Re: 922 with "tilted" wave form.


Mlynch001
 

On Mon, Apr 30, 2018 at 02:35 pm, Fabio Trevisan wrote:


Hello Mlynch,
What you describe looks like a "geometry" or Y-AXIS misadjustment.
If I understood correctly what's the equipment you have it's a T922
oscilloscope.
Looking at the Manual that is available on the TekWiki website (here:
http://w140.com/tek_921_922.pdf ), the adjustments are Y AXIS, R474 and
"GEOMETRY", R473, both are on the CRT & VERTICAL AMPLIFIER diagram, located on
the A1 INTERFACE BOARD.
Manual probably describes the correct procedure to adjust both of them, and
this is usually done in the very beginning of the calibration procedure,
meaning that most of the other calibrations are impacted to some extent by
these 2, as they change the shape of what's being shown on the screen.
Steps usually cover the following:
1. First of all, adjust the trace rotation (adjustment accessible from the
side of the scope), for a horizontal baseline (just sweep, no vertical
signal).
2. Then, using the X-Y mode, apply a signal to the Y channel enough to span
about 6 divisions height, leave the X-channel with no signal (set X input to
to GND). This will draw a vertical line on screen... Adjust now R474 so that
the vertical line, aligned to the central vertical graticule is perfectly
vertical.
3. Finally, change the scope back to normal mode, and apply a square wave to
one of the vertical channels, that is big enough to span 6 divisions
vertically, and set a sweep speed that you see about 20 cycles horizontally
accross the screen.
This setup will pretty much "fill" the screen with a square wave, from bottom
to top, left to right. Now you can adjust the geometry adjustment (R473) to
correct any "Pillow" or cushion distortions of the screen, so that the overall
shape of the displayed waveform is all "squared"...
4. The geometry adjustment, if it requires too big re-adjustment, will have
some side-effects to the Trace rotation and the Y-axis adjustment... Repeat
then Trace rotation and Y-Axis again, to make sure that the screen remains
aligned on the horizontal and vertical directions.

All that, above, assuming that - overall - the scope doesn't present other
misbehavior... However, the procedures above cannot assure you that everything
else is or will remain correct.
Since re-adjusting the Y axis and Geometry may have some impact on the size of
the waveforms displayed, by touching those adjustments, be aware that you may
need to readjust Vertical Gain and Horizontal gain... and this will require
that you have means to adjust those, or accept that they may become "off" by
some amount.

Rgrds,
Fabio
Fabio,

FIXED! Followed your instructions and everything is exactly as it should be. The little scope performs well and has a beautiful bright trace. I am pleased with my $20 bargain. Even after buying the resistor array, I still only have $50 invested, The scope is worth twice that for parts.

THANKS AGAIN!

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