Mix adjusts the amount of dry (unprocessed) signal relative to the amount of wet (processed) signal. Full clockwise is 100% wet. (On original units, this control is located deep inside the box, typically soaked in carcinogenic PCB oil.)
Variable Delay selects the delay time. Delay times vary from 0.06 to 0.3 seconds. Full clockwise is slowest.
Variation adjusts how much variation occurs in the delay. The more variation you use, the more warbled and wobbly the sound becomes.
Sustain determines how long the delay takes to die out. It is actually a feedback control similar to the one found on the Moogerfooger Analog Delay.
Echo/Doubler determines whether or not a second record head is engaged, resulting in a double echo.