Need help with Logic Datum 2 tonearm, please.

I just purchased a Logic Tempo Electronic turntable with a Datum 2 tonearm. For the life of me, I cannot figure out how to move the counterweight, it seems frozen in place and there are no setscrews anywhere, the back of the arm is not threaded, so I don't think that it screws on and off. There is an Allen screw in the back of the tonearm tube, but I don't really want to mess around until I am sure! Thanks in advance for any guidance.
There is a very small (1mm?) allen headed set-screw actually on the counterweight circumference, when removed it is about 8mm long. Slacken this right off and set up arm, tighten when done and re-check tracking force. It is very similar to a Linn Ittok counterweight.
Hope this helps.
Gary, thanks so much for the info. Unfortunately, it's been five years since I posted and the motor shorted out in the interim. On a happier note, I got a replacement motor from someone who was upgrading their Rega. Now I just have to find the time to install it and replace some resistors on the board.