Popular Mechanics/Volume 49/Issue 1/Helpful Hints from Radio Experts
These valuable kinks have been written especially for Popular Mechanics Magazine by prominent radio experts. They represent not merely ideas, but practical advice from the best technicians in the profession on methods that will help make your set more efficient in operation, more convenient to handle or easier to build.
Battery-Testing Kink
Here is a simple timesaver any radio owner can make at little or no cost which is especially useful to those set owners who have their batteries installed in a console or radio table-type cabinet where it is difficult to get at them to take a voltmeter reading. To make such a reading is required once in a while to determine the condition of the batteries and, in most cases, it is necessary to drag the cabinet away from the wall and reach into a crowded battery compartment, with the possibility of pulling a wire loose or short-circuiting something. I have eliminated all of this trouble with my set by drilling a few 1⁄8-in. holes in the side panel of the radio table close to the top and about 1 in. apart into these holes I bolted some old nickelplated switch points taken from my scrap box. To these I fastened flexible leads inside the cabinet, connecting them to each battery in turn, as shown, so that by putting my voltmeter across any two points I can determine their condition.—H. G. Nebe, chief engineer. Station WSMB, New Orleans, La.
Output Impedance for Power Tube
With the popularity of the 171-type of power tube, I think the readers of Popular Mechanics magazine might be interested in knowing how easy it is to make an output impedance to keep the B-battery d.c. from passing through the windings of the loud speaker. Not until I had burned one one such winding did I wake up to the fact that something should be done. For the audio choke I used most anything that passed enough current and still offered a fairly high impedance. I am now using the primary of an audio transformer, as shown in the diagram. The output impedance both protects the speaker and improves the tone.—C. R. Yarger, chief engineer, Station KFNF, Shenandoah, Iowa.
Bunched Leads for Sharp Tuning
The set builder who wants selectivity should bunch all leads possible, when wiring the set, and branch each lead off as it comes to its terminal point. This is a simple thing to do, considering the fact that all leads start from the terminal strip to different parts of the set. After bunching, they can be tied or laced with string at various points and fixed with a little collodion if desired. There is one thing that is very important when wiring a set in this manner and that is, that the leads from the grids of the r.f. tubes and detector should be as far away from the other wiring as possible. This also applies to the plate leads from the same tubes to the r.f. transformers; these leads must be kept clear of the grid leads. The sketch illustrates the idea, and of course it is understood that the aerial lead must also be kept clear of the other wiring. When you wire a set and string wires all over the subpanel it is highly probable that they will be energized and create a much longer field than if they were bunched together in the manner described. Use stranded insulated hookup wire and try out this method when wiring your set. I have found it highly successful.—M. M. Paggi, chief engineer, Station KFWO, Avalon, Catalina Island, Calif.
Superheterodyne Kink
In superheterodynes that employ the tuned-plate, tuned-grid type of oscillator, which is subject to spurious oscillations of a frequency other than the frequency to which the coils are tuned, the following simple remedy can be applied. We have found that a small choke coil inserted between the grid condenser and the junction of the grid coil and the grid tuning condenser effects the remedy. The choke coil need not be of special construction; a simple one may be made by coiling the busbar, at the point just mentioned, five or six times around a form 1 in. in diameter.—Wm. M. Perkins, engineer for Aero Products, Inc., Chicago, Ill.
Power Tubes Used As Detectors
One of the most valuable kinks that has come to my attention within the past few months has been the use of UX-120 and UX-171 tubes as detectors, with negative grid bias. The greatest trouble with detector tubes today is the fact that they are unable to handle the tremendous amount of current impressed upon them by radio-frequency amplifiers. Hence, the detectors distort and chatter, due to their inability to handle the necessary current.
In a set using 199-tubes, cut out the grid condenser and leak, and apply 22 volts to the plate of a UX-120 used as the detector tube. From 3 to 6 volts of negative grid bias are required when using this tube. The 171-type is used in the same manner in sets employing the large tubes with from 41⁄2 to 7 volts negative grid bias. Outside of the small additional filament-current drain, there is really less drain on the B-batteries. Far more volume and better quality can be obtained with tubes of this type as detectors, and the idea is easily applied to any set.—R. V. O. Swartwout, chief engineer, Station WCAO, Baltimore, Md.
¶ Loose or broken connections cause trouble both in manufactured and home-built sets; after checking tubes and batteries all wiring should be carefully examined.
Fixed Midget Condensers Easy to Wire
Half the usual size and one-quarter the weight of the average fixed condenser, the midget condensers now appearing on the market are easier to wire. Besides, the builder is often at a loss to find a convenient place to mount fixed condensers in the restricted space below the subpanel. The midgets are provided with presoldered lugs, making quick soldering possible with minimum of heat. They are said to be tested at 1,500 volts, and rated as safe for continuous operation at 1,000 volts, d.c. The marked capacity is guaranteed to be within 10 per cent.
