a.k.a. Collins 51J-4 is a continuous tune receiver. The unit has 30 one megacycle band positions.. This was an expensive and complex receiver developed for Military application. The 51J-4 is the predecessor to the R-390/URR and the improved R-390A/URR. Below the two pictures compare the
51J-4 to the 75A-3. Note the two top views below. The complexity of the J4 is evident. Note the position of the filters. The J4 has three filter positions mounted vertically. The 75A3 has two
filter positions mounted horizontally. Much mechanical hardware was required to give the 51J-4 the continuous tune capability. Collins loved permeability tuned systems (iron slug moving in the center of an
inductor) and the 51J-4 certainly reflected this design. Another observation is Collins always employed really sharp mechanical engineers. This was not common in the industry in this early period. Usually the
companies had really sharp Electrical Engineers that stayed away from complex mechanical designs. At Collins ME's exercised considerable impact in new product design. The success of the Collins products
indicated the impact. Both of these units are Howard Mills restorations. The outstanding workmanship speaks for an outstanding craftsman.