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Gary_whitehead
Tinkerer
Username: Gary_whitehead
Post Number: 2
Registered: 09-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, May 27, 2008 - 05:27 am: | 
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Hi All,
Can anyone give me any pointers (or best of all lead me to diagrams) on how to disassemble the Canon 300mm f4.0 FTn. My primary aim is to get to the diaphragm which is oiled stuck, secondary to the focusing mechanism to clean/regrease.
What I have noted so far;
Wide section at front of lens; there are three grub screws here which would appear to secure this to the flange below (and I assume holds the front elements). However loosening them and twisting the front section has no effect - the front section is still fast against the flange, although the entire front section (i.e. everything above the focus ring) can rotate by c60 degrees - which is a little worrying.
I am assuming that if this front section is removed the internal focusing part starts to look like a normal lens (?).
Is it possible to remove the diaphragm from the rear? I note that with the slide in filter removed there is no glass in the way.
I do not wish to go in via the front (i.e. remove rings/front elements) - firstly I do not have a spanner wrench large enough and secondly even with the appropriate tool I still see a real risk of damage.
I know that I am sounding very cautious here, but the lens is too expensive to risk guesswork/experimenting. I've done enough lenses to be confident of success if I know how to get in, but also enough to know that guesswork can lead to damage. (Worst case I will send to a pro to service, although that will itch...).
Best Regards,
Gary. |
Glenn
Tinkerer
Username: Glenn
Post Number: 359
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, May 27, 2008 - 06:13 pm: |
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The diaphragm mechanism comes out from the front, as do all the nFD diaphragm mechanisms. I have the manuals, but will have to dig them out of the loft tomorrow. From memory you have to remove all the front elements first. If I have not posted the solution by Thursday please contact me directly, and rattle my cage! |
Glenn
Tinkerer
Username: Glenn
Post Number: 361
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, May 28, 2008 - 05:04 pm: |
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Gary,
You do not state if your lens is the 'L' version, but the parts manual indicates the metal parts are similar and assembled in the same fashion. The repair manual does not list the 'L' version, so I assume the 'fit' is the same on both lenses
You will have to dig very deep into the lens to service both the diaphragm and focus mechanism. The rear/internal focus Canon tele lenses are somewhat complicated to set up after reassembly. Unless you have previously repaired a 'cam' focus Canon lens, I suggest that you have it seen to by a pro - especially as you state that you have good money invested in the lens.
The Canon repair manual is very poor - it is really a memory aid for a factory trained technician.
The front lens group/barrel is removed by removing the three grub screws and pulling off the complete barrel unit. Note completely remove the screws and give strong pull. If lens has been serviced before, thread lock can seep into the retaining groove and cause the parts to jam.
The intermediate barrel, ie the flange that the front section sits on, is screwed in and locked by grub screws. Manual indicates you have to remove the rubber focus grip and the reinforcing sleeve (revealed when the rubber ring is removed), to reveal the grub screws. Screw off the intermediate barrel.
If you now look into the lens, you will see the inner moving ring, that holds the diaphragm, sitting inside the cam barrel. The diaphragm unit is held by two screws. These can be removed through two slots in the cam barrel. Scribe lines to indicate the position of the diaphragm unit in the barrel, before removing.
No further dismantling notes are given. Only focus adjustment instructions.
Basically you just keep removing bits, and re-assemble in the reverse order. You can reach all internal screws via slots/holes in the internal barrels. The rear of the lens - the mount etc - is common to all nFD lenses, and is removed in the same way.
NOTE - If you remove the diaphragm unit, you will have to recalibrate the diameter of the maximum aperture. The manual does not include the required table of sizes!! So you will have to measure the diameter of the maximum aperture before you remove same. I use metric drill shanks as gauges, and wrap adhesive tape round if the diameter of the iris falls between two sizes of drill. You only need a setting gauge, the actual diameter does not matter, so you can reset on assembly. However this is relatively simple and should give you no problems. Going in deeper and dismantling the focus mechanism is a different kettle of fish. The focus set up procedure is somewhat complicated - scribing lines will not work.
