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01-14-2013, 03:54 AM - 2 Likes   #1
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Posts: 3
Disassembling the Sigma AF 2,8-4,5/17-70 DC Macro (no HSM)

Hi,

I have a defective Sigma AF 2,8-4,5/17-70 DC Macro, Pentax mount, bought in beginning of 2007 (the one without HSM motor).
The front lens is heavily damaged and the focal length is wrongly acquired by my K10D camera (minimum focal acquired is around 55mm).

As a result I cannot set the aperture as I wish, because
the minimum value is now f4.0 (probably because above 55mm the f number cannot be lower).
The lens has done around 12000 shots.

I decided to open the lens to look for the problem myself.
I'm going to post the photos taken during the opening, maybe they are useful for somebody else.
Please note that I am writing this only to help anybody which should decide to open the objective.
I do not advise you to perform a disassembling: it may lead to an unusable objective.

Now the photos.

01: the mount.
Three screws. (The smallest one can be left in place)

Camera
iPhone 4S

Focal Length
4.28mm

Aperture
f/2.4

Exposure
1/120s

ISO
80






02: removing the mount ring. The long stab is for controlling the aperture

Camera
iPhone 4S

Focal Length
4.28mm

Aperture
f/2.4

Exposure
1/24s

ISO
50






03: the focusing mechanism (left bottom) can be easily removed

Camera
iPhone 4S

Focal Length
4.28mm

Aperture
f/2.4

Exposure
1/30s

ISO
50






04: taking out protecting rings and a small metal stopper (fixed with the small screw in the first image)

Camera
iPhone 4S

Focal Length
4.28mm

Aperture
f/2.4

Exposure
1/60s

ISO
50






05: Longer screws are accessible. Then the focus gears and the electronics can be removed

Camera
iPhone 4S

Focal Length
4.28mm

Aperture
f/2.4

Exposure
1/24s

ISO
50






06: 2 screws for the focusing gear and 3+2 to hold in place the focal-length regulating ring




07: there are two different screws lengths

Camera
iPhone 4S

Focal Length
4.28mm

Aperture
f/2.4

Exposure
1/20s

ISO
50







08: By turning the ring slightly two more (quite short) screws are accessible. They act as mechanical stoppers (for the ring rotation) and have to be removed anyhow in order to correctly re-assemble the objective

Camera
iPhone 4S

Focal Length
4.28mm

Aperture
f/2.4

Exposure
1/20s

ISO
80







09: ring removed: contacts can be seen in the lens body

Camera
iPhone 4S

Focal Length
4.28mm

Aperture
f/2.4

Exposure
1/20s

ISO
200






10:The contacts are quite dirty, probably some grease leaked. Maybe this is causing contact issues leading to a wrong readout of the focal length

Camera
iPhone 4S

Focal Length
4.28mm

Aperture
f/2.4

Exposure
1/20s

ISO
320






11: zoom on the running contacts. ATTENTION: it's fairly easy to damage them by removing the ring

Camera
iPhone 4S

Focal Length
4.28mm

Aperture
f/2.4

Exposure
1/20s

ISO
320






12: The ring is composed of two parts. They are joined together and kept in place by "a guide and three teeth" which are positioned on the side of the ring which is closest to the lens mount.
Also here there are contacts, which can be damaged even more easily than the others.

Attention to the flexible circuit strip: it can get hung when you remove the inner ring from the outer one and get damaged.

Camera
iPhone 4S

Focal Length
4.28mm

Aperture
f/2.4

Exposure
1/20s

ISO
50





13: During the opening I have slightly bent the running contacts. They could be put back in place. However, the metal they are made of can easily crack, if you bend them too many times.
The green arrow points to the site where the post for regulating the focal length should sit.
The post is clearly visible and protruding from the main lens body. It is made out of a goldish metal.
If the post does not slip into this site, turning the ring does not move the objective focal.


Camera
iPhone 4S

Focal Length
4.28mm

Aperture
f/2.4

Exposure
1/15s

ISO
500






The most difficult part of reassembling the lens is having the two (inner and outer) focal length rings back in place, without damaging the running contacts.
The way I have done it is:

First put the inner ring in place, protecting the contacts with a piece of paper 'till the ring is in place, then carefully removing it.
At this point I fix it with the main body wit one screw (of the 3+2 set shown in pict.06)

The outer ring can be then put in place.
Let the focal length post in the main body enter through the "cut/opening/guide" visible in internal, bottom part (the side farthest from the mount ring) of the outer ring. Keep the running contacts protected with a piece of paper.
Rotate the ring along the circular guide (a circular cut just above the running contacts in the image 13) toward the site for the post, in the LONGEST DIRECTION to reach it.
Check on the upper side of the rings when the three teeth can slip into the cuts (one is visible below the running contacts in image 1), then remove the paper carefully and push the two rings together.
If I rembemer correctly now you should screw again the two short screws seen in pict. 08.
Continue rotating 'till the two rings come to a stop.
Push harder to have the post enter the site.

Then you can put everything back together.

If you do everything carefully you MAY have a functioning objective.
Mine still has problems, minimum focal length readout is now 45mm. Better but not solved.


Ciao,
leodp



P.S: I do not know for how long the photos will e accessible on my website. I attach also them as zipped file.

Attached Files
File Type: zip Sigma_17_70_disassemble.zip (903.7 KB, 150 views)
01-14-2013, 10:05 AM   #2
Ari
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