Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST ANOTHER SUGGESTED ROUTINE A few additional points are adduced by Newell N. White, of Wichita Falls, Texas, who skims the milk of eleven years of servicing as follows: "I have been in the game since 1917, doing service work on broadcast sets since 1921 with the exception of the two years I was with S. W. Bell Tel. Co. Therefore, I feel at liberty to tell the way I go about checking a set in the home of a customer. "First I will give the routine I follow on battery-operated sets. "First the A battery is checked. If it is an acid battery the test is made with a commercial current-drain tester (I find the Hoyt CellCheck good enough). A hydrometer is never used in the home. A cell checker is sufficiently accurate and there is no danger of spilling acid. Next the B batteries are tested (with no set load) with a low-resistance pocket voltmeter, which puts a heavy enough load on batteries to cause a voltage drop if they are weak. The C battery is checked next. All batteries found defective are replaced before test continues, as all other tests require voltage to give meter readings. "The next step is to remove tubes from set. When this is done all circuits should be open leading from batteries to set. A high-resistance voltmeter is connected in series with each lead from batteries. This will show up any shorted batteries, leads or filter condensers. "The next and final step in testing the set is with a Jewell a.c. -d.c. box (would not trade it for all the rest of the equipment a serviceman could carry in a truck). This is done with all batteries connected and all tubes in their sockets, except socket under test. The Jewell box tests the filament, grid, and plate circuits of all sockets. In this test, open grid suppressor, open grid coils, poor ground connections, open plate coils, open audio transformers, etc., can be detected. If still no trouble is found the loud speaker is tested by placing it in series with an a.c. plug from a convenient outlet and the a.c. voltmeter of the Jewell test box. "Next the antenna and ground are inspected. If none of these tests have located the trouble apparently the repairs will have to be made in the shop where I have an oscillator, stationary test board, output meter, etc. " I might add that while the various sockets are being tested the tubes are also tested. " In a.c. sets the troubles are less numerous as I find most calls are made on account of bad 227-type tubes or a noisy volume control. In testing these receivers the line vo'tage is first checked to give me an idea of whether the set voltages are high or low. After this is determined each socket is tested with the Jewell test box which shows the trouble very easily. Since the power pack should be under "set load" while it is being tested, I never like to test from the pack terminal strip. The antenna and ground are also inspected on electric sets. "In this letter I also wish to comment on the article by Thomas Glose, in January, 1929, Radio Broadcast. Probably Mr. Glose was using a high-resistance voltmeter to make his continuity test. If so, the sensitive meter would still show full voltage, because no current to speak of was being passed through corroded contact. " Now don't get me wrong. I would not start on a call or try to fix a set without my highresistance voltmeter, but, fellows, you better watch the things. I will bet that a meter of low resistance that had a fairly heavy current consumption, as meters go, would have shown up that poor connection." SUMMARY The amount of test equipment the serviceman should carry with him is determined to quite an extent by just how much work he thinks he ought to do in the home of the setowner. This too, is a matter of personal opin ion, but a consensus would limit this to rather elementary tests and repairs. Actually it is a compromise involving convenience and economy. Major repairs can be effected more efficiently in the serviceman's shop than on the library table of the set-owner. In summation, it would seem that the following equipment is adequate for making all the adjustments and minor repairs that should be made outside of the workshop: One a.c. and d.c. tube test set; Two screw drivers, one large and one small; One diagonal cutter; One six-inch pliers; One long-nose pliers; One small file; One sheet emery cloth; One piece of cloth; One coil of wire; One knife; One roll of tape; One Burgess snaplight ; One pencil with an eraser. The procedure should be altogether logical. Spend two minutes in finding out the past history of the receiver — then proceed to diagnose the trouble. This will fall into one of four main classes: no reception at all, weak reception, noise, and distortion. If there is no signal response, endeavor to localize the difficulty without immediate recourse to the test set. If the tubes light, hit the detector tube lightly with a pencil. A ring in the loud speaker shows the a.f. circuit to be o.k., locating the difficulty in the r.f. circuit. (No ring is, of course, indeterminate, and the usual systematic checking of power and tubes should follow.) If the trouble is in the r.f. end of the set, connect the antenna to the plates of different tubes in an endeavor to locate the faulty stage, and then go to work on it. Cases of weak reception should be similarly investigated. The possible causes of noise are many and are