Resistance C-Bias for B-Eliminator
Many of the commercial circuits, using B-eliminators, dispense with C-batteries entirely and use the voltage drop across a resistance to provide C-bias. Such an arrangement is entirely feasible when the B-eliminator gives a sufficiently high voltage output, so that part of this voltage can be utilized for the C-bias. Care must be used, however, or motorboating will develop due to coupling in the resistance. This is common to the plates of two or more of the tubes in the set. A simple instrument for regulating C-voltage is shown in the illustration. The extra bypass condenser for the detector control of the eliminator is very important. The choke coil illustrated may not be needed on some sets. but if there is any tendency toward motorboating it will be required. This choke can be of the commercial types, or the primary of an old audio transformer can be used. The four-terminal power potentiometer shown is connected as in the diagram. permitting the use of a C-bias potential of from 41⁄2 to 40 volts.
Controlling Tone and Volume
There are many sets now in use without a suitable volume control; in others, rheostats or similar devices, which are no longer in good condition, have been used for this purpose. The little device shown in the accompanying photo is said to be capable of an extremely delicate shading, from soft, whisper-like music up to powerful volume with remarkable clarity of tone. In Fig. 1 we have a circuit diagram showing the usual circuit connections before the instrument is installed. In Fig. 2 we show the instrument connected, to improve the quality and afford a means of controlling signal volume. The instrument is available in two types, one combining a filament switch and one without; the switch type is shown in the diagram. When turned as far as possible to the left, the A-battery is off. When a C-battery is desired, the circuit is broken at the point marked X, C-positive is connected to A-negative, and C-negative is connected to F-negative of the transformer secondary.
¶Popular Mechanics' radio department offers its information service free to all readers of our magazine. We will be glad to help you with your radio problems, and will promptly answer all inquiries directed to this department.
Efficient Radio Ground Easily Installed
An efficient radio ground, having a "discharge area" several times as great as a driven ground, is now available. Its walls are of heavy sheet copper, rolled into the shape of a truncated cone. A 20-ft. length of heavily enameled lead-in wire is provided, and its connection to the ground proper is made by two heavy spot welds. There are no other soldered or bolted joints and, thus, no electrolytic action is possible. The interior is filled with pure grain charcoal, which absorbs and holds moisture. The device itself, the cap being removed when it is installed, acts as a reservoir for water, providing the lowest possible resistance. Compact and convenient to handle, the unit is installed in a hole bored with an ordinary post-hole auger, no large excavations are required, and the earth adjacent to the ground need not be disturbed. The sketch shows, at the left, the comparatively small area a driven ground exposes to the moist earth and, at the right, the much greater area for discharge obtained with the new type.
Trouble-Shooting Kinks
When trouble appears in the set, investigate carefully, and the chances are that you will quickly find the cause. First examine the batteries, as run-down batteries are the cause of such troubles as the set suddenly losing volume or developing noisy reception. Test the polarity and voltage of all batteries, including C-batteries, and replace any that are defective. Tune in the strongest local station, and if the volume from the loud speaker is still weak, tap the tubes with the fingernail to determine whether or not the amplifiers are operating: if not, a ringing sound will be heard in the loud speaker. Examine all battery connections, and clean any terminal connections that appear to be corroded. If the battery tests have been made and show no trouble either in lack of voltage or defective connections, the next step is to substitute tubes of known efficiency. Try a pair of phones in place of the loud speaker. If no sound is heard in them, remove the tubes from the set and disconnect the A-battery; then, with a pair of headphones in series with a 221⁄2-volt B-battery, test the grid-to-filament circuits. This is done by putting the terminal of one of the headset leads on the grid contact of the socket, the other phone terminal going to the 221⁄2-volt battery, while the other lead from the battery is put on the filament contact of the socket. When the contacts are made on the sockets, loud clicks should be heard in the headset, except in the detector-tube socket. Because of the high-resistance gridleak here, a very weak click will be heard or, perhaps, no click at all. Test the transformer windings in the same manner. The secondary terminals are marked F and G, and the primary terminals P and B. If the trouble has not yet been located, replace the tubes in the sockets and put the set in operation. If it is possible to raise a station weakly, press down on each tube successively, and you may find one that is not making good contact in its socket. The remedy is to remove all the tubes and disconnect the batteries, at any rate remove all the tubes; then, with the fingernail or a notched stick pull up all the socket springs to insure a good contact between them and the tube prongs. Polish the ends of the latter and also switch and jack contacts, as corrosion at these points often means the difference between good and poor reception. Finally, look over the ground and aerial connections, or loop connections if a loop is used, and, if the trouble still persists, call in a reliable radio-service man.
