Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST J5T Raytheon\ KinP lanip jF for Television Reception This lamp is made in numerous types and styles, which provide suitable light sources and lightsesnsitive relays for all systems. List Price, ^7'50 if Ray the on \ \> B>tP'(ell # for Television Sending This is an extra-sensitive broadcasting tube, supplied in either hard vacuum or gas-filled types, and in two sizes of each. Information and prices on application /Raytheon BH\ LONG LIFE RECTIFYING TUBE for i6B"Power Eliminators Over a hundred different makes of "B" Eliminators require this tube, and take no other. There are millions of them in daily, satisfaction-giving use. List Price, $4?-50 Write for further information on any of this equipment RAYTHEON MFG. CO. CAMBRIDGE, MASS. The Radio Broadcast LABORATORY INFORMATION SHEETS By HOWARD E. RHODES 'T'HE ajm 0f the Radio Broadcast Laboratory Information Sheets is to present, in a convenient form, concise and accurate information in the field of radio and closely allied sciences. It is not the purpose of the Sheets to include only new information, but to present practical data, whether new or old, that may be of value to the experimenter, engineer, or serviceman. In order to make the Sheets easier to refer to, they are arranged so that they may be cut from the magazine and preserved, either in a blank book or on 4" x 6" filing cards. The cards should be arranged in numerical order. Since they began, in June, 1926, the popularity of the Information Sheets has increased so greatly that it has been decided to reprint the first one hundred and ninety of them (June, 1926-May, 1928) in a single substantially bound volume. This volume, " Radio Broadcast's Data Sheets," may now be bought on the newsstands, or from the Circulation Department, Doubleday, Doran & Company, Inc., Garden City, New York, for $1.00. Inside each volume is a credit coupon which is worth $1.00 toward the subscription price of this magazine. In other words, a year's subscription to Radio Broadcast, accompanied by this $1.00 credit coupon, gives you Radio Broadcast for one year for $3.00, instead of the usual subscription price of $4.00. -The Editor. No. 273 Radio Broadcast Laboratory Information Sheet April. 1929 Neutralizing and Compensating R. F. Circuits TJROBABLY two of the most common tasks which servicemen are called upon to perform are the adjustment of the neutralizing and compensating condensers in tuned r.f. receivers. These tasks are exceedingly important although not especially difficult. If a set is not neutralized properly it will oscillate on some wavelengths, especially down around 200 or 300 meters. Therefore, if a set does oscillate it is necessary to reneutralize the various stages. This should be done in an orderly fashion, starting with the stage nearest the antenna and following with the other stages in order. Also, all servicemen should be equipped to perform these adjustments quickly on all receivers, and in this connection specially prepared tubes of the types used in r.f. amplifiers, the 201a, 226, and 227, are a great aid. These tubes are prepared by cutting off as close to the base as possible one of the filament prongs, in the case of a 201a or 226-type tube, and one of the heater prongs in the case of the 227-type tube. In adjusting a receiver tune-in a strong local station broadcasting on some wavelength between 200 and 300 meters, carefully tuning the dials to exact resonance. Then, with the prepared tube placed in the first r.f. socket in place of the good tube, carefully adjust the first neutralizing condenser to that position which gives the minimum signal from the loud speaker. Then remove the prepared tube, and replace the good tube. Now put the prepared tube in the second r.f. stage and repeat the operation, etc. The compensating condensers in a receiver are placed across the main tuning condensers and function to compensate the slight differences in capacity between the various stages so that all the tuned circuits will be in exact resonance. Compensation should also be done with the set tuned to some station around 250 meters. When compensating a set it is best to tune-in some weak station, since slight changes in volume will then be noticeable more readily. The exact procedure is as follows. First tune-in a weak signal to maximum volume and then adjust all the compensating condensers to give the maximum signal strength Retune the main dial to the point of maximum volume and then readjust the compensating condensers again. No. 274 Radio Broadcast Laboratory Information Sheet April, 1929 Rucking Coils in Dynamic Loud Speakers T\/TANY a.c. dynamic loud speakers use "bucking -L'l coils" to reduce the hum due to the use of rectified but poorly filtered a.c. to supply the field current. This bucking coil functions as follows. Referring to the diagram, the bucking coil is connected in series with the moving coil and the secondary of the coupling transformer. The moving coil is. of course, fastened to the diaphragm. The bucking coil is wound around the pole piece of the electro magnet. Now, since the rectifier supplies to the field a pulsating current, it follows that the magnetic flux produced by this current will also fluctuate. Since the moving coil is in the field of this flux, there will be a reaction between it and the varying magnetic flux and the coil will tend to move — and its movements would have the same frequency as that of the field current. If the diaphragm moves, sound is produced and as a result we would get an audible hum. The effect of the pulsating field current is, however, nullified (more or less) by the bucking coil. The coil is also in the magnetic field and it, therefore, has induced in it a voltage corresponding in frequency to that of the pulsating field current. This voltage induced in the bucking coil sends a current around the circuit consisting of the transformer, the moving coil, and the bucking coil. The magnitude of this current is such that its effect on the moving coil is equal and opposite to that, produced directly on the moving coil by the flux. Since the two effects are equal and opposite they nullify each other and the hum is prevented. It is evident that the important thing is to get into the moving-coil system a voltage that will nullify the forces tending to make the coil move and thereby produce hum. To Rectifier , , ■< Field Winding , Bucking Coil Moving Coil ^ Paper Cone • april, 1929 page 412 ®