I do not know why you need to relube the focus mechanism. I have not worked on this lens, but have sorted many 2.8/200 lenses, which have a very similar mechanism. On these lenses I have found it is possible to relube the mechanism in situ, without stripping down. Obviously this depends on the state of the old lube and internal dirt, but this method saves a great deal of work and has stood the test of time in my well used example.
Good luck if you go down the DIY road, and contact me directly if you require further information. |
Gary_whitehead
Tinkerer
Username: Gary_whitehead
Post Number: 3
Registered: 09-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, May 28, 2008 - 05:28 pm: |
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Hi Glenn,
Many thanks for the comprehensive details. They are enough to make me think twice; as you say I have not tried a 'cam focus' lens before and this one may be a little too expensive to experiment with (an ebay purchase, good in all respects other than the diaphragm and the seller appears open to renegotiation due to the issue).
I gather from your details that (other than the diaphragm) there should be no great recalibration issues if I just strip down enough to remove the diaphragm unit and flush it (?).
Funny you should mention the 200mm/f2.8 - exactly (double helicoid) what I have in pieces on my desk at the moment; again a renegotiated ebay purchase due to (hopefully fixable) fungus infestation. Once I have that back together I will think about whether I feel lucky. (Now if I can find a 'junker' to practice on....).
Cheers,
Gary.
(P.S. What's the story with the Canon documentation? I have managed to find some very good details on a number of 'breachlock' lenses (including, as you mention, diaphragm diameters), but next to nothing outside of that.) |
Glenn
Tinkerer
Username: Glenn
Post Number: 362
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, May 29, 2008 - 10:34 am: |
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Gary,
Your 2.8/200 is the earlier nFD version, ie front focus with standard double helix. The later 'New' 2.8/200 nFD was rear/internal focus - exactly the same as your 4/300. The earlier 2.8/200 is a doddle to fix, compared with the later version.
Yes, you can just strip out the diaphragm holder complete. The two screws are at 180 degs, and coincide with slots/apertures in the cam cylinder. Get these screws out and the whole unit just lifts out for flushing. You do not even have to strip the diaphragm unit.
A point on the removal of the intermediate barrel (Canon's nomenclature) or flange piece. As stated it is screwed in and secured with three grub screws - remove flange anti-clock, all the diagrams in both the repair and parts manual seem to indicate that the three lock screws are situated under the rubber focus ring - at front of lens. This means that the rubber ring is removed rearwards. Ring is secured with a 'wavy line of Plyobond on the reinforcing sheet/cylinder. Work a thin piece of plastic down under the ring to break the bond. However I suggest you look and make sure these lock screws are not visible first, before shifting the rubber ring. I am pretty sure the screws are covered by the rubber ring, but the diagrams are a bit ambiguous. Hate to have you remove something that was not necessary!
As for Canon parts/repair documentation:-
As far as I can ascertain it was issued on micro-fiche for the nFD optics and regularly updated as mistakes were found or lenses were altered/developed - mostly as cost cutting took place.
The problem occurs in how the manuals were written/assembled.
Each lens has its own parts manual/list - exploded diagrams and part numbers. There is then a General Repair Manual. This contains general stripdown info, mount repair info and tables on focus limits and aperture sizes. As a guide, there is detailed stripdown/adjustment procedures for the nFD 2.8/50 standard lens. Then there are repair manuals for wide angle lenses, telephoto lenses and zoom lenses - the zoom lenses are split into three sections, according to zoom ratio.
I have found no freebie nFD manuals on the net, bits that are there are incomplete and in most cases useless.
I purchased mine from OTC Ltd, (Old Timer Cameras) here in UK.To get a complete set, you need to purchase a lot of manuals. This is not OTC's fault, it is how the Canon fiches are arranged, and their in house Canon expert retired a couple of years ago. Took quite a bit of experimenting to get my full set and it cost approx £140. Still it was money well spent at the time; as I had got my hands on around 40+ lenses, all in bits! The f2 wideangles that I assembled from the bits, more than covered the manuals purchase and my repair/assembly time.
I have seen nFD documentation for sale on eBay, at various sellers who specialise in camera manuals. One US seller was very good in what he had, but cannot recall his details at the moment. |
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