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Old 05 March 2021, 03:02   #3
Bruce Abbott
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Join Date: Mar 2018
Location: Hastings, New Zealand
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A bit of convergence error at the edges of the screen is normal. Each electron gun aims at the screen from slightly different position, and the screen surface is not perfectly hemispherical, so the geometry is distorted in different directions for each color. This is corrected with a combination of adjustable permanent magnets around the tube, and dynamic correction by modulating the deflection waveforms. It is very difficult to get all 3 beams perfectly aligned, particularly in the corners of the screen.

As electronic components on the PCB and parts of the tube age the convergence often drifts further out of alignment. However if your 'almost 1 line' is only in the corners that may be how it has always been.

To check convergence over the whole screen and gauge whether adjusting the alignment might be necessary, display an image with a grid pattern of white lines on a black background. Where convergence is not perfect you will see the white separating into colored lines, like this:-
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