Radio Broadcast (May 1929-Apr 1930)

A German Portable Receiver THE EXPERIMENTER'S ARMCHAm By way of departing a bit from the discussion of our own affairs, let us take a look at the German portable broadcast receiver called the "RadioBaron." It is made by Lorenz who innocently was the cause of our great crop of "wiggle-wound" coils of a few years ago. The "Radio-Baron" normally lives in a carrying case and is self contained. A multiplicity of adjustments is provided unlike our American practice. First of all the loop is wound around the frame in two layers separated by half an inch. The high inductance thus provided is necessary for the upper wavelength range of the set, which tunes-in the long-wave European broadcasting stations. A tap switch on the panel permits dropping down to the usual range. The circuit is a double-detection one (super-heterodyne) and the two tuning controls are operated by a double drum dial of standard design. Filament power is supplied by a small "dry" storage battery and controlled by the usual rheostat and switch, the latter being on the panel and the former at the back of the set. Grid and plate voltages are supplied by a combined dry-cell B and C battery. Since the highest potential available with a new battery is 90 volts, the last audio tube is special, having an additional grid. It is not a screen-grid tube but is, I believe, a pentode which provides a moderate amount of power output without resorting to a power plant in our customary fashion. The highest potential ordinarily used by the Baron is 60 volts. Circuit variations in plenty are provided for ; in addition to the range-change switch there is a lever for altering the coupling to the oscillator, a control for adjusting regeneration in the i.f. system, and provision for operating the set with 4, 5, or 6 tubes! Just what the last device does to the circuit cannot be discovered without wrecking the set. The empty sockets are jumpered by means of a double-ended plug cord. Presumably, the purpose is to save tubes and batteries on local reception. The loud speaker is peculiar. The rather large diaphragm is of something resembling celluloid and is driven from the center. It is, however, neither flat nor conical but of some curved form, possibly logarithmic. The "Radio Baron," a self-contained loop-operated portable receiver of German manufacture. By ROBERT S. KRUSE An Improved Oscillator So much for European practice. Let us now return to the United States; in fact, to a point just a bit north of the center thereof. This must serve as an introduction for Louis F. Leuck, of Lincoln, Nebraska, who describes for us a general-purpose oscillator of more than usual merit. It holds calibration, may be used as a mere "driver," as an "oscillating wavemeter," as a trans Mr. Kruse's "Armchair Chats" this month include the following: A German Portable Set An Improved Oscillator Miniature Current Transformer A 1 10-volt Potentiometer The I. R. E. Patent-Digest Letters are Welcome mitter "tone checker," as a "tuning meter," or as a receiving heterodyne. Furthermore, it is very simple, easy to construct, and inexpensive. The device with all these virtues is simply a self-contained battery-driven oscillator with circuit precautions against the effects which usually produce the worst changes in calibration. The following description is quoted in part from a longer description which unfortunately cannot be given in full. The circuit is the balanced-bridge version of the Colpitts circuit,, and was developed by Willis Hoffman of the Burgess laboratories. Examination of Fig. 2 will show that if the coils Li and L2 are alike we will have a balanced bridge whenever capacities Ci and C2 are equal. There will, accordingly, be no tendency for radio-frequency current to flow from point A to point B and very little tendency for it to flow by way of the plate supply from point C to point B. The choke, L;, can usually be short circuited without producing any effect. To obtain this condition approximately in transmitting or receiving* practice is entirely possible by simple mechanical precautions. In Mr. Leuck 's oscillator, Ci and C2 are the halves of a reconstructed receiving condenser which, in its new form, has two rotor sections connected together by the common shaf t and meshing into separately insulated stator sections. The spacing has been doubled to improve stability of calibration. Li and L2 are halves of the same split coil wound on a ux tube base whose prongs act as terminals. The bypass condenser, C3, and the vacuum-tube socket are connected directly to the ' coil socket without any intervening wires whatever. As long as the coil is pushed clear down in the socket and the tube is not changed, the tuned system, therefore, remains stable and unchanged. In support of this point, Mr. Leuck says. . . . "To get an idea of the meter's ability to retain its calibration the wavemeter was calibrated from 9xl. A month later the receiver was set on the wavelengths at which 9xl was about to transmit according to its published schedule. This was done by means of the wavemeter (i.e., the oscillator here discussed.) The signals from 9xl came in without retuning the receiver on fully half the points. This was better than could rightfully be expected, since 9xl maintains an accuracy of only -fo of 1 per cent." (This is the guaranteed accuracy but 9xl actually does better on much of its work.) The bridge circuit, together with the small size of the coils and other parts, makes the meter as free from handcapacity effects as one could wish. Shielding is entirely unnecessary and useless. The device was accordingly mounted in an old Crosley receiver case. The front of the panel has but three objects mounted on it, a good vernier dial, the filament switch, and a pilot light. The pilot light has but one object in life; when it is included even the most absent-minded person can hardly forget to turn off the filament switch. Bunning down of the A battery has very little effect on the frequency of the meter. This has been tested by exchanging a new and an old battery while listening to a beat note produced by the wavemeter and a crystal-controlled oscillator. Bunning down of the plate battery need hardly be considered as the load on it is almost nil. RANGE COVERED "The meter described was intended to cover the amateur bands with generous margin above and below. Its fundamental range is 32 to 50 meters. Since the oscillating receiver and this wavemeter both produce harmonics, the wavemeter is readily usable to determine wavelengths from 16 to 25 meters and from 64 to 100 meters. A 12volt plate battery (several This new miniature current transformer has been developed by Weston, seemingly for the experimenter. • JUNE -1 929 • • 103