The JR X3S is a 75mhz synthesized system, if you are running the 5308 model receiver box its made for a micro receiver. You will need a older style receiver box to fit that receiver. Traxxas went to a micro receiver with the 5308 which really cleans up the look but will only fit a a receiver with the dimensions of 1.5in X 1.25in not exactly sure of the height (I am thinking .75in).