how to fix my 1200 david brown tractor steering?

Presumably manual steering. Recirculating ball type steering box - (worm and nut) with 28 balls from memory.

Disconnect the drag link from the drop arm of the steering box, if no resistance to the drop arm being moved by hand take the steering box of the top off the gearbox top cover and see if there is any bits in the oil bath which lubricates the box (use a magnetic). The ball race tube may have failed on the nut assembly.

Ash.
October 2015