10 Feb
2009
10 Feb
'09
8:36 p.m.
Ha, whoops -- disregard #1 below... I was looking at the wrong section
:). Some days...
Josh
Josh Dersch wrote:
Philip Pemberton wrote:
Josh Dersch wrote:
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
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.