Controls that are sticky, either potentiometers or switches are often caused by lubricants that have hardened with age. Often these controls can be loosened by applying heat to the shaft. As the shaft heats up the hardened lubricants once again liquify allowing the control to turn freely...
Producer: John Pfeiffer. Recording: RCA Studios, Hollywood, USA, August 17/22, 1960 (Beethoven); August 15/16 & 18, 1960 (Bach); August 16/22, 1960 (Schubert)