Duo 280 darkening screens

[Tom and Lisa P]

I hve several Duo 280 screens that go dark in the corners after the 
screens are on for some time.  The last theory I heard about this 
problem was that it was the backlight going out over time.

To confirm this, I tried an experiment.  I removed the backlight and 
left the duo on for 4 hours one night.  At no time did the backlight 
seem to dim.  This would seem to indicate that a dimming backlight is 
not the sole problem.

Any other ideas ?

Mad Dog