marginal oscillator for NMR
Date: mid 20th-late 20th Century
Inventory Number: 2004-1-0408a
Classification: Nuclear Magnetic Resonance
Accessories: variety of spare parts cataloged as 2004-1-0408c-g
DescriptionHybrid (vacuum tube / transistor) marginal oscillator mounted together with an electromechanical drive assembly on a yellow wood board, with four decaying pieces of rubber tubing nailed to the bottom of the board as feet to insulate the oscillator from ambient vibration.
The circuitry of the oscillator is shielded from external radiation by a cast aluminum case screwed shut, which when opened reveals a lab-built lattice of wire, transistors, resistors, and a capacitor composed of several parallel plates attached to a rotating axis and which during part of its rotation passes through another array of parallel plates interlocking with it without touching. The oscillator also uses a single vacuum tube, mounted outside the case (presumably for heat dissipation). Also on the outside of the case are three manual controls: an attenuator switch, another unlabeled switch, and a feedback knob adjustable by discrete intervals from 0 to 10. The box also has electrical inputs or outputs labeled "low audio," "high audio," "coil" (where the Rf coil connects), and "rf out." Through at least one of these, the signal would be output to an oscilloscope (for monitoring).
Mounted beside the oscillator is an electromechanical drive assembly made up of an electric motor (with a standard electrical cord and a simple on/off/reverse controller) and two gearboxes. The motor's rotational motion was routed through the gearboxes to change the angular speed of an output shaft. The output shaft of the gearbox can be coupled and uncoupled from the capacitor inside the marginal oscillator by means of small screws. A variety of metal, wood, and plastic shims have been placed beneath the motor and gearboxes in order to more precisely align the shafts.