Advice for Tek 4051 vector-drawing issue?

[Philip Pemberton]

Josh Dersch wrote:
> 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.

If the endpoints are correct, then the DACs are probably OK. That leaves the 
deflection system, and the two filters (one X, one Y).

Take a look at 2-12 (#81 of 228) from service manual volume 2.

To rule out deflection, I'd get an oscilloscope (a Tek 454 or similar should 
be more than enough), set it to X-Y (Lissajous) mode, and wire it up to the X 
and Y signal outputs from the filters -- there are two test points shown on 
the schematic for this (TP X and TP Y).

If the display on your scope is still non-linear, go looking for faults in the 
display filters.
If the display looks good on the scope, I'd start looking for issues in the 
CRT deflection circuitry.

If you're going to hook it up to a scope, I'd try and get it to produce an 
alignment pattern that checks 45-degree lines drawn "both ways" so to speak. 
Either something like the Atari Asteroids self-test pattern, or even just a 
cross inside a square. Maybe something like this?

110 MOVE 0,0
# draw square forwds
120 DRAW 0,100
130 DRAW 100,100
140 DRAW 100,0
150 DRAW 0,0

# X forward-slash
160 DRAW 100,100
170 DRAW 0,0

# X back-slash
180 MOVE 0,100
190 DRAW 100,0
200 DRAW 0,100

# draw square backwards
210 DRAW 0,0
220 DRAW 100,0
230 DRAW 100,100
240 DRAW 0,100

# back to the start
250 GO TO 110

That's assuming the syntax of the DRAW command is "DRAW x,y", 0,0 is the 
bottom-left co-ordinate, and 100,100 is the top-right...

The idea of this is that you can tell at-a-glance if the vectors are 
converging properly in both directions in all axes.

-- 
Phil.
classiccmp at philpem.me.uk
http://www.philpem.me.uk/