RCA Theremin Service Notes

Electrical and Physical Specifications

ELECTRICAL SPECIFICATIONS
Voltage Rating:	105-125 Volts
Frequency Rating:	50-60 Cycles
Power Consumption:	90 Watts
Type of Circuit:	Special beat frequency oscillator with audio stages
Number and Types of Tubes:	Three UY-227, 1 UY-224, 1 UX-120, 2 UX-171A, and 1 UX-280-Total 8
Number of Oscillators:	3
Type of Detector:	Two-grid detector modulator
Number of A. F. Stages:	2
Recommended Loudspeaker:	RCA Loudspeaker 106
Musical Range-31/2 Octave:	Lowest note of loudspeaker to 1400 cycles approximately
PHYSICAL SPECIFICATIONS
Height tn top of Pitch Control Rod:	64 in.
Height of Cabinet:	461/2 in.
Width, including Volume Control Loop:	311/2 in.
Width of Cabinet:	19 in.
Depth:	12 in.
Dimensions of Packing Case:	141/2 in x 24 in. x 51 in.
Weight, Net:	671/2 lbs.
Weight, Shipping:	99 lbs.

INTRODUCTION

The RCA Theremin is a musical instrument operating entirely on electrical principle, and played by the movement of the hands in space. Having no limitations such as key boards, stops, etc., exceptional individuality of expression may be obtained. The instrument covers approximately three and one half octaves, the highest note being about 1400 cycles. Figure 1 illustrates a rear interior cabinet view and Figures 2 and 3 a top and bottom chassis view of the main assembly. The operation of this instrument is covered in the instruction book accompanying each RCA Theremin.

The principle of operation of the RCA Theremin is that of the beat frequency oscillator. The frequency of one oscillator may be varied by the capacity change caused in an associated circuit by the movement of the hand to or from a pitch control rod. Also an additional oscillator provides radio frequency current for heating the filament of a UX-120 arranged so as to control the volume of output, this control being due to the movement of the hand in relation to the volume control loop A detailed description of the functioning of the various circuits follows:

ELECTRICAL DESCRIPTION OF CIRCUITS

As stated in the foregoing, the musical note of the RCA Theremin is produced by two oscillators of slightly different frequency beating together. This beat note is then amplified by two audio stages. The change of note caused by the movement of the hand is due to the change of capacity across what is known as the pitch coil, this slight change having sufficient effect on an adjacent oscillator circuit to change the frequency and thus the beat note, the amount of change depending on the position of the hand in relation to the pitch control rod.