Further Tektronix 4014 debugging...

So I may have been a bit premature in my declaration earlier this week that letting the 4014 warm up for a few minutes solves the storage problems. For the time being, I have it hooked up to a Linux box (so I can 'cat' various files at it and stare in awe as it draws random things) and it seems to be performing flawlessly; everything works (including the discrete plot extensions). But I've noticed that over time as I clear the screen that garbage starts accruing around the edges of the screen -- only the middle gets properly erased. At first power-on, the area that gets cleared is a circle maybe 10" in diameter; this increases slowly over time and if I let it run for 15-20 minutes *most* of the screen gets cleared but there's always a bit on the edges that remains. I went through the portions of the alignment procedure outlined in the service manual related to storage, and all voltages were within a percentage point or two even after all these years, so not much required adjustment. There are two adjustments for the Collimation that control the size and shape of the flood that erases the screen; the service manual suggests adjusting these until the flood covers the screen. Adjusting the pots all the way counter-clockwise causes the flood to get *slightly* larger and cover more of the screen, but it's still not enough. From reading the circuit description for the erase circuits (starting on page 5-82 of the service manual), I note that the duration of the flood (as controlled by the Collimation circuits) is controlled by an RC network and I suppose it's possible that one or more of the capacitors is out of spec -- but I don't know if the length of the flood has anything at all to do with the area it covers -- can anyone shed some light on this? I suppose it's more likely that the tube's just showing its age. I suppose I should be happy it works as well as it does. At any rate, if anyone has any insight here, I'd love to learn more... - Josh