Which of the following is NOT a method of controlling the refrigeration compressor output?

The capillary tube is a specific component used in refrigeration systems primarily to control the flow of refrigerant between the condenser and the evaporator. It is a fixed orifice device that allows refrigerant to expand and cool based on the pressure differential but does not actively control the output of the refrigeration compressor. Instead, it simply regulates the amount of refrigerant entering the evaporator.

In contrast, cylinder unloaders, suction throttling, and speed control are dynamic methods that adjust compressor output based on the cooling load requirements. Cylinder unloaders can help vary the displacement of the compressor, allowing it to reduce its capacity when less cooling is needed. Suction throttling involves manipulating the suction pressure to control the energy that the compressor draws, effectively regulating its output. Speed control adjusts the compressor's operational speed to match the cooling demand, allowing for more precise control of the system’s performance.

Therefore, among the options listed, the capillary tube is the method that does not control the refrigeration compressor output, making it the correct choice in this context.