Radio Digest (July 1924-Apr 1925)

26 RADIO DIGES T— Illustrated February 7 , 1925 2 TUBE KNOX REFLEX (Continued from page 25) ■while the writer now had a howl-less set, it was far from an efficient one. The local stations came in loud on the A tubes and the tone was excellent, but the DX reception was far from satisfactory and that of first making a wooden hub 2% inches in diameter and 1 inch thick, after which 15 holes are drilled in the circumference of this hub at equal distances apart. These holes are to be M inch in diameter and Vt inch dowel rods 2% inches long are fitted into the holes. These should be a snug enough fit that they will stay in while the winding is in progress, but not so tight that they cannot be readily removed when the winding is finished. When the required number of turns have Figure 3 it was felt that there was some way of improving this. About this time an article came out which stated that some of the most popular reflex circuits were prevented from oscillating by using a small number of turns in the primaries of the Radio frequency transformers, but that in doing so the impedance of the plate circuit was reduced to such an extent that the plate voltage variations were very small, which reduced the efficiency of the set. This seemed perfectly reasonable and although it was known that by raising the impedance the tendency toward self oscillation would be increased, it was felt that regeneration was now under strict control. The writer decided to try a 50-turn honeycomb coil in the plate circuit of the first tube; this raised the impedance all right, but it also started the howling and whistling again. However, we were on the right track, so it was decided to vary the action of this coil by shunting a variable condenser across it, and discovered that the regeneration was at last under perfect control with tuned impedance. The set still retained its excellent tone quality, but was now also an excellent distance set. Los Angeles is one of the most difficult cities in which to receive DX stations as there are many strong locals, and other conditions which seem to prevent getting out of town, yet the receiver now went through the locals and put stations 500 to 800 miles away on the loud speaker with good volume. Constructing Spiderwebs As can be seen from figure 2, there are six spiderweb coils used in this receiver and, as these are the only parts which must be made at home, we will take up their construction first. There are two methods of winding and using these coils. The easier method is that shown in figures 3 and 4 in which a thin fiber form is used with a solid center about 2% inches in diameter, while the outside diameter is 5 inches. The wire is wound on this form in the manner shown and to the required number of turns, and when the 'coil is finished the form is left inside the coil and used for mounting. The second method which was used by the writer is LIST OF PARTS 3 Variable condensers, .0005 mfd. vernier $16.50' 1 Variable condenser, .000045 mfd. midget 1.50 1 Crystal detector, panel mount.. 1.75 2 Sockets 2.00 2 Rheostats, 20-30 ohm 2.00 6 Spider web forms, 5" 3.00 1 14 lb. spool No. 26 sec. wire 1.40 1 % lb. spool No. 30 sec. wire 85 2 Audio frequency transformers.. 8.50 7" x 18" panel 3.15 7" x 17" base-panel 2.95 Filament switch 1.00 Open circuit jack Binding posts Pixed mica cond., .001 mfd Fixed mica cond., .006 mfd Fixed mica cond., .0005 mfd Fixed mica cond., .0001 mfd. Cabinet 7" x 18" x 7" Resistance unit, 24 ohm Miscellaneous bus bar screws, etc .80 .60 .40 .75 .35 .35 9.00 .76 2.00 $59.60 All coils, with the exception of L5, may be wound with number 24, 26 or 28 single cotton covered wire, or double silk if preferred, but coil L5 should be wound with wire not larger than number 30 and either number 30, 32 or 34 may be used. There should now be six spider web coils each on its own form and ready for mounting. (Next week Mr. Knox gives the panel drilling layout and baseboard layout with detailed instructions on connecting the coils. — Editor's Note.) REFERENCE LIBRARY (Continued from page 19) station. Occasionally, "test" to see if the detector is properly adjusted (provided you are not listening to a station). Battery Supply for Crystal Sets Battery supply for crystal detectors was used as far back as 1903 — four years before I heard or saw a Radio station in operation. Battery supply will aid silicon detectors and several other types, but it is primarily intended for carborundum crystals. It is strange indeed that so few carborundum detectors are being used. It will hold its adjustment for days and is fairly sensitive. It is the kind of crystal detector that "saves your religion." Note the sturdy construction suggested in figure 5. A battery is connected in the circuit as shown in figure 4. A "dead" dry cell and a potentiometer (400 to 600 ohms) are the only additional parts required. The battery must be connected in the circuit as regards polarity in such a way that it aids reception. To the broadcast listener it is suggested: try the battery first in one position in the circuit and then reverse the connections. Then place it in the circuit in the position in which best results were obtained. The writer has never been successful in adding battery supply to galena crystals. Another carboborundum hook-up is shown in figure 6. (Next week Mr. Jones goes into the factors which limit Radio reception and enable some fans to get both coasts while a neighbor has difficulty getting out 500 miles. — Editor's Note.) GLOBE 1<W LOSS TUNER LOG ANY STATION IN THE U. S. Figure 4 been put on this wooden form they may be either coated with cement or held firmly in place with heavy waxed thread interlaced in the windings. The third possible method is to wind the coils as shown in figures 3 and 4 and then go around the center disc with a fine hack saw, cutting off all of the spokes except one which is used for mounting. The turns are, of course, secured with thread or Radio cement before cutting out the fiber. Coil L2 may be wound first of all with 41 turns, on either of the forms above described. "When winding coils LI and L5 string should be wound on first to a depth of about % inch after which 17 turns of wire are put on one form which makes coil LI, and 17 turns are put on another form to make L5. Coils L3 and L4 are wound each on their own form without string, L3 to contain 27 turns and L4 to contain 51 turns. Coil 6 is wound on another form and contains 33% turns. Boosting Weak Signals • Weak signals from distant stations received on a set using a loop antenna as the intercepting medium can be greatly intensified and strengthened by grounding the negative side of the A or filament battery. The increase will be particularly noticeable on sets employing only two stages of Radio frequency amplification because the infinitesimal amount of energy picked up by the loop cannot be sufficiently amplified. Distant stations that can barely be detected can often be raised to good audibility by grounding the receiver through the negative side of the A battery. While having, the aforementioned advantage it also has the drawback of raising the static and noise level along with the weak signals and in many cases of broadening the tuning. TUNING BROADCASTERS (Continued from page 9) times it is found good to shunt (place in parallel) heavy capacity (0.5 to 2 mfd.) fixed condensers across the A and B battery leads to the receiver, especially where these leads are over one foot in length. This prevents energy, so picked up, from going on through the receiver. After all is said and done, the increasing number of stations, many of which are of the super power type, has placed the Radio section of the department of commerce where it is literally "up against it." It remains therefore for the manufacturers and Radiophans to improve their sets to meet the demands placed upon them. Radio-Madrid Spanish Radio company at present operating the big broadcasting stations in that country, has recently undergone a complete organization. — Patented Dec. 9, 1924 — Suitable for use in all standard ^ — hook-ups. 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