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In the service manuals Sony says that this dihedral procedure is seldom necessary. But the method is explained and the alignment cassettes include the pattern used for the adjustment. Dihedral is having the two opposing video heads on the spinning disk located exactly 180 degrees across from each other. This improves the capability of the heads to exactly start each recorded scan in precisely the right place, every time. If it is off the picture will not be sharp and the edges will be jagged.
This is because half the television picture is displayed in the first thirtieth of a second and the second half in the other. They are interlaced together, first one and then the other. The human eye cannot see individual frames at a rate faster than 24 per second so this change is not noticeable. This is why there are two heads, one for each of the scans for a single frame. Having the two start at the right place each time and line up perfectly is very desirable. On the alignment tape there is a recorded section called the monoscope. It is used for testing the dihedral. It has the switching pulse that starts each scan of the picture recorded in the upper third of the screen so it can be seen. It usually resides at the very bottom where it is not visible. Examine the image at the top. It is acceptable but not perfect. If adjusting the tracking won't line up the two scanning patterns and sharpen the picture then the dihedral is off. I check every unit that I service and find that the dihedral can be improved about five per cent of the time. I adjust it even when it falls in the acceptable range because perfect makes for the sharpest image. Check out the bottom screen after adjustment and notice it is sharper. To check out how this is done and the tools used click on the picture. To proceed to the next section that describes some CORRECTIONS AND IMPROVEMENTS click here
 
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