I can give it a try at adjusting the joke, but am afraid to make things worse.
Can someone please give me some advice about how to go with this (I guess
small) adjustment? I have been advised by the seller to avoid untighten the
joke screw... he suggested to remove these rubber black things (that seem
glued) and try and rotate it a bit in one direction or the other,
but I really am not sure and very afraid of making it worse.
Thanks
Rick
Rotating the yoke is going to rotate the entire image, which won't solve your problem. And don't remove the black wedges! That's terrible advice. What you want to try is changing the angle that the yoke sits on the tube neck. This is taken from a 4600 manual, but all yokes work the same so it should give you an idea of what to try.
On the top two drawings, you can see that when the the yoke is angled upwards or downwards, it changes where red and blue cross over each other and the green at the edges of the screen. Angling the yoke left or right changes how they line up in parallel. It looks from your pics that you have a bit of both going on, but I'd try adjusting the up/down axis first before messing with the left/right one. The movements you are going for are very small.
The wedges are used to scoot the yoke angle a bit one way or the other, and when the convergence looks good they are glued in place and the yoke is tightened down. But if yours are already glued in, leave them there. They were glued in that spot to hold the yoke at the proper angle, and if you remove them you will have to start the entire process from scratch, which trust me you do not want to do. Adjusting convergence can be a frustrating process even for those who have done it dozens of times, and more often than not, you'll make it look worse than it did when you started... having the wedges in place means that if you screw it all up, you still have your "starting point" marked and can return it back to the way it was. So leave them there!
If the black wedges are still in place, what I would do is loosen the yoke screw enough that you can move the yoke, and gently move the yoke in towards the wedges to make sure it's up against all of them like it's supposed to be. Then turn it on and see how it looks. After that you can try angling it slightly one way or the other to see if that helps. You'll want the monitor out of the game, so you can watch the image as you move the yoke... convergence is pretty much impossible to do with the game in the cabinet unless you're a true expert.
You can safely touch the yoke itself with the monitor on, it
does not carry high voltage is insulated (edited for safety/clarity). But other parts of the monitor (including, as MKL notes, the uninsulated yoke connection) are dangerous, so be extremely careful of what you touch and manipulate. If you aren't comfortable with it, don't do it. The best thing that can happen if you touch the wrong thing is you crap your pants and your arm aches for a week... that's the best thing.