The Sloan-Kettering Institute CS-15 compact cyclotron was the first prototype built by the Cyclotron Corporation. The description of the RF system for this machine has been reported by Hendry( The original automatic frequency control used a stable tuned frequency discriminator. However, for various reasons, into service. this unit had never been put In addition, unlike the newer model machines which employed a series pass tube for anode voltage control and regulationc2), our anode supply did not have such a device. Therefore the long term drift down of dee voltage had been quite appreciable. As a result, the external beam current was very unstable and a full time operator had to sit before the console constantly making necessary adjustments.