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01-10-2012, 11:31 AM   #1
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Auto aperture repair for Cosina Vivitar Series 1 28-105

New member here, hope I'm posting this in the correct area!

I recently obtained a copy of a Vivitar Series 1 28-105, and it is the later Cosina
version with the 72mm filter.

I stripped it down, cleaned out a bunch of grit, repaired some stripped mounting screws, and reassembled.

Works okay, except that the "A" setting for auto aperture does not seem to work. Took it apart again, checked all the contacts and the actuating slider, all seems okay.

In aperture priority mode however, I cannot change the aperture setting. To verify it is not a camera problem, I checked with another lens which performed correctly.

The lens is stopping down fine in Manual mode, no restrictions on the aperture blades.

In program mode, it also appears to stop down correctly.

Exposed image EXIF data however, always shows exposure as either f/0, or f/2.8, even if it is stopping down fully.

EXIF shows it as an "A" series lens though, so at least it is getting that part right.

Carefully reviewing the auto aperture mechanism, it appears that there may be a part missing.

I'm guessing that it might be a little plastic shim or a metal lever or tab, that allows a molded stop on the aperture collar to depress the A contacts on the lens mount circuit board.

Does anyone have any experience, part lists, manuals, or even just a photo of this area on these Cosina Pentax mounts? I am assuming it was a detail used on many of their lenses.

If I knew what was supposed to be there, I can probably fabricate a replacement.



01-10-2012, 02:00 PM   #2
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Since it registers that it's an A lens, perhaps the data the contacts are sending back is off- maybe they're shorting or not linked up correctly?

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01-10-2012, 02:09 PM   #3
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Yes, I think that is the case.

There is a resistor in the circuit, perhaps it reads a value off of that to confirm auto aperture is available.

I think I will manually close the circuit with some foil, and see if the lens becomes more functional that way.

If I could just keep it in Auto as Aperture Priority, that would be a step up from where I am now.

01-23-2012, 11:33 PM   #4
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May be this can help?

Kiron 28-105 repair manual for your inspiration...
Good luck.

Attached Images
File Type: pdf kiron 28-105 k.pdf (529.6 KB, 452 views)
01-24-2012, 11:53 AM   #5
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Amazing toine, I've spent the better part of a week over the past month looking for a pdf of this same lens assembly. Where did you come across such a thing?
01-24-2012, 02:43 PM   #6
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It was on my hard drive. I cannot remember from which site I got it though - sorry.

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