Here are the instructions for my loose zoom fix. It's a good idea to cross reference these with Richard Haw's disassembly instructions for the lens at the link provided by Dieter above.
The first step is to move the rubber grip on the zoom ring forward about a centimeter. There is no need to remove it entirely, but only to move it enough to expose the three screws at the bottom of the zoom ring. If you've never removed the grip on this lens before, it can be hard.
Begin by inserting a thin but rigid plastic thing under the front edge of the grip:
You don't want to use something metal for fear of scratching the branding ring. Once you've got the shim wedged under the edge of the grip, lever the grip outward enough to grab the edge with the tips of your fingers, and pull. Don't pull too hard or too much, or the grip may rip. Just try to get the grip barely over the branding ring at that point. Then move the shim a little, grab the grip again, and pull. Repeat until the whole grip has been shifted forward enough to expose the three screws at the bottom of the ring, one of which is shown below:
Now, before removing these screws, you will want to mark which screw is adjacent to the infinity symbol on the distance scale. In the picture above, I scratched two tick marks above that screw. Having done that, remove the three screws using a JIS 00 cross-type screwdriver. If the screws prove too tight to remove, apply a drop of acetone to each screw and let the acetone alone for a few minutes to soften the cement securing the screw, then try again.
Once the screws are gone, it's time to remove the zoom ring. This is the step where you have to be the most careful, because you're going to be separating the mated focus helicoids. You MUST mark the point where the helicoids come apart. If you fail to perform this step, then when it's time to rejoin the helicoids, you will not know which of the many angular positions at which the helicoids can be mated again is the correct one. And if you don't get this right, the lens will either not be able to achieve infinity focus or not be able to focus as closely as it should.
While keeping the lens's DoF markings and white focus index line facing you, start SLOWLY turning the zoom ring. You will see and feel the ring move up as you turn. While slowly turning the ring, keep a slight upward force (toward the front of the lens) on the ring, so that you'll know when the helicoids come apart the instant it happens. Remember to keep the white focus index line in view while you turn, because this will be your reference point for marking where the helicoids separate. Note that the angular position of the ring that has the focus scale is immaterial, since it turns independently of the barrel; therefore, pay no attention to where the infinity symbol is, but only attend to the straight white focus index line.
As soon as you feel the helicoids separate, STOP turning the ring. Now put the lens down gently on the workbench and scratch a mark on the bottom of the zoom ring at the point inline with the white focus index line, as in the picture below. Later, when you reassemble the lens, you'll line this mark up with the focus index when screwing the zoom ring back on.
Now remove the zoom ring. This is what the lens will look like with the zoom ring gone:
Stand the lens up on its mount end. Do you see the three screws on the top surface of the helicoid ring, as shown below?
You will need to remove these screws...but first you need to mark the angular position of this helicoid ring relative to the inner barrel that it attaches to. For this purpose, I use the dimple that is next to one of the screws. I scratch a mark directly below the dimple on the inside of the barrel. (You will need to move down the ring that the focus scale is on in order to expose the inner barrel so you can put a mark on it.)
Having marked the angular position of the helicoid ring, go ahead and remove the three screws and pull the ring off. This is what you'll see then:
The lighter-colored ring that surrounds the darker-colored ring in the picture above is a spacing ring. There is another spacing ring right under it. One of these spacers is the part you'll modify to implement the fix for the zoom creep. You're going to affix a few pieces of Scotch tape to the spacer with the goal of minutely increasing its thickness.
How does the modification work?
If you slide the ring that has the focus scale up and down, you'll notice that the metal sleeve that has the spiral grooves in it turns at the same time. But, obviously, the helicoid ring that you just removed does not turn, being fixed to the inner barrel with those three screws. Normally, when the grooved sleeve turns as a result of zooming, one or both of the spacing rings above it will also turn, while the helicoid ring above the spacers does NOT turn. The thickness of the spacers is calculated to provide just a little resistance against the turning of the grooved sleeve.
But in the case of a lens with zoom creep, since the resistance normally provided by the felt strips under the zoom ring has been reduced, we can add a bit more resistance to the turning of this grooved sleeve, and this should fix the creep.
Gently remove the spacing rings from the top of the grooved sleeve. Then, gently separate them. (The purpose of being gentle with these steps is to avoid bending and putting a kink in any of the spacers.) My copies of this lens each had two spacers, one thinner than the other. Your copy may vary. Assuming there are two or more spacers, choose the thickest one to put the tape on, since it will be the most resistant to damage.
Cut three short strips of Scotch tape and put them all on the same side of the spacing ring, equidistant from each other (at 120 degree intervals). To make the tape easier to see in these photos, I've chosen a pink tape. But standard Scotch tape is good.
Now lay the ring on your workbench such that the sticky side of the tape is facing up. Use an X-Acto knife to cut the excess tape off, leaving just the tape that is directly in contact with the spacer. As you cut, keep the knife right against the edge of the ring, because we don't want any excess tape overhanging the ring. The next picture shows how it looks with just the outer parts of the tape cut away.
And here's how it should look with all of the excess tape cut away:
Put the spacers back together such that the tape goes between them, and then place the spacers back on top of the grooved sleeve. Make sure they fit perfectly around the inner barrel, with no overlap.
Screw the helicoid ring back on. At this point, you need to test the sliding resistance of the ring that has the focus scale on it. Its resistance to being moved up and down should be noticeably greater than before. If the resistance is very high, so that the ring seems stuck or is actually hard to slide, then the tape was too thick. See if you can find some thinner tape and try again after removing the old tape.
But it's more likely that the resistance, while increased, will still be too low. Certainly, if the ring with the focus scale on it just slides down under its own weight after you pull it up, the resistance is far too low. But even if there is just enough resistance to prevent that, it's probably not enough, because once you put the zoom ring (with its rubber grip) back on, the increased weight may be enough to make the combined part slide under its own weight.
Thus, if the resistance is, or even seems, insufficient, you need to put more tape on the spacing ring. You can put three more pieces of tape on the ring, in between the first three, filling the gaps. It's important to put all the tape on the same side of the spacer. If you put tape on both sides, the spacer will be too thick for sure, and sliding resistance will be far too high. So, hopefully, six pieces of tape (all on the same side) will be just right. But if that proves to be still not enough, you'll have to find some slightly thicker tape and try again.
Once you have the sliding resistance of the ring with the focus scale on it just right, it's time to reassemble the lens. Twist the zoom ring back onto the focus helicoid, first lining up the mark you made on it with the white focus index line. Put the three screws back at the bottom of the zoom ring. Before pulling the rubber grip back into place, try zooming the lens and see if you still like the sliding resistance.
This fix can take many iterations to get just right. But when it's right, it's great because the turning resistance of the zoom ring is not affected at all. Only the sliding resistance.
