R-390A

The R-390A military shortwave radio receiver was the result of a project undertaken by the U. S. Army Signal Corps to replace the existing R-390 receiver then in use. The R-390 had done its job so well that the Corps decided continued use of this type of receiver necessitated an improved, reduced-cost version.

Accordingly, in 1951, the Signal Corps met with the receiver’s original designer, the Collins Radio Company of Cedar Rapids, Iowa. Two objectives emerged from this meeting: improve or redesign existing circuitry, and simplify the receiver where possible without sacrificing performance. Meeting both objectives was expected to result in a reduction of the cost of the receiver. With the Signal Corps’ specifications in hand, the Collins engineers got to work. At least four working prototypes were submitted to the Signal Corps before Collins submitted its final progress report in 1952. The Signal Corps engineers accepted most of Collins’ recommendations. Production of the new model, called the R-390A, began at the Collins factory in 1954. In later production runs, the receiver was subcontracted to other manufacturers, among them Motorola and the Electronic Assistance Corporation (a division of Hammarlund). Even the famous cosmetics firm of Helena Rubenstein, in an odd bid to win military contracts, produced a small run of the receivers.

DESIGN OF AN ELEGANT RECEIVER

The R-390A is a general coverage radio receiver capable of receiving amplitude modulated, code, and frequency-shift keying signals. Its tuning range is from 500 kilohertz to 32 megahertz, in 32 one-megahertz bands. The circuit is the superheterodyne type, double conversion except on the lower bands where triple conversion is used. It employs 21 vacuum tubes, a larger than normal count for most general-coverage receivers. The receiver weighs 85 pounds and can be operated on ordinary 120-volt current. It fits neatly into a standard 19-inch relay rack and can be remotely operated.

Tuning of the R-390A’s radio frequency and intermediate frequency front end is synchronized by means of an ingenious mechanical system of racks, gears, and cams. When the front panel tuning controls are rotated, this system raises and lowers ferrite slugs in and out of the receiver’s tuning coils. This ensures that all front-end circuits are tracked, meaning all circuits are tuned to the correct frequency to maintain excellent selectivity and sensitivity.

The receiver’s construction is modular for easy servicing. Each major area of the receiver is contained in easily removable subassemblies, and these can be repaired or replaced as needs be. Though the R-390A is mechanically and electrically complex, alignment and servicing are not difficult with the Signal Corps’ published procedures.

The R-390A’s input circuitry is so well designed that it can receive signals down to its –143 decibel noise floor, which is close to the galactic limit. A vacuum tube originally designed for television service, type 6DC6, provides much of the receiver’s RF performance.

A major part of the R-390A’s utility, and its fame, lies in its mechanical digital frequency readout. In appearance, the display resembles an automobile odometer. To tune the receiver, the operator first selects the desired megahertz band with the "Megacycles" control. Then, the entire one-megahertz range of the band may be tuned with the "Kilocycles" control. As the operator tunes through the selected band, the display gives an exact (to within 100 hertz) digital readout of the frequency to which the receiver is tuned. In the pre-electronic digital era, this represented a major advance in receiver technology, doing away with analog dial calibration and interpolation.

Six panel-selectable bandwidths are available to assist the operator in reception under difficult conditions. Those bandwidths are 0.5, 1, 2, 4, 8, and 16 kilocycles. A crystal filter circuit provides the 0.5 and 1-kilocycle bandwidths, and the remaining bandwidths are provided by means of mechanical filters. These filters were designed by Collins Radio for the R-390A, and were so efficient that they became an industry standard for filters. Collins mechanical filters are still available today from Rockwell-Collins, the successor to the Collins Radio Company.

The receiver also provides audio response filtering capability. There are two positions: a "wide" bandwidth for use under most conditions, and a "sharp" bandwidth to assist with radiotelegraph code reception. The R-390A’s audio response is at best adequate; most users either add an external audio amplifier for improved quality, or modify the existing circuitry.

The remainder of the R-390A’s design is standard industry practice. A beat frequency oscillator, necessary for code or single-sideband reception, is available. Two audio amplifiers are provided – one for local loudspeaker output, and the other for line output. There are two analog meters mounted on the panel, for monitoring line output and received signal strength.

THE R-390A TODAY

The R-390A was deployed, over time, to most branches of the military and was as successful as its predecessor. It remained in general use through the 1980s; the last major update to its documentation was in 1984. As the military has procured newer receivers, many R-390As have been released to surplus while others were thoughtlessly destroyed. Some receivers were retained by the services, however, when they found that the R-390A’s vacuum tube circuitry could easily survive an electromagnetic pulse. There are reports, possibly apocryphal, that R-390A receivers are still in use aboard U. S. Navy submarines since the receiver can withstand the strong radio-frequency fields found aboard ship.

Many of the R-390As that exist today are in the hands of collectors and amateur radio operators. The receiver has lost little of its famous reputation; few modern solid-state communications receivers can equal its performance. There is a wealth of information, both printed and electronic, devoted to R-390A restoration and maintenance. The R-390A is a marvelous receiver, and a fine example of the best of vacuum tube technology.

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