Ok, good news, CRT is ok
I have some problem with color decoder. Yesterday when I adjusted the color saturation I see the test bars changing color, like if I adjusted the hue control... After a suggestion by the old owner of this Zenith, I inverted the position of the two 6KT8 (1st color amp/cathode follower and sound amp/sound limiter) and now I have the correct chroma when I adjusted the saturation knob.
But seems I have an AGC problem, the saturation changing during watching, probably some tube has gone (like 6HS8 AGC/sync sep.)