Minivator 950, I think - can't find a model number anywhere?

My Minivator will descend as normal but, when I try to raise it, It emits a long beep and will only lift one chain link at a time. I have loosened the two bolts on the back of the vertical support, swung the handle, etc downhill, and can see no evidence of a blockage of any sort. The batteries are near-new. Also tried the previous fix of lifting the chain from the top - no joy.