Radio Broadcast (May 1927-Apr 1928)

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Lynch, Inc. 1775 Broadway New York City Gentlemen: Please send me a copy of your new booklet "Resistance the 'Control Valve' of Radio." I am enclosing '25 cents. Name Street City State R. B. 151 Radio Broadcast Laboratory Information Sheet January, 1928 The Three-Tube Roberts Reflex CIRCUIT CONSTANTS '"PHERE have been many requests from readers for further information on the Roberts 3-tube receiver illustrated in the August, 1927 issue of Radio Broadcast on page 209. This receiver is a reflex set consisting of a stage of r.f. amplification, a regenerative detector, one stage of reflexed transformer-coupled audio amplification, followed by another straight audio stage. The circuit, which was not given in the article mentioned above, and which many readers have requested, is published on Laboratory Sheet No. 159. The list of parts is given below. Li, L2 — R. F. transformer. L2 may consist of 45 turns of No. 24 wire wound on a 3-i'nch tube. Li should contain 40 turns of No. 24 wire with a tap at each 10 turns. Li should be wound alongside the filament end of L2. L3, L4, Ls — Interstage r.f. transformer. L3 and L4 have the same specifications as Li and L2 with the exception that L3 should be wound with No. 26 or No. 28 wire and should only be tapped at the exact center instead of at every 10 turns. That end of L3 nearest the grid end of Li should connect to the plate of the r.f. tube, the center tap connects to transformer T2, and the other end of Ls connects to the neutralizing condenser. L5 is a movable tickler coil consisting of 20 turns of No. 26 on a lj inch tube. Ti, T2 — Any good audio transformers. Ts — Any good output transformer. Ci, C2— 0.0005-mfd. variable condensers. 51 — Antenna tap switch. 52 — Filament switch. Ji — Double-circuit interstage jack. J2 — Single-circuit jack. V — Volume control, 50,000-ohm variable resistance. C3 — Neutralizing condenser, 0.000015 mfd. Cj— Grid condenser, 0.00025 mfd. Rd — 4-megohm grid leak. Ri — 10-ohm rheostat. R2 — 0.5-ampere fixed filament control resistance. C4 — 0.001 -mfd. fixed condenser. d — 0.0001-mfd. fixed condenser. Cs — 0.00025-mfd. fixed condenser. Eleven binding posts Three sockets Hook-up wire For best results a power tube should be used in the last socket. If a 171 type tube is used with 180 volts on the plate, the C bias required is 40.5 volts. When the receiver has been completed it should be neutralized by tuning-in some station, adjusting the tickler until the detector oscillates and a whistle is heard and then varying the neutralizing condenser until the whistle changes in pitch the least amount (its loudness will change considerably) as Ci is varied. 5. 159 Radio Broadcast Laboratory Information Sheet January, 1928 The Three-Tube Roberts Reflex Radio Broadcast Laboratory Information Sheet January, 1921 Fading HOW IT MAY BE PLOTTED ON THIS Laboratory Sheet is published a curve showing how the signal strength from station \\G\ varied over a period of about 10 minutes during a fading test on this station made during the early part of November. Anyone can make these measurements. To make fading measurements of this sort the only instrument needed is a 1.5-or 2-mA. milliammeter. The «; meter is connected in the B plus lead to the detector E tube; it will read about 1 mA. if the detector is a 2 201-A type tube using a grid leak and condenser for detection with 45 volts on the plate. When a signal is tuned-in the meter deflection will decrease, the amount of the decrease depending upon the strength of the signals. If the meter deflection with the signal tuned-in is subtracted from the meter deflection when not receiving a signal, the difference will be the amount the meter deflection has changed due to the signal. If the normal plate current is 1 mA. and the signal causes the values to decrease to 0.6 mA. then the deflection due to the signal is 0.4 mA. If this value varies with time it indicates fading and can be plotted as a curve, as shown on this Sheet. An examination of this curve indicates that at the start of the test the meter deflection due to 0.1 the signal was 0.5 mA. but that after about one 1 minute the signal strength quickly fell to 0.25 mA. and then increased and decreased several times in rapid succession. 05 05 ; 04 o 0.3 0.2 1 i r-J 1 t RAI I )I0 B R0AJ UTC 3CAS RY T 3 4 5 6 7 TIME IN MINUTES