WHY SOME HOME-BUILT SETS FAIL

Radio Record, Volume I, Issue 28, 27 January 1928, Page 3

WHY SOME HOME-BUILT SETS FAIL

CAUSES OF UNSATISFACTORY CONDITIONS

I don’t like to be too direct in my accusations. In fact (writes "XN.Y.Z." in the Melbourne "Listener-in’’), I don’t like to make accusations at all, and would much prefer to have any of niv Statements falling into this category termed as assertions, Nevertheless, there.is a question in my mind which has been there ever since I began experimenting with radio mally years ago, and it has never been answered. In vain, I have tried to seek the information desired, only to be sidetracked in the end, until I found it necessary to arrive at my own conclusion in the matter, which conclusion is as follows :- Ever since the early davs of the crystal set and the single valver, we have had radio authors. And eyer since tlhe advent of radio authors we have had "distance records’? which peor, ordinary mortals tried to equal or beat with but little success. Times nnumerable I have casually glanced through a story describing 4 new circuit or receiver, and, invariably, when I reached that part of the article dealing with "What it Has Done," I became sold through the prospect of receiving interstate stations every night in the week, and sometimes in the afternoon as well. Now, as I said in the beginning, I don’t like to make accusations, but doesn’t it seem queer that the designer and author of the story on the "Blank" circuit plus a very few of the builders of this set incorporating the circuit, are the only ones’ to receive stations ‘thousands of miles away,’’ while the majority of people who build the set do well to receive broadcasters within a 250wnile radius? It did to me, I know; but never having been able to secure a satisfactory explanation to the question, I set about to learn what I could for myself, and, as a result, I found both contracting parties to be equally to blame, though neither of them suspected that guilt rested on their spotless characters.

STUDYING THE FAN. Let us view for a moment the actions of the set builder. In the majority of cases the investigator will find him to be a more or less regular "circuit fan." This type of addict is one, like yourself, who, when he sces a new circuit, immediately sets about to "throw it together" and see what it can do, The first step followed by the "circuit fan" is casually to glance over the list of parts and to summarise what is needed to build the set. One look and he discards at least 50 per cent. of the apparatus listed. "Two audia transformers," he reads, "Tt have a couple of old ones home ‘that will be good enough for a trial,’’ he concludes, "\ fiye-megohm grid leak is what they specify for the detector, Huh! I don’t suppose it would make much difference if I used that old two-meg-ohm grid leak that I have laying around somewhere. They specify two .00035 infd. yatiable condensers for tuning the antenna and detector circuits, Well, I can use those old .0005’s if I take off a few turns from the coils. And there’s another thing, the coils they used are wound on two-inch tubing. That old four-inch cardboard tubing I have at home will serve the purpose just as well if I leave off a few more turus. "Gosh, this set uses four .5 mfd. bypass condensers. Those things don’t meat much, anyhow, and, besides, it’s only a test I’m making, so I guess I can do without them, as well as without the radio frequency choke coil in the plate circuit of the detector valve. As far as rheostats are concerned, I won’t haye to use them in this test, ‘Pit put six volts directly across the filaments; it ought to work just as well." —

eee Se friend hurries home that night aud spreads all his apparatus on a Ieead board, and begins to wire the set with No. 86 wire, or worse. We might Le able to cut the tip off’ little fohnuy’s nose and still be able to tecognise him, but when we twist his ears, blacken his face, cross his eves and pull all Lis teeth, the poor child Will tind little maternal comfort by _saymg, "Well, mamma, here I am.’" And so it is with « radio set. We can take & few liberties and get away with it, but when we start redesigting it, it naturally follows that we have an entirelv diflerent set. A set with a bundred Kinks that must be ironed out, and Which, even after the flattening precess, may be far from desirab].e Let us review just what our friend has done to change the characteristics of the set to such a remarkable degree. First, the use of the old audio tians. formers in place of the modern type will naturally result in inferior quality. Second, he substituted a two-me,zohm grid leak for the fiye-megohm grid leak Which was specified. If vou know anything at all about radio receiving sets, you will appreciate the importance of the grid leak and, particularly with the regenerative receiver, the rather critical requirements in the matter of the resistance value of this unit. In some of the more unstable regenerative receivers of bygone days, reception from a distant station could be destroyed entirely, merely bv changing the value of the grid Jeak. ‘Thus we find that the set builder, simply by making this substitution, has detracted at least 5 per Cent. from the efficiency of the circuit as described. . MAKES DRASTIC CHANGE. In his next substitution, he performs an operation upon the vitals of the re-ceiver-the radio frequency tuning and neutralising system, In the first place, let us suppose that the receiver’ called for a detector tuning coil, comprising | a primary and a secondary wound on a 2-inch former, the latter coil consisting of 75 turns of a certain size of wire, tapped at the sixteenth turn for neutralisation purposes. The set-builder makes a few comparative calculations and decides that sixty turns would be about right for the .0003 condenser. fe then competsates for the fact that he is using a larger diameter tubing and does a little more caiculating, reducing the number of turns on the secondary to thirtv. However, itl many instances, he fails to take note of the fact that the tap for newlralisaton must also be changed. and instead of changing the specifications in this resnect, he places his tap at the sixteenth turn as originally specified. i We makes his ext mistake in eliminating the by-pass condensers, which, despite his belief, do play an important part in the operation of the receiver, though their effect may not be noticeable to the ear on reception from

