Hi there, could really do with some help!

Records are playing on my system but the arm is trying to pull the stylus back to it's starting place so the record sounds distorted and keeps skipping backwards. Is this something to do with the counterweight or something? Turning it around to 0 seems to reduce the problem somewhat but it still gets stuck in the groove and sounds very crackly. When i push down on the stylus end of the arm a little the crackling goes away - too much weight at the back of the arm?

I just got the system through ebay and the previous owner assures me it worked fine. Any advice would be greatly appreciated.