Resolution lines on Zenith roundie

[iong]

I have 2 Zeniths. One is a 26kc20 and the other is a 25cm33. Both seem to have picture tubes with good emission that can go bright. however if the picture is set to a good brightness I see the Horz retrace lines near the top. I tried setting the 3 screens once to med. brightness as some of the manuals say and to very low brightness and I still get those annoying lines. I also have the drive controls low. These are not copy protection lines I can move them a little with the vert hold control. How can I get rid of these? Any ideas anyone.
Thank you,
Perry