Quantum Data 801GC, 801GF, 801GX manual

Test	Pin and barrel distortion correction
Purpose	If perfectly linear sweep signals are sent to a perfectly
	wound deflection yoke that’s 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 and
	resembled a pincushion. This distortion occurs
	because the geometry of the deflected electron beam
	does not match the geometry of the tube faceplate.
	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 needs
	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 test 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.

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