cuit derived bias voltage across a 100,000 ohm resistor, bypassed for audio, in the grid circuit of the last if tube. The voltage developed there at high RF levels was applied to the grids of preceding stages. The system preserves the modulation envelope pretty well for subsequent detection. (When the detector diode feeds a delayed AVC line, extraction of power from the if circuit by the AVC only at the top ol the modulation envelope tends to distort the audio peaks. This drawback was overcome in the latter AN/ARC-5 sets b> using unde-layed ÀVC clamped by a shunt diode which provides the required ''delay" in the AYC action) "blocking," which the SCR-274-N and AHA sets were designed to defeat would have given spurious navigation information by giving decreased output with increased input beyond a certain RF input leveli
T he HAT RAV and ARA designs were virtually identical with the SCR-274-N (N for Navy) production through 1942, but changes were in the wind.
Crystal control had become practical. Stability was of the essence. Command sets were be rn,H used for fleet communications over much longer distances than the design basis of short-range plane-to-plane work. Combat pilots could not twist volume control knobs continually. Special instrument landing system equipment was used with teh Command system, m
The author lias alreadv described the Naval m
Research Laboratory* Crystal-controlled com-
mand set—with AVC—circa 1943. Official Naval correspondence reveals the problems
On the left is the Model I, Serial I low frequency Command receiver, circa 1936, Note the slightly different front panel control panel, the phone jocks and different placement of the antenna and ground fittings. The dial was adapted from the Navy RU series receiver.
which cried desperately for improved gear to handle the new demand of the services. The Army it so I! sent A.R.C. President Dr. Lewis VI. Hull to Europe with Ceneral Harold McClelland to see what could be done to improve communications in the Eighth Air Force.
Dr. Hull's highly-classified recommendations; remove all controls except volume from lhe pilots' panel; locktune or crystal-control I Iki Command Set, and add AVC to make even the volume control unnecessary'.
Back in Boon ton A. B.C. engineers attacked both problems. Under a contract calling for receivers to operate with the ZA ILS system, with the assistance of Air rack Corporation engineers, the low frequency command receivers were overhauled. Delayed AVC of a more conventional type was applied to the 190-550 and 520-1,500 kc receivers and cathode follower outputs were provided to the ZA equipment in the aircraft.
The product was the R-20, R-21, R-22, K-23, R-24 receivers. The first three, covering 1.5-9.1 mc retained the SCR 274-N AVC, while the navigation sets were more drastically altered. All five bands were given new, redesigned, temperature-compensated front ends, additional use of ceramic dielectrics, and new external controls. A type 12SF7 if tube was specified in order to provide proper bias on the cathode of the AVC stage in the new design, and to avoid interaction between the AVC and the BFO circuits.
Before R-20, R-21 and R-22 production was j >ut into high gear the Navy decided to put full AVC into all the1 command receivers. I he
Top view of the first Q-5er, The set used only two 90 kc if circuits. Designer Paul O. Farnham said recently "the use of three tuned if coupling units on the two lower frequency bands was not deemed proper because two such tuned units appeared to give adequate gain and selectivity. We changed our minds later/' Tube lineup in this set included 6K7 RF and if tubes, 6L7 mixer, and 6H6 detector. Audio and BFO tubes were experimental Raytheon CK-45 power pentodes with 6,3 yolt heaters designed for A.R.C, modified sets were labeled R-25, ft-26 and R-27 in the AN/ARC-5 series.
rhe tuning capacitor in the ARC-5 series was a completely new component. It was so built—out of brass and invar (36% nickel) steel, as to have a slight (.000015 pf/pf/de-gree C.) negative coefficient, i.e., with normal warm up heating if would decrease in capacitance. (Air tin-compensated capacitor would be expected to increase markedly in capacitance witli temperature*)
The warm up of the oscillator tube and the oscillator tank coil would tend to increase the circuit capacitance. The negative tuning capacitor behavior would thus tend to offset that change. Residual changes were absorbed by an additional small (3 pf) "padder" with a negative coefficient of 750 parts per million per degree centigrade.
Unfortunately the R-23 and its sister R-2 4 (520*1,500 kc) receiver suffered from low audio output problems. Back at the drawing boards the A.R.C. Engineers under designer Dr. Frederick Drake made more changes.
Output with the improved circuit was increased from 120 milliwatts for the standard input of 10 microvolts at 1000 cycles modulated to 30 percent to better than 400 milliwatts.
