According to some embodiments, motor-tuned nuclear magnetic resonance (NMR) probes may be tuned manually without overcoming the tuning motor holding force. An NMR probe includes a switchable manual-mode/motor-driven mode capacitance-adjustment assembly for adjusting the capacitance of a variable capacitor connected to an NMR RF coil. The capacitance-adjustment assembly includes a tuning shaft coupled to the variable capacitor through a gear assembly, and a mode-switching coupler coupled to the tuning shaft. The mode-switching coupler includes a first terminal coupled to a piezoelectric motor, and a second terminal coupled to the tuning shaft. In the manual mode, a user pushes up the tuning shaft, decoupling the two terminals of the mode-switching coupler and thus decoupling the motor from the tuning shaft. The user then manually rotates the tuning shaft. In the motor-driven mode, a spring tensioner presses the two terminals of the mode-switching coupler together, coupling the motor to the tuning shaft.