Thats because the ferrite slugs get frozen in there, and they are brittle from the factory new... So you have to be very careful. I had a problem with that on my CC, i broke a slug inside the yoke which I have to match up and replace. 

I did the very same thing with my Zenith round CRT color set from the 60s, on the convergence board... Yuck. 

Penetrating oil would do wonders here.