Dimensions of the RCA Oscillator
by Reid Welch
December 25, 1997
Both RCA oscillators are identical, two-layer coils wound on a tubular fiber form about 1.5" diameter by 1 3/4" long.
The wire is single silk covered ("SSC") enameled wire approximately #30 B & S gauge.
Turns per layer- about 75.
The winding length is around 1 3/4"
A couple of turns of black "book cover cloth" is wrapped between layers.
Inductances: inner layer =158uH actual. Outer layer= 177uH actual.
Note: The inner and outer inductances are unmatched because the outer layer, having a greater diameter, happens to offer more inductance.
General hook up: The coil layers are wound in opposite directions. To wit:
If the inner layer is CW, the outer layer is CCW. Also, the oscillator system will not work if any one layer is connected in reverse.
RCA stock connections:
Note: As said before, if connections of either layer are reversed there can be no oscillation. OTOH, the layers are interchangeable, but that's not necessarily optimal.
Inner layer "start" goes to cathode. The '27 cathode is standing on a bypassed 5k resistor to ground.
Inner layer "finish" goes to the grid.
Outer layer "start" goes to the bypassed B+ rail.
Outer layer "finish" goes to the plate.
RCA uses a cap of about 1100 pF between grid and plate on each of the fixed and variable oscillators. About 50pF of adjustable capacitance is in parallel with the larger C for the purpose of trimming oscillator frequency. The frequency runs about 175kHz.
RCA-style oscillators are easy to make. The first (and only) ones I've made were wound on the paper tube core from a roll of towels. These operate today in the Stout. I merely approximated the RCA specs, using smaller wire (32 DCC) and a little less inductance (150uH) than the RCA and a C of about 900pF. The frequency is 212kHz.
Larger wire is surely a good idea.
An LCR meter is handy if you work with coils. Equally valuable is a frequency counter. The Radio Shack 22-168A multimeter measures frequencies up to 1mHz.
Place theremin oscillator coils as far apart as possible (six inches minimum) and preferably not on or near a metal chassis. Orient the coils 90 degrees from each other. Oscillators drift with the slightest temperature change, so if there is a slow, continuing temperature rise inside the cabinet pitch drift will continue indefinitely.
[Copyright 1997 by Reid Welch]
There are no comments.