Single-Signal Receiver for Byrd Expedition
By McMurdo Silver
(Reproduced from the October 1933 issue of Radio)
McMurdo Silver's advertising during the early 1930's often noted that Silver radios were selected for use on Admiral Byrd's polar expeditions. The apparent implication was that the sets advertised were the same as used by Byrd. Not so, as this article certainly proves.
PROBABLY the first use of the quartz crystal resonator for extreme selectivity in radio receiver design that came to popular knowledge was Dr. Robinson's Stenode Radiosrat circuit. Unfortunately, this system contributed greatly to "apparent" selectivity (difficulty of tuning), but very little to "actual" selectivity (elimination of interference), except in almost direct proportion to loss of fidelity. Time seems to have proven that a crystal resonator has no place in a high quality broadcast receiver.
But for C.W. code reception, it has a very definite place, particularly for the amateur bands, the width of which a good crystal resonator receiver (single signal) will effectively double. But a crystal resonator cannot be added to any superheterodyne in a haphazard, hit-or-miss manner. There are plenty of tricks in getting it into a receiver design and making it work as it really will if the design be properly engineered.
The receiver here described and illustrated was selected by Dr. McCaleb of Harvard, acting as Admiral Byrd's radio advisor as the communication receiver for the Admiral's 1933-1934 Antarctic expedition. It and all other short-wave and broadcast receivers for this expedition were designed and built by the writer. It is strictly a custom-built, single-signal superheterodyne, employing a properly designed quartz crystal resonator which effectively eliminates one audio image (one side of heterodyne signal) from every C.W. code signal, actually cutting in less than half the space in the frequency spectrum occupied by any C.W. code signal. It is also an excellent and advanced superheterodyne, which with the crystal switched out, is ideal for short wave broadcast or phone reception.
It covers a range of 200 to 10.1 meters (1,500 to 25,000 kc.), can be used with regular or doublet antennas, has a C.W. beat oscillator, is entirely self-contained with no plug-in coils, A.C. operated, and has band spread tuning functioning anywhere in its range- on amateur, commercial or broadcast bands. 17-1/2-in, long, 10-1/2-in, deep and 8-3/4-in, high, it is self-contained in its own easily removable shielding case, and will fit a standard 19-in, relay rack if desired.
Its sensitivity is better than 1/2 microvolt absolute, its selectivity with crystal cut-out absolute 10 kc. or one channel (22 kc. wide 10,000 times down) or absolute single-signal with crystal in series circuit, its fidelity flat to 4 db. from 40 to 4000 cycles with crystal out, and its undistorted power output three watts (5% harmonic distortion).
The circuit employs a '58 tuned r.f. stage (V1) on all four bands, and a 2A7 first detector and electron coupled oscillator (V2). The r.f. and first detector circuits are tuned by the left hand six-to-one vernier dial, and the oscillator by the similar right hand dial, The center dial is the oscillator vernier, or band spread tuning-the only control used in tuning over any short wave band.
The new 2A7 tube is the first combination tube which actually does a better job than will separate tubes to perform the same functions. It is a remote cut-off (no cross talk) screen grid first detector, and an electron coupled signal frequency oscillator. Its conversion gain and frequency stability are superior to separate tubes used to perform its two functions.
The tuned r.f. stage preceding it eliminates the image frequency or repeat point found on all short-wave receivers starting with only a first detector tube, and also constitutes amplification which tends to minimize oscillator hiss found in sensitive superheterodynes not so equipped.
Separate coils for all of these circuits, in separate aluminum shields, are selected by a positive, long-lived, five-gang, four-position band selector or wave-change switch at the lower center of the panel. The tuning dials are six to one reduction ratio, and employ an automatic take-up gear drive, free of backlash or play.
The first detector is followed by two stages of 465 kc. i.f. amplification (V3, V4). The i.f. transformers utilize Litz coils of excellent "Q," tuned by a new type of mica and isolantite compression trimmer, providing better selectivity by virtue of a more favorable LC ratio.
The crystal (XL) is placed in the i.f. amplifier input circuit, and is controlled by a switch having off (for broadcast), parallel (for intelligible phone), and series (for single signal code) positions. The selectivity it provides is variable, being controlled by the lower right knob (C5-C6) which actuates the air tuning condenser of the crystal input circuits. A 465 kc. Bliley crystal and plug-in air-gap holder are used.
The second detector is a '56 triode (V5) to the plate circuit of which is coupled the '58 electron coupled beat oscillator (V7) for C.W. code reception or location of weak phone or broadcast stations. This oscillator is turned on or off by the upper left toggle switch (S7) and its audio beat note is controlled by the vernier condenser actuated by the lower left knob (C8).
Tube V6 is a '56 diode A.V.C. tube, giving the full benefits of automatic volume control for phone or broadcast reception. It can be cut out when desired for code reception by the lower right toggle switch (S8). The lower left center knob is the audio volume level control (R1) and an off switch (S9), while the lower right center knob is the sensitivity, or manual volume control (R2). The lower left toggle switch (S7) cuts off B supply to prevent blocking when the. receiver is used close to a powerful transmitter.
The audio amplifier consists of a single '59, 3-watt pentode (V8), resistance coupled to the second or audio detector, and having an output jack on the rear of the receiver chassis for head phones or magnetic speaker, and a four-pin plug for the eight-inch Jensen speaker furnished. Use of head phones cuts out the dynamic speaker.
The power supply is conventional, using an '80 rectifier (V9) in a condenser input filter system employing two filter chokes-one in the chassis and the second, the five watt speaker field. Semi-self-healing dry-electrolytic filter condensers are used.
Throughout the design of the receiver, electrical symmetry has been rigidly held to in the placement of all parts, so that each circuit progresses through the shortest possible leads on into the next circuit. The result is absolute stability and the entire absence of regeneration, resulting in a most favorable signal to noise ratio.
As for results, foreign amateurs and broadcast stations at excessive loud speaker volume are "duck soup" to the 5A single signal receiver, while selectivity can be made anything from 50 cycles and less to 10,000 cycle band width by means of the crystal switch and selectivity control.