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The cause of the dimming monitors is the CRT cathode wearing out. The most common type of CRT (and the type used in most NeXT monochrome monitors and all of the NeXT color monitors) uses what is called an oxide cathode. A thin coating of oxide is deposited on the cathode to allow the electronics which form the picture to get off the cathode easily. The oxide gradually boils off the cathode itself, and when the oxide is gone, the CRT goes dim.
Typically, the oxide will last from 10,000 to 20,000 power on hours (screen savers don't help the cathode, they only prevent phosphor aging). Unfortunately, the black monochrome monitors fall into the short end of the life range thanks to Toshiba who made the CRT's. The aging is more noticeable in Unix machines because they tend to be left on. Note that there are about 8,000 hours in a year. If you leave your monitor on all the time, all oxide type CRTs will be dim in three years.
The other type of CRT cathode is the I-cathode or dispenser type. This type of cathode is porous and continually brings new activation material to the surface. Its lifetime is 40,000 hours or more. The last of the NeXT monochrome monitors (N4000B) used this type of CRT and they don't go dim. There aren't many of that type around because NeXT quit the hardware business after producing only a few thousand. If you can get an N4000B monitor, you won't ever have to worry about a dim monitor.
Many manufacturers are going to dispenser cathode type CRTs in their monitors with Panasonic leading the way. The best advice is to turn off the monitor when not in use. If that is impractical, try to purchase one with the long life cathode.
Spherical Solutions ([email protected]) has a supply of new N4000B long life monitors for sale in either ADB or non-ADB configurations. If you need to repair or replace a monochrome monitor, that is by far the best type to use.
If you read this far, you probably know more than you ever wanted to about CRT aging, but I hope this helps.