Radio Broadcast (May 1928-Apr 1929)

our readers suggest Uses for Damaged Meters THE writer recently had on hand two Weston model 425 thermo-ammeters, reading up to 1.5 amperes, that had been used as antenna ammeters. Accidental overloads destroyed the thermo-j unction but left the galvanometer movements' undamaged. Test showed that the movement gave a fullscale deflection with a current of about 2 milliamps. The meters were accordingly taken apart, the thermo-junctions discarded, and the leads from the moving coils attached to the external connecting posts of the meters. One of the meters was then employed as the grid-current meter in the modulated oscillator described in June, 1924, Radio Broadcast, a nichrome shunt of about ten ohms being connected across the meter to provide a little damping. Otherwise the meter would be working practically on open circuit, and the needle would oscillate freely owing to the absence of electromagnetic damping, making the observation of readings difficult. The remaining meter was used as resonance indicator in an absorption wavemeter for transmitting work, the circuit of which is shown in Fig. 2. A "low-loss" coil and condenser form the tuned circuit. The meter in series with a carborundum crystal, is tapped across about one-eighth of the coil. Carborundum is used for two important reasons: it is robust, holding its adjustment indefinitely, and not being liable to burn out on accidental overload; and it usually has a high resistance of the order of thousands of ohms, so that it does not unduly damp the tuned circuit, which would make the point of maximum response very broad and indefinite. For the same reason, the crystal and meter are shunted across only a small portion of the tuned circuit, and in practice the instrument tunes so sharply, especially on the shorter waves, as to make a slow-motion control necessary for comfortable working. It is quite sensitive, giving a good deflection at 80 meters when excited by the modulated oscillator at a distance of a foot. A shunt of ten ohms or so, wound on a match-stick with fine nichrome wire, is needed to steady up the needle, the best value for which should be found experimentally. _ A picture of the instrument is shown in Fig. 1. The writer used a crystal of carborundum set in solder in a cup, with a steel phonograph needle attached to a stiff spring as the "catwhisker" (use plenty of pressure), but no doubt the commercial carborundum detector cartridge would be quite satisfactory. The connections to the meter may have to be reversed to get the polarity right. No attempt was made to calibrate this instrument in wavelengths, in view of the possibility of a fresh point in the crystal being needed occasionally which might affect the calibration. The point of maximum response to the transmitter is found, and the actual wavelength is then obtained by removing the instrument without disturbing its setting, and coupling it to a modulated oscillator, which This department of Radio Broadcast is utilized each month for the presentation of miscellaneous short radio articles which are received from readers. These abbreviated manuscripts describe "kinks," radio short cuts, and economies that the experimenter runs across from time to time and that can be made clear in a concise exposition. Although some of these notes have been submitted by engineers and professional writers, the editors particularly solicit contributions from the average reader. All material accepted, including photographs, will be paid for on publication at our usual rates with extra consideration for particularly meritorious ideas — The Editor. has been carefully calibrated, and is kept as a standard wavemeter. F. G. Cannine, Melbourne, Australia. High-Frequency Tuning I HAVE read in Radio Broadcast numerous excellent articles on short-wave reception and transmission. In most receivers the tuning condensers specified are usually of 0.00014 mfd. capacity, or thereabouts. For easy tuning, especially on the very short wavelengths, a small capacity condenser is to be preferred. In my receiver I use a 50-mmfd. General Radio midget condenser cut down to 4 plates (about 12 mmfd.), and I find this sufficient for all bands from 18 meters up to above 85. A hair-splitting vernier isn't necessary, and with an ordinary 4-1 vernier the tuning is not in the least critical. Harry F. Washburn, Jr., New York City. Fig. 1 — External view of home-made wavemeter. The resonance indicator is a d.c. milliammeter. A Handy Connector OFTEN the experimenter finds it desirable to connect several pairs of headphones in series, or to make other temporary connections. It usually takes considerable time to connect the phone tips, and I have hit upon a plan which greatly simplifies the process. I carefully removed the clips from several discarded B batteries and placed them in my tool box. Whenever it is necessary to connect two phone cords or wires all that is necessary to make a tight connection is to insert the tips into the clip from opposite sides, and the connection is tight until you are ready to release it. Irmel N. Brown, McAfee, Ky. Increasing Charging Rates ATUNGAR or Rectigon two-ampere charger can be made to charge up to three amperes simply by removing the outer cover or shield. This metal shield around the transformer absorbs considerable energy, particularly the old-type cast-iron cover on the Rectigon. To give the battery a quicker charge, simply remove the cover. R. B. Barrows, Portland, Me. STAFF COMMENT The same applies to six-ampere chargers. The charging rate may be still further increased (apparently without damage to the tube) by remounting the elements of the charger on a wooden base. This will also eliminate most of the noise associated with many commercial types of tube rectifiers. Home Broadcasting IT ERE is a surprisingly simple and inexpensive way of converting your present radio set into a speech amplifier or publicaddress system. Such an outfit is not only amusing for use in the home but has many valuable applications for special events, large gatherings, etc. A single open-circuit jack is wired across the grid and negative filament terminals of the detector tube socket, into which the leads from the microphone transformer are plugged. In this way the detector tube functions as the additional stage of audio-frequency amplification desirable for best results with a microphone. The microphone in this case is a standard telephone transmitter and the transformer is a Jefferson No. 1603 bellringing transformer. The 110-volt primary leads from the transformer are connected to a regulation loudspeaker plug and inserted into the jack connected to the grid circuit of the detector tubes. The telephone transmitter, with 3 standard dry-cell batteries in series, is connected across the two outside binding posts on the transformer (the terminals of the 18volt secondary winding). • february, 1929 page 260 0