Some further progress, I have obtained a grub screw and installed it. The drive pulley still wobbles around somewhat when driving the arm. It appears to have a round opening on one end and the D shaped opening on the other. I suspect that I need to position it so that the left hand side is on the round portion of the worm screw drive shaft and the right hand side with the locking component on the D shaped portion.

It might be the case that the end of the worm screw drive shaft is bent. Previous shipping damage? That is going to be harder to fix. Any suggestions?

In any event, the arm works, albeit that the worm screw drive shaft gets jammed at a couple of points as it travels across the platter. I suspect that it needs cleaning as it is coated in a layer of old blackened oil or grease. The platter motor does an auto shut off when the arm returns to the starting position.

Next steps, clean the worm screw drive shaft, re-position the pulley and check that the shaft is straight.

I have confirmed that the worm screw drive shaft for the arm mechanism is bent. Any suggestions as to how to straighten it?

I don't think there's any better way than to simply remove it and bend it back. It seems to be brass and should be bendable with no problems, but if you try to do it in place you might damage something else.

Screw some suitable nuts (M4?) on it if you grab it in a vise or something so you don't damage the threads.

Thanks Mika. I don't think that there is any other way either. I will give the approach to straightening it a bit of thought.

Does anyone know whether or not a BG 4002 worm drive shaft will fit in a BG 4000? It would appear to solve a few problems for a number of people if it did.

The shaft has the same part number (2830049) in both service manuals, so I suppose they would be identical...

I don't think it would be very difficult to manufacture a replacement from generic threaded brass either, perhaps using the pulley end of the old shaft.