bagaki: Here's the response from the Sony FAQ website
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Description:
There Is A Thin Gray Line In Either The Top Or Bottom Section Of A Sony Trinitron Monitor.
Solution:
There is nothing wrong with the monitor, the thin line(s) is normal. The thin line is a shadow cast by the horizontal Aperture Grille Damper wire. The damper wire is used to stabilize the Aperture Grille, which is featured on all Sony Trinitron picture tubes.
The number of damper wires vary according to the size of the monitor. For example, 15" monitors only have one thin line while the 17" and 20" monitors have two thin lines.
Through the adjustment of the brightness and contrast on the monitor, the line(s) may appear less noticeable.
There is a mention of the horizontal Aperture Grille Damper wire in the Sony User's Guide which came with the monitor.
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The ONLY time I notice them is when I've opened a new Word document.