Zenith 12" B&W horiz width troubles

[Adam]

13A12S chassis. The H width adjustment is that sleeve around the neck of the CRT. It's already pulled all the way out. And I tried several different HO tubes. Also notice how the picture isn't square, it's sort of pulled in in the center both horizontally and vertically. I've already adjusted those rings behind the yoke to minimize this as much as possible. And if you look closely at the individual lines, overall they're curved inwards, and they become sort of wavy towards the left side where the picture is pulled in. You can hear a vibrating noise that I think is coming from the V out transformer, but I've had sets that did that before and it didn't cause any problems. I'm beginning to suspect the yoke itself, but I don't have another one of these Zenith 12" yokes to try. Am I right to think it's the yoke, or is there anything else I should look at first?