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Repairing a Sigma 70-210 f/2.8

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Gubbe View Drop Down
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Post Options Post Options   Quote Gubbe Quote  Post ReplyReply Direct Link To This Post Topic: Repairing a Sigma 70-210 f/2.8
    Posted: 27 November 2006 at 14:38
Well, 'repairing' may be too strong a word, but the lens is usable and I only ended up with one extra screw and a piece of extra glass... ;)



I bought a Sigma 70-210 f/2.8 APO from a Dyxumer for about a hundred EUR. It had trouble with focusing whereby the focus mechanism would jam and sound very rough. In essence, AF was completely useless and even MF was barely usable.



I opened it up a long way from the rear (mount) side, removed the mount, the rear barrel, the rear group with the diaphragm and electronics, some AF gearing and spent a whole lot of time removing the Zoom ring mechanism, but still couldn't gain access to the front of the lens.



I didn't see any obvious methods of opening the front besides a filter ring -type thin ring that held the front elements in place. When I tried to remove that ring, it got horribly stuck and I had to cut it with a dremel to get it off (did it quite nicely actually). After doing that I noticed to my humiliation that it wasn't even supposed to be possible to remove it that way because the threads it was on were recessed beyond the filter threads slightly. Yet, I still couldn't get the lens open so it was left collecting dust.



Then one evening, just a few days ago, I was getting ready to go to sleep and found a neat screwdriver that would let me try and open some screws in a very difficult to reach place inside the lens. That turned out to be a false hope since the screws were glued tight. I took the lens in my hand and really started to ponder how the outer shell could be removed. I twisted and turned everything that I had already twisted and turned a dozen times before and suddenly a threaded ring came loose near the focusing ring and it was all obvious. The front barrel came off and I could access the focus threads and clean them thoroughly, applying just a dab of grease in the process. After cleaning the front elements well (they had got dirty during all this) I started to put the lens back together. This was at about 3 AM so I was just a wee bit tired. I dropped the lower front element in the lens barrel where it was supposed to be dropped, but did so carelessly and to my horror a piece of glass chipped off the edge of the element! I assessed the damage and came to the conclusion that although that wasn't a Good Thing(TM), it was insignificant enough to not cause major problems with the light transmission. The pics you'll see will show what the chip looks like.



Well, I finally got the lens reassembled and went to bed. The next day it seemed to work quite ok before the AF motor suddenly went "Whrrrrrrrr!" The AF motor axle had decoupled from the focusing mechanism somehow and I could hear a rattle inside the lens. At this point I was more amused than annoyed so I opened the back of the lens again to find that one of the gears from the AF gearing had popped off. The gear, although it stayed quite nicely in its place at rest, became twisted when torque was applied because its axis was loose from the mounting plate. I managed to reattach it with a spike and a hammer and now it should stay in place quite fine. In retrospect, this loose gear was probably the main reason that the AF jammed, although it had probably become loose because of the rough focusing threads.



Although a lot less clunky than prior to the gear popping, the focus still seemed a bit heavy so I opened it up from the front again and wiped off all the grease from the bottom of the threads, removing a lot of minute metal dust in the process. I tried a different grease, but ultimately wiped that off too. I left it with a very thin layer of residue grease and now it seems to move relatively smoothly - well enough for me to be happy with it, although the focus mechanism was probably a lot smoother when it was new. During all this I managed to re-smudge the front elements so I cleaned them again, but I took short cuts and some grease was left on the edges. I'll probably take it open once more and clean those off too. There are probably some small scratches on the glass as well from the handling. I managed to light it nicely to show and exaggarate all the ugliness in one of the pictures I'll link to later.



The autofocus mechanism works now and doesn't make me fear of overheating the focus motor, but it seems to do strong hunting. In other words, it overshoots, then compensates and overshoots in the other direction, going back and forth across the proper focus distance. I'm hoping that cleaning the front elements will reduce this behaviour. I will also try blocking light from entering the lens through the chipped part, just in case that causes havoc with the focusing.



I'll post some pictures at the address below. See the 'Large' directory for full res shots.

Pictures!



I'll probably keep this baby, but if someone wants to buy it, make me an offer.



Thanks for reading all the way through! Feel free to ask questions.

Edited by Gubbe - 08 December 2016 at 18:37
Arto Rantala
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Gubbe View Drop Down
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Post Options Post Options   Quote Gubbe Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2006 at 15:04
Waaah! I just broke off that same little gear again! @#&*!
Arto Rantala
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Bob J View Drop Down
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Post Options Post Options   Quote Bob J Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2006 at 15:09
Ahh, the saga of dis-assembley without instructions... very often the removal of the broad rubber focus/zoom bands reveals a way in. I don't think the chip will do much damage to contrast - the grease is probably much more of a problem. Your story highlights one of the things I've found from taking lenses apart - the focusing linkages on some of the third-party lenses (chiefly cheap Tokinas in my case) can leave a little to be desired. Even the cheap Minolta lenses (such as the 35-80 with the integrated lens cover) are impressively built inside.... That reminds me, I've still to find a way into that 28-105 xi zoom with the sticky aperture...

Bob
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Frans View Drop Down
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Post Options Post Options   Quote Frans Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2006 at 15:11
Maybe this teaches us that fixing lenses with a hammer doesn't work that well?

