Kiev 6C / Vega lens not stopping down

I have a working Kiev 6C, which is in fantastic mechanical condition. I understand that I got very lucky 11 years ago when I bought it.

The Vega 125 2.8/90 lens that I have for this camera has ceased closing the aperture. The aperture ring moves as expected but the diaphragm doesn't close, either when attached to the camera or by hand when removed.

Any ideas? I am reluctant to open up the lens as I am not the world's best repair man but I'm willing to try if I can get an idea of what might need to be done.

Thanks!

Hi!
Manual is here: http://www.butkus.org/chinon/russian/kiev-6s_60_ttl/kiev_6s_60_ttl.htm

Please post a picture of the camera end of the lens with the taking camera perpendicular to the subject lens, aperture indicator should be at 12 o'clock position.

Sounds like oily aperture blades, both vega lenses that I've owned have had that trouble. Though they were both 80mm F2.8's. I never did get around to opening one up. On mine I was able to use the stop down button to close the aperture, the blades were sticky enough that it would stay closed until you rotated the aperture ring back to 2.8. So I was basically using it as a pre-selector lens, it takes a little getting used to and is obviously not correct, but for me it was better than taking a gamble buying another. Or spending the money to have mine repaired. I eventually switched to Carl Zeiss Jena lenses on my Kiev 60 and left the problem behind.

Here are a couple of photos of the lens.