Further Tektronix 4014 debugging...

[Josh Dersch]

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