Not sure how the complete story was meant but reading it make me chuckle here and there.
Dynax 7D - VC-7D - AF 28 F/2.8 - AF 50 F/1.7 - AF 135 F/2.8 - 3600HS (D)
AF 17-35 F/2.8-4 (D) - AF 28-75 F/2.8 (D) - AF 35-70 F/4 - Sigma 170-500 F/5-6.3 APO
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Gubbe View Drop Down
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Post Options Post Options   Quote Gubbe Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2006 at 16:57
Who cares if fixing lenses with a hammer doesn't work that well? It's fun!
The gear axle is actually made of very soft metal and its attached into the plate by expansion of the end, done by striking a pin into the dimple at the end of the axle using a hammer. Unfortunately I've mangled the axle by using "suboptimal" equipment and decided to try different methods.
The epoxy is currently curing on the gear axle. I'm hoping that will hold it in place. If it doesn't then this thing will probably become a manual focus lens.
I'm wondering if that loose gear, assuming it came loose right away back when I last assembled the thing, could be the culprit in the AF hunting.
I'll have to test it again after the epoxy cures.
Arto Rantala
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Post Options Post Options   Quote paulobro Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2006 at 17:46
Mmmm...

Somehow I don't see Sigma frenzied to hire your services for their shop.
Nor a crowded, way overshot ebay auction...

But then, I'm oftenly wrong.
Paulo Brochado
 



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Post Options Post Options   Quote Bob J Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2006 at 18:01
Any chance you could get a spare bit from another Sigma lens?

Bob
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CKsam View Drop Down
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Post Options Post Options   Quote CKsam Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2006 at 18:57
Originally posted by Gubbe Gubbe wrote:


I'm wondering if that loose gear, assuming it came loose right away back when I last assembled the thing, could be the culprit in the AF hunting.


I agree that fixing lenses can be a lot of fun! It's hard to completely ruin them if you're careful. I love picking up dodgy broken lenses and getting them working again to (nearly) full capability.

The AF hunting could be due to bent sliding contacts on the circuit board/ROM bit. I had a similar problem when I took a Minolta 28-135 to pieces to fix something different. It's easy to bend the little contacts a bit on the reassembly without realising it. Worth a try, perhaps, but I could be completely wrong.
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Post Options Post Options   Quote Gubbe Quote  Post ReplyReply Direct Link To This Post Posted: 27 November 2006 at 19:04
Thanks for the tips! I'm sure I could use a spare, but finding a donor lens might be really difficult.
I'll have to start looking if this epoxy thing fails.

I would investigate CKsam's idea too, but there are no electronics in the focusing part. Only the zoom ring has a cable coming out from it and even then it's only for the focal length data which I understand is of no concern to the AF system. If you think I'm missing something, please inform me.
Just to be on the safe side though, I'll be sure to test that the focal length data is correct by taking shots at different zoom settings and looking at the EXIF info.
Arto Rantala
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Post Options Post Options   Quote Gubbe Quote  Post ReplyReply Direct Link To This Post Posted: 28 November 2006 at 21:06
CKsam you are a bloody genius!



I took pictures with all focal lengths and the exif data for the focal length from 150mm onwards reads like this:

150mm 160mm 135mm 120mm 120mm 210mm.

I twisted the zoom ring with considerable force for the last pic.



Incidentally, AF works fine up to around 150mm, only hunting beyond that. (Yes, I fixed the gear.)



BobJanes also deserves a big wet kiss straight on the mouth. I removed the rubber on the zoom ring and found a hatch that gives direct access to the slider that shorts out the contacts for the focal length. I just needed to clean the contacts and adjust the brushes a bit and voila! The focal length is now correct in the EXIF and there is no large-scale hunting left whatsoever!

Had I not known to look under the rubber, I would still be disassembling the zoom ring trying to find access to the brushes or spraying contact cleaner everywhere.



Right, about the gear. Like I said, I epoxied the axle and it came out fine. When screwing the plate back in its place I noticed that the gears turn very smoothly until the leftmost screw is fully tightened. I placed a small piece of very thin plastic under the corner of the plate and then tightened the screw and it's now smooth! Yes, the AF and MF are both now much smoother than they were before, I'd say they're almost like new!

Oh right, the epoxy. Although it came out well, I still didn't want to risk it since if the axle popped off again, it would be very difficult to re-attach. So, I cut off a piece of a toothpick, placed a piece of the same plastic on top of the gear as a cover and epoxied the stick so that it supports the top end of the axle. I also did it because I had broken the clip that holds the gear in place. Now the axle is supported from both ends and although the whole thing looks ugly, the axle will never come loose again. EVER.



Now all I need to do is clean the front elements and aside from the chip in the glass, this thing will be good as new!

Edited by Gubbe - 25 October 2012 at 20:40
Arto Rantala
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Post Options Post Options   Quote CKsam Quote  Post ReplyReply Direct Link To This Post Posted: 29 November 2006 at 10:36
Great news! Now for another project, eh?

Sam.
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Post Options Post Options   Quote Gubbe Quote  Post ReplyReply Direct Link To This Post Posted: 29 November 2006 at 11:01
Yes indeed. You've all seen my WTB thread and I already have a Sigma macro coming in, should be here any day now.

I'll go clean the front elements now, hopefully for the last time. ;)
Arto Rantala
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