen control on the flyback. If it is unmarked, then try both of the controls on the flyback - one will be the screen control and the other will be focus - the effects will be obvious. If you did touch focus, set it for best overall focus and then get back to the section on focus once you are done here.
If there are individual controls for each color, you may use these but be careful as you will be effecting the color balance. Adjust so that the raster lines in a black area are just visible and dark neutral gray.
Now for the gain controls. On the little board on the neck of the CRT or on the video or main board there will be controls for R, G, and B gain or contrast (they are the same). If there are only two then the third color is fixed and if the color balance in the highlights of the picture was OK, then there is nothing more you can do here.
Set the user contrast control as high as it will go - clockwise.
Now adjust each internal gain/contrast control as high as you can without the that particular color 'blooming' at very bright vertical edges. Blooming means that the focus deteriorates for that color and you get a big blotch of color trailing off to the right of the edge. You may need to go back and forth among the 3 controls since the color that blooms first will limit the amount that you can increase the contrast settings. Set them so that you get the brightest neutral whites possible without any single color blooming.
Now check out the range of the user controls and adjust the appropriate internal controls where necessary. You may need to touch up the background levels or other settings. Check at the other resolutions and refresh rates that you normally use.
If none of this provides acceptable brightness, then either your CRT is in its twilight years or there is something actually broken in the monitor. If the decrease in brightness has been a gradual process over the course of years, then it is most likely the CRT. As a last resort (untested) you can try increasing the filament current to the CRT the way CRT boosters that used to be sold for TVs worked. Voltage for the CRT filament is usually obtained from a couple of turns on the flyback transformer. Adding an extra turn will increase the voltage and thus the current making the filament run hotter. This will also shorten the CRT life - perhaps rather drastically. However, if the monitor was headed for the dumpster anyhow, you have nothing to lose.