fixing Pollock chair height adjustment  

I have a Pollock executive chair (1985), and the height adjustment mechanism is broken. This is the version that is supposed to raise and lower by pressing in the ring on the bottom and spinning it. I am able to raise the chair, but it does not lock in place and falls back down if any pressure is applied. Is it possible to repair the mechanism, or at least keep the chair from falling back to it's lowest height? I don't need to change be able to change the height often, but do want to raise it so I can use it comfortably at a desk.