A Be IT PORE ae SILT | tocal stations. Elis climination of the choke coil in this particular cireuit is fatal, for regencration is accomplished by feed-back through a midget condenser, which instrument is generally useless without assistance from the choke-coil in forcing back the radio frequency currents into’ the grid circuit. In snmmation of all the substitutions we find the following:-First, the quality of reproduction in the recéiver thus "thrown together" is terrible; second, the detector is not functioning at maxinium efficienecy due to impro-. per value of the grid leak; third, the radio frequency stage is either unbalanced, resulting in squealing, or overbalanced, resulting in weak signals-_ botl: conditions being due to improper neutralisation; fourth, unstabilisaton, duc to failure to by-pass stray radio frequency currents, and, finally, lack of sensitivity is brought about through absence of regeneration in the detector circuit and incorrect grid Liasing because of the absence of proper filament control. AUTHOR TO BLAME, TOO. However, the designer and author is not entirely without blame for the condition mentioned in the ontset of this article. The design and perfection of the receiver he describes may have taken hin: anywhere from a week to more than a year, and the time elapsed between its completion and the writing of the story describing its construction may be considered, during which period it is quite possible that the receiver or copies of it have been tested many different times, and in many different locations. Making allowances for the enthusiasm of the designer and author, it is easy to see that he would naturally credit all feats performed by a receiver incorporating his circuit td the system Which is of his own invention or sign, Perhaps he does not fully take into consideration the location in which the receiver has been tested. Let us assume that three or four receivers incorporating a new circuit are distributed to various tocations for test. ttior to the description. of set for 41 benefit of the public. One of the sets will probably be kept in, say, Melbourne, while another might be sent to a friend residing in, say, Sydnev, certain sections of which are notable for their reception of distant stations.

ee The third set may be sent to a friend in Mildura, where stations to the north aml south are easily received, while the fourth set might possibly find a testing place in Gippsland. = TESTS LAST WEEKS, Let us assume that the test extends over a period of several weel:s, during Which time the receivers in the outlying districts are piling up some very enviable distance records. When the tests are completed, it is only natural that the designer of the set should ccmbine all the reports of distant ‘reception and credit his receiver witha these performances. In the broader sense, however, the circuit is not entirely responsible for the reception it is credited with, since the locations in which the tests were made played a very important part. For greatest accuracy, it might be more equitable when mentioning records made by receiving sets incorporating new circuits to specify the particular location in Which the test was made, the time ofday and the comparative performances of other receivers, the characteristics of which are more generally known to the average fan. Thus, we find that both parties are contributors to the ‘disappointments that sometimes follow the building of a& new receiver. {n analysing my own actions, as well as those of other fans, I have gained a pretty fair idea of the set builder’s position, and in fairness to all concerned I am sure that the builders are far more to blame than the designer, since the latter’s error, if there.is one, lies only in a slight over-enthusiasm, while the builder not only often disregards thie carefully planned specifications set forth in the Story, but in many cases mav be ac. cused of carelessness in building the set. Radio experiments conducted .. under the direction of Professor N. I. Adams Jr., of Sloan Laboratory, Yale University, U.S.A., disclose the possibility of using a pair of matched piezo-electric crystals to control two or more broad« casting stations so that they can oner. ate simultaneously on tne same wave« length without producing heterodyne howls, This system, Professor Adams believes, may enable networks to use one wave and clear many channels, making it unnecessary to eliminate 800 broadcasters, as has been proposed by Tederal Commissioner Pickard,