Radio Broadcast (May 1928-Apr 1929)

SEPTEMBER, 1928 A "ONE-SPOT1 SCREEN-GRID SUPER 251 C1 D1 RZ P R3 D2 C2 FIG. 3. HOW THE INSTRUMENTS ARE MOUNTED kilocycles. For a 545-meter signal (550 kilocycles) obviously only the sum of the oscillator and signal frequencies can be had in a practical set, which is 950 kilocycles. Thus the oscillator tuning range must be 1900 to 950 kilocycles, or 157 to 315 meters, approximately. The 315meter setting (950 kilocycles) will heterodyne either a 550-meter station, or a 1350-kilocycle station, which would be at 222 meters. Thus it is seen that a 400-kilocycle intermediate frequency will provide a super-heterodyne that will be truly "one-spot" for all stations above 222 meters, and that the oscillator settings for stations below 222 meters will be so far away that the input tuning circuit is only called upon to discriminate between signals at the extreme opposite ends of the broadcast wavelength band, which is not difficult with a good input circuit. CONSTRUCTION OF THE SET HP HE parts used in the writer's receiver are 1 listed at the end of this article and, costing about $92.00 on the open market, justify the hope of having a receiver low in initial cost. The set is put together using a bakelite chassis, B, §" thick and 21 ts" by 9 rf" wide, this size being chosen so that the set can be housed either in any standard table or console cabinet or in the new Silver-Marshall metal shielding cabinet which has been found to contribute much to the quiet operation of the receiver when operated near X-ray machines or other sources of local interference. At the rear of the chassis all r.f. and a.f. transformers are placed in a row, with the oscillator coil, Li, at the left rear end of the chassis held in place by having its six connecting lugs pushed down through holes in the chassis, turned over on the under side, and all connections made to the turned over lug ends. The oscillator coil consists of a bakelite tube \\" long and \\" in diameter upon which are wound at the bottom a tickler consisting of 32 turns of No. 29 enamelled wire, while just above the tickler is a grid winding of 45 turns of No. 29 enamelled wire. Inside this tube is held another bakelite tube if" in diameter, upon which is wound the pick-up coil of 32 turns of No. 29 enamelled wire, the pick-up coil being connected in the first detector grid circuit. The pick-up coil is spaced equally under grid and plate coils. The tube sockets are a part of the bakelite chassis, consisting of standard socket springs riveted or screwed to the bakelite at proper points. The two variable condensers, Q and Q, are fastened to the chassis in the front center and to them are in turn fastened the drum dial brackets to which is attached the control escutcheon. To this escutcheon is fastened the filament rheostat, (with on-off switch attachment), R3, and the 3,000-ohm potentiometer R2, which, controlling the potential on the screen grids of the intermediate amplifier tubes, also serves to adjust volume and sensitivity. The neatness and simplicity of the wiring arrangement is very easily duplicated due to the convenient location of the various soldering lugs and connection points. All battery connections terminate in the multiplug at the right rear of the chassis. Battery connections are made to the set through a battery cable with receptacle plug. Antenna and ground connections are made through binding posts at the left end of the chassis. No panel has been shown for the receiver as it may be placed in any cabinet with wood, metal; or bakelite panel which must simply have a center slot 5!" wide and 55" high cut so that the panel will slip behind the bronze escutcheon plate of the set. If the Silver-Marshall shielding cabinet is used, the receiver chassis is simply placed upon the wood moulding and the cabinet slipped down over the whole assembly. The receiver requires for operation any good B-power unit capable of delivering 22, 45, 135, and 180 volts, such as the Lincoln no type, as well as a 6-volt A battery or good A-power unit such as the Kodel, Abox, Balkite or Sentinel types. Pour ux-222 screen grid tubes, three ux21A tubes, one UX-171A, a 45 and a40-volt C battery complete the accessories, outside, of course, of a suitable loud speaker, antenna and ground. If the best possible results are desired, a slight improvement may be had by substituting a UX-112A detector tube for the UX-201A type at a slight additional cost. In operation the receiver is simplicity itself. The rheostat need merely be turned up, the volume control knob adjusted to just below the oscillating point for the intermediate amplifier, and the two drum dials rotated at about the same dial settings' to tune in station after station, near and far alike. LIST OF PARTS TP HE data for the coils used in this set is given \ in the text. They may be home constructed, if so desired. The rest of the parts are of standard design, and substitutions may be made. The complete parts are obtainable in kit form. B — 1 Micarta sub-base, 2 if" x 95" x \" BPi, BP2, BP3— 3 Binding posts O — 1 Lincoln 104L (left) 0.00035-mfd. condenser Cs — 1 Lincoln 104R (right) 0.00035-mfd. condenser C C C ) r' r1' n\ — 6 Potter 104 1.0-mfd. condensers Co — 1 Aerovox 0.002-mfd. condenser Go — 1 Aerovox 0.0001 5-mfd. condenser with clips Dv— 1 Silver-Marshall 806L (left) drum dial D2 — 1 Silver-Marshall 806R (right) drum dial Ji, J2, — 2 Yaxley 420 tip-jacks Li— i Lincoln 102 oscillator coupler L2 — 1 Lincoln 103 antenna coupler L3 — 1 Silver-Marshall 275 r.f. choke P — 1 Lincoln escutcheon control panel Ri — 1 Aerovox 2-megohm grid leak R2 — 1 Yaxley 3000-ohm potentiometer, No. 53000 R3 — 1 Yaxley 10-ohm midget rheostat R-i — 1 Carter 0.57-ohm resistor, type Hf R5, R6, R7, Rs — 4 Carter 10-ohm resistors, type RU10 SW — 1 Yaxley switch attachment, No. 500 Ti, T2, T3, T4 — 4 Lincoln 101 tuned i.f. transformers T5 — 1 Lincoln 105 1st. a.f. transformer T6 — 1 Lincoln 106 2nd. a.f. transformer W — 1 Jones 8-lead battery plug and cable (special) Tube socket assemblies Wood sub-base supports, 2 if" x if" x f" Small parts assortment consisting of screws, nuts, lugs, bus-bar, spaghetti, flexible wire, Belden flexible shielding braid and screen-grid clips