Construction of a Semi-Replica RCA Theremin

By Dayfan - 12/27/2005 - Tech and DIY

Introduction

Acknowledgements – Arthur Harrison provided enough data on his website to inspire me to attempt constructing a replica RCA Theremin. I knew that I would never own one and I had actually never seen one except in pictures. Making one was as close as I could get.

History

The history of the invention of the Theremin by Leon Theremin is well known so I won’t go into it again except to say that his invention opened up a new world of electronic musical instruments from the Theremin to the Ondes Martenot to the modern synthesizers we see today. My inspiration probably started with the movie Forbidden Planet, which still has some great Theremin-like sounds. My first Theremin was a design by Moog published in Radio and Television News in 1954 which I built in the mid 1970’s. Parts for tube equipment were hard to find even then and I had to have two of the TV horizontal sweep coils required in the circuit wound by the original manufacturer. This Theremin ended up in a rock band.

Preparations

Gathering data – My first efforts went into gathering data. I searched the web for all the data, pictures, and general information I could find. The most useful was the RCA Service Manual and Arthur Harrison’s website. His website had specific information on some of the coils and capacitors that were not spelled out in the RCA Manual and a redrawn schematic that made deciphering the circuit much easier. Unfortunately there was no source for some of the components but I decided I would just have to figure them out.

Gathering parts – I have restored antique radios for about 10 years and have been a amateur radio operator since 1955 and being the type that never throws anything away, I have a large collection of vintage parts. I dug through what I had and found most of the power supply and a lot of the main chassis parts. The critical missing components were the chassis, audio transformers, some tubes, and the coil forms. I used a cardboard mailing tube for the main coils and the oscillator coils were wound on another cardboard tube. Fortunately most of the missing components were available from Mouser Electronics or Antique Electronic Supply. Dimensions of the chassis and any other parts were scaled from drawings or photographs that I found on the web.

The coils I could wind but the tubes and the audio transformers had to be found. All the original tubes are available but three, the two 171A’s and the 120, are getting scarce and very expensive. I decided to try a 31 in place of the 120 and I had two 71A’s, a more modern version of the globe-shaped 171A’s, along with the other tubes.

I knew that I couldn’t exactly duplicate the original instrument because many of the parts and values were unavailable and unknown and I had never seen a real instrument. My goal was to use what I could find that most closely duplicated the instrument both physically and electronically and maintain the overall feel of the original Theremin.

Power Supply

The power supply is typical of the time (1928) utilizing three chokes to do much of the filtering because high value capacitors were very large. The chassis I used was 10 in wide, 6 in deep and 1 in high. The size was scaled from photographs and the Service Manual. I tried to use the same layout as RCA used with adjustments for the physical size and shape of the parts I had. Nothing in the layout is critical. The power transformer voltage I had was too high so I had to use a large power divider resistor to get voltages suitable for the vintage tubes. (Photo 1)

Tags: Theremin Construction, RCA Theremins, DIY