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Dennisdietz
Tinkerer
Username: Dennisdietz
Post Number: 1
Registered: 08-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, August 09, 2008 - 06:14 pm: | 
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Hey all, new here and I have a question that lead me to this forum....
I have a Nikkor-H 300mm f/4.5 sn:409622 that has a "stiff " focus. It is stiff for the whole focus and occasionally it feels like there is a little grit or sand rubbing, though you can only feel that occasionally. I've tried working the action for a couple hundred turns but it has not really helped. The problem is worse if the lens is pointing up and extending. If you point the lens down and extend or retract it is much less noticeable.
I was thinking of disassembling to the focus mechanism and either cleaning/regreasing or simply trying to add a tiny bit of Ronsonol or Rem-oil to try and soften and "rehydrate" (well hydrate would refer to water but you know what I mean). I was able to get off the front element assembly by removing a single screw at the base of the assembly, just under the "Nikkor" wording. When the focus is fully extended, ie closest focus, there is another screw on that barrel just above the focus grip ring. I removed it and tried unscrewing in both directions bit nothing seemed like it would give and I did not want tp force it. I have read that stiff focus problems are best dealt with by going in from the front of the lens but don't know how much farther to go. I am hesitant to go to far and have an adjustment screw or internal part be hard to get back together.
I have searched the net a lot that last few days but have seen absolutely nothing about disassembly of this lens. I can't even find a diagram to buy.
I have seen mention of using Ronsonol poured in around the focus ring from the top but I do not want to risk getting anything inside the lens or to the aperture blades. The focus "grip" has six screws around it. It seems like that is the next step after removing the front element to get to the focus assemble (helicoids?). Any thoughts?
Here is a photo of the lens with some notes:
<img>
Removing screw 1 allows the front element to unscrew at the red dashed line. Removing screw 2 does not seem to allow anything to unscrew, I tried both left and right threading. I tried pretty hard bot did not force it.
I wonder if removing the screws (6) around the focus ring will allow the focus ring to be removed if the front element is removed first?
Turing the focus ring causes the front element to move in the direction of the gray arrow and at the same time, the focus ring moves forward in the same direction as the gray arrow. I wonder if there is grit between the silver part of the lens and the focus ring and or, if there is grit between the black part of the barrel and the ring.
The aperture blades are located between the front and back elements, at about the back grip portion of the focus ring.
Any disassembly thoughts or experience?
Best! |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 644
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, August 09, 2008 - 07:47 pm: |
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Not sure if this is the same lens or not, it's the 300/4.5 AI version. |
Dennisdietz
Tinkerer
Username: Dennisdietz
Post Number: 2
Registered: 08-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, August 09, 2008 - 08:05 pm: |
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Rick,
Thanks. I appreciate your quick response and in searching around before posting, I read several of your replies. Glad you are chiming in.
It looks similar but a little different. I have a pre-AI version. I think your drawing shows a rubber grip on the focus ring, yes? Either way, would you think the next step in getting to the focus threads/helicoids (is helicoid teh right word) would be to remove the six screws around the focus ring?
I also thought about adding a couple drops of Ronsonol from the front of the focus ring, between it and the barrel and working the focus to try and thin the grease a bit. Maybe try the same from the back of the focus ring to try and loosen the grease and maybe flush and sand/grit. My though was to only try a drop or two at a time in order to loosen but not flush out the grease.
One thing I don't know is how sealed the mechanism is behind the focus ring. Is my idea simply a good way to get ronsonol/oil/grease into the lens and onto the blades and glass? |
Dennisdietz
Tinkerer
Username: Dennisdietz
Post Number: 3
Registered: 08-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, August 09, 2008 - 10:03 pm: |
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Well I tried taking out the six screws on the ring and nothing seems to come apart. I also tried taking out the other screw in your diagram but was unable to unscrew that upper section either. Neither direction seems to want to budge, and I'm a pretty strong guy.
