What ever alternative you go for, make sure you're working the intended muscles i.e. the back muscles. Lying on your back and raising a pair of weights won't work the muscles targeted by the pull up. One idea is to use a door frame. Place a hand on either side of the door frame and pull yourself in, feeling the tension in the muscles between your shoulder blades. Other people have placed a broom stick between two chairs (not sure how stable that is!). Good luck.
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