Radio Broadcast (May 1928-Apr 1929)

Fig. 2 — Circuit diagram of absorption icavemeter This arrangement will be found to have excellent voice-frequency characteristics and the output will be very lifelike and natural, depending of course upon the quality of your audio amplifier. As the microphone is very sensitive it must be placed some distance from the loud speaker, preferably in another room if used indoors, to prevent mechanical feedback and squealing. A volume control in the audio amplifier will be found helpful in controlling the output as well as any tendency to squeal due to proximity of microphone and loud speaker. T. F. McDonough, Los Angeles, Cal. STAFF COMMENT Mr. McDonough is perhaps a little tolerant of the quality in the arrangement he describes — which by the way, works nicely. However, the standard telephone microphone, aside from being the property of the Bell Telephone Company, is hardly partial to the frequency requirements of good loud-speaker speech reproduction. Much better quality will be secured by plugging the winding of a horntype loud speaker into the jack provided in the detector circuit — without a transformer of any kind. The loud speaker is used as the microphone. You may speak into the horn, or the horn can be removed and a small mouthpiece substituted. No batteries are used. The loud-speaker unit functions as a magnetic microphone, and will output excellent quality. A Simple Wire Shield "Our Readers Suggest — " for last November contains a contribution on reducing hum in a.c. sets by running the a.c. leads in grounded bx cable. Another and perhaps easier way of securing the same results, is to wrap the leads in "talking tape," and grounding as usual. "Talking tape" is the familiar indoor antenna tape. H. Cranston Jones, Brooklyn, N. Y. A Simple Lightning Arrester AN EFFICIENT lightning arrester may be made from old phonograph needles. Obtain a small piece of bakelite and mount on it two strips of brass copper or other metal. The strips may be held in place by two terminals, as shown in Fig. 3. The phonograph needles are soldered on to each strip, towards Bakelite Solder To Ground each other, with a gap of 2V between them. The leads from the antenna and the ground are connected to the terminals on the strip. A heavy discharge of lightning will jump the gap and will not injure the receiver. Edward Piranian. Philadelphia, Pa. An Emergency Output Circuit A SIMPLE emergency repair of a set in which the primary of the output transformer had burned out was accomplished as illustrated in Fig. 4. The secondary of the transformer was connected as a choke coil and the loud speaker fed through a 2-mfd. condenser. The primary was left unconnected. Break connections at points marked "X" and make the new connections indicated by dotted fines. As the repair was an emergency one, -and some doubt was felt about the particular condenser at hand standing up under the voltage applied to the last audio tube, the return from the loud speaker was connected to the B-plus side of the choke, so that the Fig. 3 — Old phonograph needles may be used in the construction of a practicable lightning arrester. Loud Speaker Fig. 4 — Diagram shows method of using burnt-out transformer as an output choke. B supply would not be short-circuited if the condenser did blow. If a condenser of proper voltage rating is used, the return may be connected to the negative filament or to the center-tap connection of the filament transformer secondary if the tube is lighted from a.c. John O'Donnell, The Bronx, N. Y. C. STAFF COMMENT This department receives more suggestions on output arrangements than on any other phase of receiving technique. The editor considers that this is indicative of unusual interest in the subject, and so will continue to publish the majority of such contributions, even with the possibility of an occasional duplication. High-Resistance Voltmeter THE high-resistance voltmeter is second only to output devices in contributions to this department. The following suggestion is submitted by D. J. Valentine, of Bangor, Me. The average radio fan or set-builder seldom can afford the price of a high-resistance voltmeter, although there are many occasions where the use of such an instrument is practically a necessity. For best results the B voltage applied to the plate circuits from a power-supply system should be adjusted carefully with a high-resistance meter, instead of by guess work, as the average fan must do. The voltmeter described below was made by the writer at a cost of less than $10.00, and it Screw driver Fig. 5 — Bending the plates at the right-hand side of the condenser affects tuning on both long and short waves, while bending them at the left-hand side will affect only the long waves. has a resistance of 1000 ohms per volt, which means that only 1 mA. is drawn for full-scale deflection. The parts were mounted on a small piece of rubber panel, with the necessary binding posts, and the whole enclosed in a cigar box which was covered with artificial leather. The meter is a milliameter with a range of 0-1 milliampere and the resistors are Daven " Davohms " of 200,000 and 500,000 ohms. The resistors are guaranteed accurate within plus or minus 1 per cent. By throwing the singlepole, double-throw switch (such as a Yaxley Antenna Switch No. 11), either the 200,000ohm unit or both units are thrown in series with the meter (Fig. 6), and the range becomes 0-200 or 0-700 volts accordingly. Of course, other values of resistances may be used, and the range of the instrument extended to 1000 volts or more. The accuracy depends on the accuracy of the meter and of the resistors. If desired it may be calibrated with a standard although the one built by the writer proved to give sufficiently true readings for all ordinary uses. STAFF COMMENT The voltage will always equal the reading on the meter divided by 1000 times the resistance in series with it. Balancing Gang Condensers THEBE are many instances, particularly in factory receiver construction, where trimmers are not provided to compensate tuning discrepancies in tandem-tuned circuits. The proper adjustment of such circuits may be effected by inserting a screw driver between the plates, close to where they are attached to the frame, and prying one stator plate nearer to an adjacent rotor, thus raising the capacity of the section without materially altering the tuning characteristic (an advantage over the trimmer system). It is also possible, by means of this method, to compensate tuning on the long wavelengths without affecting tuning on the short wavelengths, as suggested in the drawing of Fig. 5. A. T. Lequear, Erie, Pa. 200,000 ohms JVWWWW|. Meter 500,000 ohms Fig. 6 — Circuit of home-made highresistance voltmeter. • februarj , 1929 page 261