71D Circle Stuck

I'd suggest the problem is either in the small reversing gearbox that is mounted in the A-frame, that reverses the direction of the control shaft .. or in the circle gearbox itself. I'd say maybe a bearing has broken up, or has misaligned the drive components, resulting in the drive actuating in the one direction, but twisting out of gear alignment when reversed, thereby jamming the gears.

Also you might look at the spacer blocks on the circle and make sure one of those hasn't come loose and jams the circle one way.

Thanks Oz and Ol'Grump, will try to see what I can discover as soon as I can get a little free time.

Ol' Grump wins the prize. One of the adjusting nuts had fallen off from the circle guide shoe. The guide shoe then worked like a cam. It allowed enough slack for the circle drive mechanism to disengage from the circle teeth. Had to pull it back it to place with a few discrete tugs from the 930 with a chain on the moldboard. Then I replaced the missing bolt and adjusted the shoes. Now the circle turns true and I am ready for the next snow storm due tonight.
Thanks for the help guys. This is the best place to come for good advice.