Advice for Tek 4051 vector-drawing issue?

[Josh Dersch]

Inherited a Tektronix 4051 this afternoon (really, really cool old 
machine - 6800 CPU, vector storage-tube display) and after cleaning it 
out and reseating the socketed chips it appears to work almost 
correctly.  (Even the tape drive works after cleaning the head... amazed 
that the pinch roller hasn't turned to goo.)

The one issue is that "long" vectors do not get drawn as straight lines 
-- they end up curving in the direction of the destination.  I've taken 
a photo to demonstrate:

http://yahozna.dyndns.org/computers/tek4051/unvector.jpg

The above is _supposed_ to be drawing a line from 0,0 to 100,100 and 
back, but as you can see, it's not really doing a very good job.

Short vectors seem to draw OK, as do perfectly vertical and horizontal 
ones.  Text gets drawn fine.  I have the schematics but I've never dealt 
with a vector-based display before (only other vector display I have is 
in my Vectrex, and I haven't had to tweak that one yet.)  Any ideas 
where to start?  I figure the D/A converters on the CPU board are 
working correctly since text positioning works, and the endpoints of the 
vectors seem to be correct.  Bitsavers has the schematics at:

http://www.bitsavers.org/pdf/tektronix/405x/070-2286-00_4051_Service_Vol2_May77.pdf, 
(see page 92 for the start of the display schematics.)

And a higher-level overview of the functionality at:
http://www.bitsavers.org/pdf/tektronix/405x/070-2065-00_4051_Service_Vol1_May77.pdf 
(See page 147 for the start of the display overview.)

Thanks!
Josh