VFD burnout?

[rpm1200]

The discussion of Adam's early GE digital clock radio got me thinking about one of my clock radios that has a vacuum fluorescent display (VFD). It's a Panasonic with a mirrored face, mid-80s. The clock digits are all becoming very dim. On the low brightness setting it is pretty much impossible to read, on the high brightness setting it is visible in low light. However, the radio dial (which I believe is part of the VFD) and the small indicator that lights when you set the time are still bright. Are the phosphors wearing out on the VFD or is there an electrical problem? It does appear that the segments that get the most use are the dimmest. Should I check voltages and if so, what are typical voltages I would expect to see on the various display pins? Is it possible to rejuvenate these displays?
Thanks!