The old Proline servos required a negative going pulse (all current servos use a positive going pulse). The old Airtonics servos are the only ones that I know of that had the positive wire in other than the center connector pin. If you swapped out the leads to put them with the center pin positive, they also work on all current receivers. Also, now the standard is a 1.5 millisecond neutral position with a .5 millisecond excursion either way for full control. Some of the early servos used a different pulse width for neutral.