You can try Finger nail polish on the ball joint, let it dry for a few hours or apply it for you go to bed and in the morning it will be dry and snap the arm back on. This should create some friction for the ball and socket. Not sure how long it will stay tight though. Most loose joints on mine transformers, i apply a thin coat of nail polish each night and go from there. I hope this helps... Also, if this is not a ball joint, but a pin, you can try furniture polish to stiffen it up, but i have yet to try this method.