[techkan]

I recently purchased a Pentax-M 1.7 50mm lens. I am able to rotate the aperture ring between f22 and f2.8 - it is not going to f1.7

Any idea what the problem might be and how it can be repaired ?

[Na Horuk]

Look into the lens and rotate the ring, see how much the aperture blades close (move the little lever on the backside). Does that work normally? This way you can tell if its just the ring or the mechanism.
How much did you pay for the lens? You might want to at least get some money back if the seller didn't mention this problem.

Also, I think there is a guide on how to disassemble that exact lens on these forums, which might be helpful if/when you decide to take it apart.