FD 100-300 aperture issue

I have an FDn 100-300mm f/5.6 that won't stop down. I mount it to any of my

Cameras and the aperture no longer responds when the DOF is engaged. Yes the

lens is off the A setting. I was able to depress the two pins and rotate the

lens mount and the two aperture tangs move freely, but the aperture doesn't

respond. It stays wide open.

Is this an easy fix, or am I better off taking it to the local Canon repair

shop and pay the $50 to have it fixed. I do have a donor FDn 50 f/1.8 that I

can try to practice on first.

Thanks in advance.