Radio age (Jan 1927-Jan 1928)

16 RADIO AGE for October, 1927 the greatest sensitivity and selectivity with tone quality, the super-heterodyne becomes only a matter of the assembly of a few more pieces of equipment than the ordinary receiver, with considerably greater efficiency and enjoyment. Just how the Selectone Intermediate Units are kept at a uniform standard is an interesting procedure. The coils of the transformers are tested before assembly onto bobbins for open windings and short circuited turns using two oscillators driving at about 100 kc with a common plate supply and headset so included in the circuit that changes can be readily detected. One of the oscillators is adjusted slightly below or above the 100 kc oscillator so as to give a beat note of 500 cycles, easily readable in the phones. The coil is then plugged in the test circuit and variations in the frequency of the beat note are listened for, if there are such. A change in the tone of the beat note indicates imperfect winding and such are unsuitable for use in the Selectone IF Units. The condensers used to peak the transformers are carefully tested with a stabilized oscillator and a vacuum tube voltmeter on a standard transformer. Changes in capacity differing greater than five per cent above or below standard reject the condenser. Under the close scrutiny of expert engineers, skillful assemblers put the coils on the bobbins, and affix thereto the condensers which peak the transformers. The terminals are then carefully soldered into place on the new style bases, and the transformers undergo their first tests for amplification, peak frequency and spillover point. If the units conform to the standards demanded by the specifications of design, they are removed to the impregnating and mounting department where they are dipped swiftly and skillfully in a special compound and mounted in the new highly finished bakelite housings. The impregnating operation is one of the most important of all, as a special compond is required and the mixture must be kept at the proper temperature so as not to alter the characteristics of the transformer. This operation seals the windings, the condenser and the bobbins against atmospheric influences and further serves to lock all leads and windings into place so that the characteristics cannot alter, due to dropping or handling. The transformers are then ready for matching into sets. The apparatus used for this consists of a special stabilized oscillator with a uniform output over the entire scale of its variable capacity and an ultra-sensitive vacuum tube voltmeter. Great pains and precautions are taken to keep this oscillator stable and uniform, and the finest precision meters obtainable are used in the vacuum tube voltmeter which is also of special circuit and construction. The oscillator and voltmeter are heavily shielded to prevent pick-up of energy other than that obtained from the voltage drop across a resistor in the oscillator unit. The vacuum tube voltmeter is connected across the secondary of the transformer to be tested, the List of Parts 1 Remler .00035 mfd Variable Condenser 1 Remler 3-gang .00035 mfd. Variable Condenser 2 Remler Universal Drum Dials 2 Remler R. F. Chokes No. 35. 2 Selectone Transformers B-500 2 Selectone Transformers B-510 2 Selectone R. R. Transformers B-520 1 Selectone R. F. Transformer B-530 1 Selectone Oscillator B-540 2 Thordarson Audio Transformers R-200 1 Thordarson Out-put Transformer R-76 1 Carter 400 ohm Potentiometer 1 Carter Heavy Duty one ohm Rheostat 1 Carter 15 ohm rheostate with fila ment switch 2 Carter Tip Jacks 2 Carter .00025 Grid Condensers 1 Carter .002 Fixed Condensers 1 Carter .0001 Fixed Condenser 1 Silver-Marshall Type 340 Midget Condenser 10 Benjamin sockets 1 Pair Benjamin No. 8629 Brackets 4 Tobe 1-mfd. By-pass Condensers 1 Jewell Pattern 135 0-8 Voltmeter 1 Pkg. Kester Radio Solder 1 Formica 26x7x3/16 inch Drilled and Engraved Panel 1 Formica 25x10x3/16 inch Drilled Sub-panel 60 Kellogg Soldering Lugs 40 Ft. Acme Flexible Celestite Wire 2 X-L Binding Posts primary of the same transformer being coupled to a standardized 201A tube, operating under actual load conditions. The signal generated by the oscillator is impressed on the grid of the test stage or first tube, and the gain between* the test tube and the vacuum tube voltmeter is then observed on the very senitive milliammeter in the plate circuit of the' vacuum tube voltmeter. When resonance is effected, that is, when the oscillator is driving at the peak frequency or point of maximum effciency and amplification of the transformer, the operator reads the setting of the oscillator, and with the aid of a powerful microscope observes the exact maximum deflection of the milliammeter in the vacuum tube voltmeter. The deflection of the meter is recorded as well as the setting of the oscillator condenser. The latter reading represents the peak frequency of the transformer, and the milliammeter reading gives the gain of the transformer. The transformers are then sorted into sets having the same oscillator reading (peak frequency) and are then sorted into sets consisting of two B-510 and two B-500 transformers having the same reading on the vacuum tube voltmeter. The tests do not stop here however, as Mr. Scott feels that the one final and certain method to use, to absolutely prevent any defective units from leaving the laboratory, is to give them an actual air test on a standard receiver. A set of transformers can be dropped into their proper places on the special test set in less time than it takes to insert a set of tubes in their sockets. Every transformer is thus tested for selectivity, distance and tone quality before leaving the laboratory. To continue with the description of the recdver Grid bias rectification is used in the second detector for the very apparent reason that the heavily amplified signal impressed on the grid would overload the tube and cause distortion. Better tone quality is thus obtainable. The second detector is also outfitted with a Remler RF Choke and bypass Condenser, to block and bypass IF strays in the plate circuit, so that only the direct current component passes into the audio amplifier transformer winding. The audio amplifier is standard in design with the departure that the last stage employs a CX-310 power tube in order to handle the tremendous amplitudes delivered by the RF, IF and first audio stages. Thordarson R-200 Audio transformers are used in the usual cascade circuit in conjunction with an R-76 Output Speaker Coupling Transformer which protects the speaker windings from the 400 volt plate current used to actuate the CX-310. The tonal quality obtainable with this widely known amplifier is now a tradition among radio enthusiasts, and needs no further description. Battery connections are made with a Jones Cable, having 10 leads coded as follows : A positive red, A negative green, B negative yellow, B positive 45 volts blue, B positive 90 volts pink, B positive 400 volts brown, C positive black, C 6-9 volts negative orange. The filament current for the UX-210 power tube, which is taken off the Eliminator is carried by twisted leads to two binding posts, and twisted leads connect the filaments of the 310 tube to the socket. One of the unusual features of the New World's Record Super 10 is the specially designed eliminator to suit the receiver and its various load. Mr. Scott strongly recommends that this eliminator be used with the New Super 10 and will ensure the very finest results. The construction of the eleminator is simple, in fact is decidedly less involved than the usual type of B power supply. The New Carter bypass condensers, (which are guaranteed to stand continuous operation at 1.000 Volts D. C.) and resistances are used in this circuit in connection with Thordarson T-2098 Power Transformer and T-2099 Choke Coil. Two CX-316 B Rectron Tubes are used, one on each side of the AC cycle so that full wave rectification is effected with ample current for all the loads required with the receiver. The circuit of connections appears in the blueprint and requires no additional mention. The CX-301A tubes are used throughout the receiver, except as has been noted, the last stage which requires the CX-310. The filament current is from the usual storage battery. While considering this part of the circuit, it is strongly advised that a relay switch be used for automatic control of the A & B power supply. In actual practice this is almost a necessity because of the precautions that must be taken with heavy current surges in the B Power Supply if the filament current is shut off before the eliminator is disconnected. It is well to remember that good quality and sensitivity do not keep company with dead A batteries. This generally describes the most