Buying the part yourself and installing it can save a lot of money! Search for a great deal online and you're more than likely going to find it. It's always good practice to check out the prices for a spare part for your Chamberlain 850CB from several different places. That way you can be sure you're getting the best deal. There are usually more than one company producing the Chamberlain garage door opener part that you're looking for, especially aftermarket-type spare parts. Find the right part and fix it right!