An electronic scrambler was a device used to interfere with electronic signals.
In 2267, Spock constructed an electronic scrambler to disable an alien entity called the Companion. The Companion was chiefly composed of ionized gases and electricity; Spock's device was designed to scramble its electronic impulses.
The device was very simple; a red on-off switch near the right side of the front panel was the only important control. The scrambler evidently caused the Companion some degree of pain or discomfort, as it reacted immediately, seeking and destroying the device and very nearly killing Kirk and Spock. Only Zefram Cochrane's intervention prevented it from killing the two officers. The device was either not powerful enough, or not sophisticated enough, to cause the creature any permanent harm. (TOS: "Metamorphosis")