New Tube Is Self-Neutralized
Insert, Photo of the New Tube, Showing Control Grid Connection at Top
The tube shown in the photo is one of the few really new developments in the radio field for some time. Designed to give much greater amplification in the r.f. stages, this newcomer, announced by Mr. Elmer E. Bucher, of the Radio Corporation of America, is to be known as the UX-222. The tendency to oscillate just short of the point where maximum signal strength is attained has been the outstanding fault of the standard tube. While oscillation is very desirable and necessary in generating the broadcast carrier wave, its presence in the receiver amplifying circuit renders the standard tube useless as an amplifier until oscillation is suppressed. The voltage-amplification factor of the new tube, due to its unusual construction and methods of application, is said to be from 30 to 150 without a squeal; the average r.f. amplifier gives an amplification of 6. The B-voltage is 135 maximum, the filament voltage is 3.3, just .3 volt more than that required for the 199-type tube, and the amperage needed is .13. The new tube is at present designed for battery operation only. Its use is limited to the improvement of amplification in the r.f. stages, and it is not adaptable to present-day sets. Each stage of r.f. must be thoroughly shielded, as shown in the circuit diagram, and three tubes should be used to obtain maximum results. The tube has two grids in addition to the usual filament and plate, the extra grid being said to make the circuit more stable by preventing feedback. No neutralizing methods are required in the receiving circuit when the four-element tube is employed. Hence, the construction is simplified and the set is more easily controlled without the customary howls and squeals. The point on which the greatest interest is focused is the amplification factor, which is 30 to 50 in circuits of the tuned r.f. type and to 150 when used in sets of the superheterodyne type. The circuit for the grid return and necessary grid bias is shown in Fig. 1. The screen grid inclosing the plate, and connected to the prong in the base ordinarily connected to the grid in the standard tube, is led to the 45-volt tap of the same B-battery that supplies energy to the plates of the standard tubes in the set. The regular grid, which now becomes what is known as a "control" grid, is connected to a terminal at the top of the tube, and its outside connection is made as shown in Fig. 1. The plate voltage recommended is from 90 to 135 volts, preferably the latter.
Figs. 2 and 3 show cross sections of the elements in the tube. In Fig. 2, the heavy vertical line in the center represents the filament; the narrow solid spiral encircling it represents the control grid, similar in character to the grid of a standard tube. The dotted spiral that incloses both these elements represents the inner screen; the circular plate is shown broken away to expose the inner elements. The outermost dotted spiral is the outer screen. Both inner and outer screens are connected, thus shielding the plate on both sides. Fig. 3, also a cross section, shows the various elements in their relative positions. In Fig. 4 we have a suggested typical screen-grid r.f. amplifier circuit. consisting of two stages of r.f., employing the new tubes and a standard detector tube. Just what the future holds in store for this new tube remains to be seen. but experimenters are very enthusiastic about it and are looking forward to its early release. At the time this article is written no definite release date has been announced by the manufacturers, although samples have been supplied to several experimental laboratories for the purpose of circuit development.
Light-Sensitive Tube
Left, Light-Sensitive Tube: Right, Diagram for Using the Tube with One Stage of Amplification
Designed to fill the need for a rugged light-sensitive valve, or photo-electric cell, which can be used conveniently in apparatus by experimenters and manufacturers interested in television, talking motion pictures, alarm systems, color sorting, etc., this new tube replaces many of the makeshifts formerly used by experimenters. The cell is mounted in a spherical bulb, 2 in. in diameter and 5 in. in over-all length, the lead-in wires being brought out to a standard Edison screw base, as shown in the photo. The internal construction is designed to utilize all those features which are best calculated to produce a cell which is free from "spurious" or "dark" currents and from "lag" or "fatigue," in laboratory parlance. It is also said to be free from the influence of temperature and to be highly sensitive to light change over the entire spectrum. The active medium used is potassium, on the surface of which is formed a film of colloidal potassium hydride. This surface is said to have a sensitivity almost a hundred times as great as the pure potassium. Further sensitivity is obtained by the presence of neon gas. The anode is a small disk placed edgewise toward the window, so that no shadow is cast upon it. With a single step of amplification, as shown in the diagram, a deflection of 30 milliamp. has been obtained with a 100watt lamp as a light source. In this circuit a UX-112 is used as an amplifier tube with a 90-volt B-battery, 6-volt A-battery and an 8-volt C-battery. A fixed resistance of 10 meg. is used in the grid circuit. Technical information in the nature of formulæ, laws, and constants relating to specific problems will be furnished by the manufacturer.