Quantum Data 881 - Manual (Page 1181)

881/882 Video Test Generator User Guide (Rev A.22) 1065
Measure the width of the image at the top and bottom of the display. Any difference in
readings should be within the specification limits. Measure the height of the image at both
sides of the display. Again, any difference in readings should be within specification limits.
If either of the differences is out of specification, the trapezoid distortion of the monitor is
out of specification.
Add or adjust magnets on the yoke to correct the problem. The pin and barrel distortion
correction should be repeated to make sure that it is still in specification.
Test Pin and barrel distortion correction
Purpose If perfectly linear sweep signals are sent to a perfectly wound deflection yoke mounted on
a perfect CRT, you would not necessarily get a perfectly formed raster. Instead you would
likely get a raster that had its corners stretched away from the center, resembling a pin
cushion. This distortion occurs because the geometry of the deflected electron beam does
not match the geometry of the tube face plate. Also, imperfections in the yoke or CRT may
affect this problem. In some cases one or more corners may be pulled towards the center
of the raster causing it to look like a barrel. Uncorrected raster distortion carries over as
distortion of the displayed image.
Method A slot gauge may be used to determine if the amount of pincushion or barrel distortion is
within limits. A basic slot gauge may consist of a piece of opaque film with at least two (2)
transparent slots in it. One slot is used for top and bottom distortion and the other is used
for the sides. By positioning the correct slot over each portion of the border line, the entire
line should be visible. If this cannot be done at all four sides, the monitor requires
correcting.
There are two main ways of correcting pincushion distortion. The first involves placing or
adjusting magnets on the yoke. This is a trial-and-error method. However, skilled
operators develop a feel for how strong a magnet to use and how to place it in order to get
the desired correction. If any correction is performed, the trapezoid distortion correction
should be repeated.
The other correction method involves adding correction signals to the deflection signal
driving the yoke. This method is usually found in color monitors, where adding magnets to
the yoke would cause problems with convergence and purity. The type and number of
adjustments depends on the monitor being tested.