In a cable-steerable endoscope, borescope, or guide tube in which opposing cables are displaced to deflect the tip of the instrument, a self-adjusting mechanism in the control section ensures that a maximal differential force and a minimal total force is applied to the two steering cables. The self-adjusting mechanism has a frame that is affixed within the control section and a slider plate that is slidably disposed on a longitudinal rail of the frame. The slider plate projects outwardly to engage ends of the cable sheaths for the steering cables. The slider plate engages the frame rail when there is force on only one steering cable to lock the slider plate against sliding motion. When there is no force, or equal forces on the steering cables, the slider plate moves proximally on the frame. This arrangement, along with a reset spring, automatically takes up unneeded play or slack in the cables and also avoids the simultaneous tensioning of the two cables that can produce high steering forces and that can damage the steering section.