Trying a little harder and the focus ring will twist and pull off. I've taken notes and photos. There is definately some sand/grit under the focus ring. It shows a double row of reverse direction "threads" that are very steep. There is a brass stop that works with the focus ring to stop it at min/max focus. The grease is very black and feels a little gritty, plus there is virtually no grease on the actual focus threads. The actual focus threads/helicoid assembly is not fully apart yet but I can screw/unscrew it by hand, as if I were holding the focus ring. I am unable to completely unscrew the threads and will try more tomorrow, making sure to mark exactly where they come apart. I'll post some more with photos tomorrow.
Any cleaning and grease recommendations?
Thanks so far for your help! |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 645
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, August 10, 2008 - 07:49 am: |
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Sorry I can't be more specific, the AI is the only 300 Nikkor I've worked on. You want to get the helical as completely clean as possible, especially if the grease is as nasty as it sounds in your description. I usually use lighter fluid on cotton swabs, and it takes a lot of them. If you can't get the helical completely separated, clean what you can reach and then let some solvent wick into the inaccessible part and work it back and forth to work out as much dirt as you can get. I use white lithium grease to relubricate focus helicals, there are also silicone damping greases made for the purpose. I apply this also with cotton swabs and work it in to make sure that all the threads are uniformly covered, and wipe off any excess that may come out the end. |
Dennisdietz
Tinkerer
Username: Dennisdietz
Post Number: 4
Registered: 08-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, August 10, 2008 - 08:16 am: |
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Thanks again Rick. That is exactly the cleaning method I was going to try. As for the white lithium grease, I read somewhere someone suggested the high temp stuff for brake parts. Is that the stuff? Would that make it more or less stiff than the typical Nikon grease?
One last thing, if I let a little (and I understand that a little really means a little) ronsonol wick up, is there any chance it could get into the lens itself? One reason I hesitated and asked questions before trying to disassemble the lens was not knowing how sealed the lens is behind the focus mechanism.
Thanks so much again. I have been taking photos and notes as I go and will certainly put it all somewhere more accessable to people in the future |
Dennisdietz
Tinkerer
Username: Dennisdietz
Post Number: 5
Registered: 08-2008
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, August 18, 2008 - 06:40 pm: |
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Thought I would/should update this. Cleaning as described above worked well. The focus is still a little stiff but much improved and very smooth. (I used a sparing amount of regular white lithium grease and notice no real difference in focus stiffness between clean threads and greased threads.) There are two screws that can be removed to get off the front element. One is visible when the lens is fully extended and the other is at the base of the widest part of the front barrel, under the lens hood. It is visible if you extend the hood. Either screw will allow you to unscrew the front element and remove the focus ring. The ring removes just as Rick described in his drawing, except the pre-AI does not have a rubber grip and the screws are plainly visible. The ficus ring almost snaps on and off once the screws are removed.
I tried to get the focus threads apart further by removing the screws that hold the mount on. No luck there. The back mount/aperture adjustment lever is all one piece. The is a long barrel and an even longer fork that extends down past the rear element and engages the aperture blades. Carefully note the positions of everything as you pull the mount off so that you can index the fork back in the right place on re-assembly. There are no visual clues as to where the fork goes except screw positions and then, the lever must be moved counterclockwise to engauge the fork to the blades if you use the screws for alignment. You can see the back half of the inner set of focus threads with that off and I was able to remove a little more dirty grease with q-tip.
There is definitely a stop in there somewhere that prevents the focus threads from completely unscrewing. I cannot find it yet and may not bother further at this time. I can only assume at this point that removing the silver out "shell" on the back of the lens would allow a little more disassembly, but that's only a guess.
Also, thanks again Rick for the advice. Using similiar thinking allowed me to get a pretty grungy Nikkor 35mm f/2 into perfect shape again. Nice smooth focus and a now spotless inner element. That lens is a lot easier to get apart. You must go into it from the back but it was very easy. The focus comes completely apart, so careful indexing was necessary right at the end to get it back together right, even then, it too me two tried to get the right set of threads.
Thanks again. |
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