treated to some extent elsewhere in this department. If the trouble is distortion, the a.f. circuit should immediately be suspected, and tests made for emission, and incorrect A, B, and C potentials. The point to be emphasized is that service time can often be cut by using a bit of common sense before resorting to a test set — invaluable as this may be. As John Dunham points out, in his article in March Radio Broadcast, a knowledge of radio engineering fundamentals is a servicing tool second to none. Bad Tubes and Service Troubles W\ S. HARTFORD, of the Kellogg Switchboard and Supply Company, makes a concise and interesting observation: "You may be interested in knowing that 65 per cent, of the reported cases of no reception were traced to incorrect insertion of one or more tubes. About 15 per cent, of the cases were due to low B voltage. In consumer complaints, many owners felt that advancing the rheostat to the tubes will cause shortened tube life. The actual effect is to impair the selectivity and sensitivity of the receiver with a gradual weakening of the tubes." The Arborphone Company broadcasts a somewhat similar warning on sheet 8 of their service manual: "We wish to call your attention to the fact that the greater percentage of power pack trouble may be attributed to defective tubes. After through tests and reports from various sections we find the following troubles are caused by defective tubes: Lack of sensitivity, lack of volume, a.c. hum, noisy reception, and fading. However, the most serious of all the defects is that many of these tubes, after being in use a short time, will develop a short in the plate circuit, thereby frequently burning out the power transformer." Miscellaneous How Much Current Does My Receiver Consume ?: The serviceman is often asked just how much current the particular a.c. set he is servicing consumes — a reasonable bit of curiosity on the part of the owner. Malcolm Chase of Taunton, Mass., suggests a simple way of determining just what it costs to operate any a.c. set, as far as current consumption is concerned. " In order to determine the approximate cost of electricity consumed by an electric set without any accessories the following method may be used. A fifty-watt lamp which consumes five-hundredths of a kilowatt hour per hour should be turned on and, if a kilowatt costs say eight cents, multiply eight by fivehundredths. This will give you the cost per hour to operate a fifty-watt lamp. Now consult your electric watt-hour meter and count the number of revolutions the aluminum disc turns in one minute. "Now turn on your radio set or battery charger. Suppose the disc makes one and onehalf times as many revolutions as it did for the fifty-watt lamp in one minute. Obviously the radio set will cost one and one-half times as much to operate as the fifty-watt lamp. Make sure of course that the only load on the line is the device you are interested in computing the operating cost of." Selling A.C. Tube Insurance: The Fordham Electric Company, dealers and service in the Bronx, New York, are selling the Ward Leonard Vitrohm Linevoltage Reducer, shown on page 391, to many clients in outlying a.c. districts where the line voltage often rises above safe limits. This is good business. The good will of the customer is more than worth the sale of replacement tubes. The latest merchandizing scheme at this store will interest the small-town serviceman who is also generally the main source of radio supplies. Hundreds of burned out tubes are heaped in the window surmounted by a sign reading: "For Sale Cheap." A second sign tells the story of the fine-voltage reducer as a means of preventing blown-out tubes. Dozens of customers have been brought in the store by this window display and sold on the spot. Many of them spent only two dollars for one of the line units the first time, but came back later to spend much more money for a set and accessory equipment. Elimination of Noise THE following trouble is quite common, although the first time it is encountered it might cause a lot of extra work. It starts out like a microphonic howl and grows to a roar of rather low pitch. The usual routine tests indicates everything o. k. except that when a certain 226-type tube is inserted in the settester the howl stops. However, when it is returned to the set the noise starts again. Upon examining the tube it will be noticed that the filament and grid are nearly touching each other. When the tube is in the set the vibrations from the loud speaker cause the filament and grid to short, thus causing the howl. When the tube is inserted in the tester, which was away from the set the vibrations do not affect the tube and everything seemed o.k. The . remedy is obvious. A good way to find the offending tube is to hold each tube with one hand and tap it lightly with the other. A defective tube will cause a prolonged scratchy noise in the loud speaker. H. Weimar, Appleton, Wis. DATA FROM KING MFG. CO. The service bulletins of the King Manufacturing Company, of Buffalo, N. Y., contain some interesting "dope" on the elimination of noise. "Some radio tubes are microphonic in action and when used will cause a ringing sound in the loud speaker. This will be especially noticeable when the set is jarred or when the lid is closed. Usually microphonic tubes cause the greatest trouble in detector and first-audio stage. • april, 1929 page 390 •