Advice for Tek 4051 vector-drawing issue?

[Josh Dersch]

Philip Pemberton wrote:
> 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.
Thanks for the suggestion!  Made two discoveries:

1) The schematics on Bitsavers do not correspond to the revision of the 
machine I have (mine appears to be earlier).  I have a printed set of 
them, so I'm OK (I mistakenly assumed both were the same).  There are no 
X/Y test points on my Tek's display board, but it was easy enough to 
hook up to the X/Y connector (J55, for those playing along at home).

2) Having done so, it looks like the display filters are at fault.  The 
oscilloscope display is distorted in exactly the same way as it is on 
the Tek's display.

Now to start debugging from here... (and maybe I should get these 
schematics to Al for archiving...) would dried up capacitors be a likely 
culprit?

Thanks,
Josh