View Single Post
Old 5th Nov 2017, 11:57 pm   #7
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Question about scanning and "sync pulses"

I could have mentioned that even if the very slight down tilting of the Horizontal lines was bigger, it wouldn't matter. The H scan lines can be made perfectly horizontal by rotating the yoke on the neck of the tube.

When you get used to repairing these sets you will release the clamp on the yoke and rotate the yoke to get rid of any picture rotation error.

Also many TV sets have a pair of magnets near the rear or attached to the rear of the yoke assembly to centre the scanning raster on the tube face. Some sets can run a DC current via the yoke coils to centre the picture that way, but that is less common and done in some computer monitors. In vintage sets with focus coils or focus magnets, the focus coil or magnet can often be moved to help centre the picture.

If you do release and reclamp a deflection yoke, never do the clamp up too tight , just enough to stop the yoke rotating easily.

So in magnetically deflected TV sets to rotate the image you rotate the yoke which rotates the scanning raster and therefore rotates the picture that is modulated onto that raster.

With Electrostatically deflected TV sets, many vintage American ones use the 7JP4 CRT, the deflection plates are part of the CRT, so the only way to rotate the picture is to rotate the whole CRT. This is also one of the reasons why "square faced" electrostatic CRT's for TV work were not made, it would have required super precision alignment of the gun/deflection plate structure with the tube face, and then rotation coils for fine adjustment.

Although square face CRT's are common for oscilloscopes and the image (scan line) is rotated by "rotation coils" that are added around the neck of the tube, because a square face tube can't be rotated.
Argus25 is offline