Mike- I'm using a RR 501 unit that came with Activehome.
It's a little larger(stronger built) and can accept commands off the A.C. It has'nt given me any problems so far. I've got a couple of 751's on different codes and they work fine also, except that dame top screw pulling out;a little tape and their fine. If your 751 is still under warranty explain to them the problem and they might send you one without having to send back the defective one. Good Luck