Glad you found the problem. If those are retrace lines, there is a circuit modification you can make to suppress them.
The modification for this and many other Admiral models can be found in the Riders publication.
http://oldtubes.net/library/free_man...ures_Vol_1.pdf
Or they might be from copyright encoding.
Also that the image is in 4:3 format and should not fill the whole screen vertically.
Something more like this taken from my set with the same chassis.
https://flic.kr/p/23iBMxx