Radio Broadcast (May 1929-Apr 1930)

z: RADIO BROADCAST X A few words about lead-cell storage batteries may not be amiss at this point. A voltage reading taken across the terminals of such a battery without the set load is valueless for the practical purpose of determining its condition and state of charge. The no-load voltage may be high, but the voltage with even the small load of the tube filaments may be very low. Neither is a voltage reading under that load a sufficient indication of the state of charge. The change of terminal voltage, even under normal load, is not a linear function of the change in ampere-hour capacity. During discharge, for example, the voltage curve is only a gradual slope downward until After the fault has been localized it may be found by watching the voltage across the tube in the diagnoser socket, with the diagnoser plug in the set socket, while vigorously moving and pulling the particular wires to which the trouble has been traced. nearly the end of the discharge, when it finally begins to fall rapidly. It is possible for the terminal voltage to be high enough to supply five volts at the tubes when the battery is more than three-quarters discharged and, therefore, in need of recharging. Likewise, a hydrometer reading alone is not a sufficient indication of the condition of a battery. A cell may be reversed, or contain damaged or shorted plates without giving any indication of that condition by the specific gravity of its electrolyte. Therefore, a voltage test under the set load should be made and the state of charge should also be determined, either by measuring the specific gravity of the solution with a hydrometer, or by employing one of the various ammeters put on the market for that purpose which place a comparatively heavy load across the cell and measure the current on a scale which is roughly calibrated to show the approximate state of charge. The latter is preferable for the practicing serviceman. While not as accurate as the hydrometer, it is sufficiently so, and it is more compact, does not break easily, and cannot drip acid solution. Trickle Chargers WHEN a trickle charger is used, its rate of charge should always be measured by inserting the one-ampere range of the diagnoser ammeter in series with one of the leads between the charger and the battery. If the rate is adjustable, a good deal of possible future trouble can be avoided by adjusting it to approximately the rate which is needed for that particular set. Careful questioning will usually extract from the customer a fairly good estimate of the average number of hours the set is used daily. If he is not home to be questioned, and in the absence of other evidence, an average of four hours is a safe assumption. Multiplying that figure by the current drain of the tubes gives the number of ampere-hours taken from the battery every twenty-four hours. Dividing that figure by the remaining hours of each twenty-four, during which the battery is being charged, gives the rate of charge needed to replace the same number of ampere-hours. That figure plus 30 per cent, is the rate to which the charger should be adjusted. The computation may be expressed in the following formula: hours of drain X drain in amperes + 30% = charging hours of charge rate. If the charger is adjustable in steps instead of continuously, it is better to use the rate which is a little higher, instead of the one which is a little lower than the desired rate. If the rectifier is one of the wet types, and does not give the output which is necessary, the only reasonable remedy is to throw out the inexcusable thing and substitute for it a good dry rectifier replacement unit, such as Elkon or Kuprox, made especially for that particular charger. Dry rectifiers do not, of course, last forever — because it they were made that way the manufacturers' future replacement market would not exist — but at least they may be replaced, when exhausted, by others of the same make and type without causing the serviceman that feeling of guilt which he has when he supplies any accessory or part which he knows to be inferior. A similar dry type is also made to replace Tungars, and is preferable because of its economy in operation and life, and its quietness. Charger Connections IT IS rather important to remember that the positive terminal of the charger must be connected to the positive terminal of the battery. If that order is reversed the battery will be discharged rapidly, and then very slowly charged again in the opposite direction, a process which materially shortens the life of both the battery and the charger. When a storage battery using a trickle is found to be fully discharged, or partially or fully charged with the polarity reversed, it is well to be sure of that point. The foregoing would seem to be information of the kindergarden variety, but the number of times QRV servicemen have discovered exactly the described state of affairs is amazing. When the condition is found, the only remedy is to have the battery sent to a battery service station for proper recharging. When an ammeter test does not show any output, or an intermittent output from a trickle charger, a very probable place of trouble is at the points of the relay. Those points are slightly burned by sparking each time contact is broken, and, therefore, require cleaning occasionally. A very few strokes with fine sandpaper or a very fine file, held flat against the points by closing them against it, will suffice to clean them properly. The relay switch arms can get out of adjustment, so that they do not close firmly in one or the other direction, and may need bending back into their original shape. The spring against which the switch arms work may gradually lose its tension and require shortening. The tension should be adjusted so that good firm contact is made to close the charging circuit, but still light enough to permit the arms to be pulled sharply, and held In series-filament circuits much time may be saved by testing the continuity of each tube filament separately with a C battery. firmly, against the opposite points when the magnet is energized. We have spoken so far only of a.c. trickle chargers. There is no difference in method of testing or connection where d.c. is the supply, but there is one caution about such equipment which is important. Whenever a d.c. trickle charger is installed, fixed condensers of any convenient value larger than 0.001 mfd. should always be put in both antenna and ground leads, between the set and the lightning arrestor, before the charger is connected to the line. This is also essential when installing d.c. sets, with the exception that the manufacturer usually takes care of putting a suitable capacity in the ground lead, within the set or power pack. If there is none for the antenna within the set, then it can be connected externally by the installer. Observance of that rule will avoid the evaporation of relay switch points, parts of primaries of antenna input transformers (in the event of a grounded antenna or shorted lightning arrestor), parts of tube filaments, and lighting circuit fuses, at a cost which is compara V fl,' J/ ® * e fm 1 A common cause of "dead" d.cpowered receivers is ignorance on the part of the housemaid concerning the manner in which the plug should be placed in the wall socket. tively low. If you don't know the reason for those possibilities, draw a circuit diagram starting at the line, going through the relay, charger and battery to the filaments, showing the antenna-ground system with its connection to the filaments, remembering that either side of the line may be grounded and that, when the relay switch is in the charging position the points do not open until after the set has been turned on at the filament switch. The same situation exists with a set operated from the d.c. line. Perhaps some enterprising manufacturer will put a variable-rate d.c. trickle charger on the market before battery-operated receivers become obsolete. Until that time, in cases where the rate of the present manufactured article is not reasonably near that required, and where the battery and charging equipment can be kept entirely out of sight, a good substitute for it is a porcelain lamp socket screwed to a wooden base, into which may be put a size of lamp which has approximately the right resistance to give the desired rate. Then, if it becomes necessary to change that rate, a different size of lamp may be substituted easily. Series Filament Circuits THE testing of series d.c. filament circuits is just as easy and may be done just as quickly as the testing of parallel circuits, if a logical routine is followed, When the tubes are not paralleled with resistances, there is but a single series circuit extending from one filament terminal on the strip to the other filament terminal, and consisting of alternate sections of tube filaments and single wires connecting those filaments, like a string of sausages. There may also be a section atone end consisting of a rheostat. Testing for voltage from the negative end (negative terminal on the strip) first to the positive end, and then successively to each joint between sections, will quickly disclose the point at which the open occurred. While it would be just as effective to use the positive end as the point in the circuit from which to start testing, it saves time to get into the habit of always starting at the same point, and the negative end is preferable as the base of operations for all circuit tests, except when testing a.c. circuits — which obviously have no polarity to an a.c. meter. It is, of course, necessary to keep the tubes • may, 1929 page 27 •