I'his new model—the R-23-A/ ARC-5—was accepted by the Navy in 1945 despite the fact that the test aircraft at Anacostia Navat
Air Station in Washington was equipped with a 14 volt electrical system, (The R-23-A was a 28 volt receiver.) Engineer Norman J. Anderson recalled recently that a planeload of Navybrass found the set highly improved despite the half-voltage on the tube heaters.
Shortly afterwards the Navy ordered the IM48/ARC-5*, a 14 volt model of the R-23-A.
p had disappeared. The low-frequency command receiver became a standard item on all military aircraft for range nagivation even though the transmitters and receivers for other bands were abandoned and the CW oscillator joined the ZA in disuse-
Although the R-23-A was now standardized, low-frequency navigation was disappearing. By 1960 it was a relic in North America and obsolescent overseas, YHF Omni had replaced it. The R-23-A remained as a little-used standby in transport aircraft, a role it still plays, today, in 1966, thirty years after it was designed.
The final improvement in the receiver was noise-limiting, proposed by the Navy in 1946. The 12SR7 BFO tube and the BFO transformer* plus the ZA output circuits were dropped. In their places were added a 12116 double diode and a noise-limiter control relay. The 12A6 output tetrode was replaced by a tetrode-diode tube under the RCA experimental number A-5023.
Noise limiting had first been applied to command receivers in the 1944 VHF receiver desitrn, following techniques developed by the British in wartime research. It had been the subject of considerable research in both private and government labs in this country, with detailed work done by Maguire Industries under a Navy contract.
The R-112 and R-112/ARC-5 receivers used diode-connected (riode noise limiting circuits, but the R-23-A Was built with the 12H6 twin diode tube. AYG in the set was delayed by using the diode !lj the 12Sf 7 if tube as o clamp to prevent AVC voltage from appearing at the RF and if grids until enough AVC voltage had been generated to override the 30 volt cathode bias on the 12SF7 tube.
The modified R-23-A was examined at Wright Field and at NHL in 1945 and 1946. At the same time competitive bidding on R-23-A procurement brought in the Lewyt Corporation and Strom berg Carlson. The former failed to fulfil an 1,100 set contract according to specifications and the latter finally dropped command set production. A.R.C. officials were Stung by the postwar military procurement depression and moved into the civilian market The Command Sets were finally demobilized und dressed in gray peacetime paint.
Engineer Paul O. Farnham, the designer of much of the command gear, told this writer he was disappointed to see the command design changed, the dial discarded, the plug-in RF and if feature abandoned, and cost made a higher consideration than maintenance convenience.
Hut in many ways the postwar gear was very good. The best materials were no longer scarce. New ceramics and modem finishes including better insulating varnish and other top-quality components were now available. Locktal tubes such as the 14AT and the 14R7 were ready. They eliminated the moisture-holding tube base of octal tubes and offered improvements in reliability.
Early R-l I (civilian) receivers returned to the twin-triode NL circuit, but YHF gear was built with a double triode AYC-NL-deteetor designed by Farnham, using an unbypassed audio cathode resistance, Later ( 1958) a highly effective squelch was added to this circuit.
The dial-less, ceramic-insulated locktal-tube command set was basically designed in 1946 by Engineer Norman J, Anderson with only slight changes from the modified H-23-A to adapt the postwar tubes.
The last 15 years of command set production was devoted chiefly to these civilian versions, R-10, R-ll, R-13, R-l 5 and R-l 9 which were bought in large numbers bv the military for use in the Korean conflict in 1950-54, Aircraft Radio Corp. was rescued from the postwar aircraft equipment depression by the demand of the Army and Navy for lightweight radio gear to be used in helicopters and spotting planes like the L-17 Navion, the L-19, and aircraft as large as light twins and jet trainers such as the T-37.
Much of the business went into Cessna-built light planes, an association that culminated in 1957 in the purchase of A.R.C., by the Cessna Corporation. The transaction came at (he time that crystal control was becoming mandatory in civil flying and transistors were revolutionizing electronic design.
Most of the old hands at A.R,C, have now left Boonton, President Lewis Hull, Field l£n-gineer AI Parkes, Paul Famham, Norman Anderson. John Johanson, and the father of the command sets, Dr. Fred Drake were all retired from A,R.C, by 1961 when the last Command receiver left the white clapboard plant in Boonton, NrJ, The Command Sets, and an era, had ended.
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