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980 DP 1.4 Video Generator / Analyzer - User Guide
16.117.13
Method
Measure the lengths of the two (2) diagonal lines. Any difference is an indication of parallelogram distortion. The
difference in readings should be within the specifications of the monitor.
If the difference in the readings is too far beyond specification, the monitor should be rejected and sent back for
repair, rather than trying to magnet a defective yoke.
16.117.14
Test
Trapezoid distortion correction.
16.117.15
Purpose
This image gives you a way to measure trapezoid distortion in your monitor. If the distortion is not too severe, you
may be able to correct it by adding or adjusting magnets on the yoke.
16.117.16
Method
Perform the yoke winding orthogonality check and parallelogram distortion check first to avoid wasting time on a
monitor with a defective yoke.
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.
16.117.17
Test
Pin and barrel distortion correction.
16.117.18
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.
16.117.19
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
Page 433
Rev. A1
June 16, 2017
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