I have a Sears Model 139.535155R garage door opener and unfortunately I have a Sears 139.53400 owners manual. (They both came with the house). Recently, the door has been opening and closing, but I have not been anywhere near the remotes. I would like to change the signal code, but cannot figure out how to change the receiver signal code. The owners manual for the 139.53400 indicates that the receiver has a "code switch block". The model I have does not have a "code switch block". How can I change the signal code of the receiver?