Radio Broadcast (May 1928-Apr 1929)

MAY, 1928 A FLEXIBLE A.C. SUPER HETERODYNE 41 Cable Block-, Resistor Strip 0.002 mfd. "Time Amplifier OOO .1 mfd © © 220 0 ( Choke V Osc. Cond. Sec. Cond. LAYOUT OF PARTS 50,000 W Ant. Cond. ----45j---: FRONT PANEL LAYOUT > < > < > < ( if Y -Hi"* e - • -13$" ->■ «- 15s" -> 7*4" RESISTOR STRIP FIG. 3 of filament leads, the pins that normally would supply filament power being dummies. These leads are twisted together and connected to the filament terminals of the oscillator tube socket, the detector then turning on or off with the rest of the intermediate tubes. The grid and plate make the normal connections to the circuit through the adapter pins while the cathode is brought to the pin normally used for negative filament. The wire connecting to the corresponding post on the socket inside the can should be unsoldered and a new wire run out of the shield from this post and connected to the cathode post of the audio amplifier tube socket. The removal of this connection, which is merely a connection to the shield, does not disturb the circuit in any way. The bypass shown across the B and P terminals of the amplifier is most important, as well as the radio-frequency choke in the output lead of this amplifier. The receiver is now ready for test. To do this properly the power unit that is to be used must be at hand. A few suggestions anent this unit will undoubtedly assist those who are not familiar with this adjunct to quality reception. The combination of a full-wave rectifier, a good filter circuit, and a voltage regulator tube provide the best possible B voltage supply. As the unit is also used to secure C bias, its freedom from ripple is important. Naturally a push-pull cx-310 (ux-210) stage represents the last word for a tremendous amount of undistorted output although a cx-371 a (ux-171) stage will be found sufficient in the average home. The first step in testing is to throw the switch to the single-circuit receiver position. A minute should elapse before the receiver starts functioning. The 3000-ohm resistor should be set at the halfway position. Failure to operate may be readily traced in this simple two-tube set. Hum audible over a foot from the loud speaker indicates either oscillation, in which case the tickler should be adjusted, or an open circuit. A check of the B voltage at the plate and a test for open grid circuits is sure to locate the trouble. The panel resistor should be turned to the left to obtain greatest volume. After tuning up and down the dial a few times the antenna tuning should be tested by opening the switch. Its effect will be noticed more readily on the weaker stations. The dial setting may be matched within a few divisions of the detector dial by putting a few turns of wire in series with the antenna. This will be necessary only where a short antenna is employed. Up to 1 50 feet may be used without fear of broad tuning, assuring a stronger signal from distant stations. The oscillator is tested by tuning-in a station in the middle or lower portion of the wave band and temporarily shortcircuiting contacts Nos. 4 and 5 of the switch. After the tube has warmed up the dial should be rotated. A loud heterodyne squeal on the incoming signal indicates the proper functioning of the oscillator. A check up on the connection and the continuity of the grid and plate circuits will readily correct any trouble. The third step in the testing is that of the intermediate amplifier. The iooo-ohm resistor should be set at a quarter turn from the zero bias point, the 15-ohm potentiometer at the midpoint, and the B resistor slightly below the fullvoltage position. A turn of the switch and a few moments wait should see the super-heterodyne in operation. A rapid succession of "birdies" when the oscillator is tuned indicates oscillation in the intermediate stages. This is the only trouble that will be encountered if the wiring has been done in the correct manner. It is readily corrected by reducing the filament voltage. An o. 5-ohm rheostat, which should be located at the filament transformer end of the cable, is the remedy. A slight readjustment of the bias and B-voltage resistor will also be of assistance. It should be borne in mind that the amplifier should be adjusted so that maximum amplification is obtained at all times. The input is controlled by the antenna coupling while the resistor across the transformer primary may be used as an auxiliary means of reducing the tremendous volume that may be obtained, to a reasonable level. While the rheostat takes care of the voltage applied to the cx-326 (ux-226) tubes in excellent fashion, care should be taken that the cathode tubes are not run at an excessive voltage. If the receiver is to be operated from an A battery and is to derive 122.50 2.75 7.50 2. 85 10.50 its plate voltage from either batteries or powersupply unit, various changes must be made. Three S-M No. 51 1 sockets will be required in place of the three No. 512 ones indicated in the list of parts. Two Carter I R-6 6-ohm rheostats, R9 and Rio, are also necessary. A Carter M-400-S potentiometerfilament control switch, Rg, is also wired in the circuit. The list of parts for the a.c. receiver: List of Parts Ci, C2, C3 — Samson No. 65 0.0005-Mfd. Condensers I Li — Samson No. 71 Antenna Coil L2, L3, L4 — Samson No. 3 1 Coupler L5, L.6 — Samson No. 41 Oscillator Coil Three Marco No. 421 Illuminated Controls S^M No. 440 Time Amplifier (Includes Three Intermediate Stages and 2nd Detector) T — S-M No. 220 Audio Transformer Lv— S-M No. 276 Choke Coil Three S-M No. 512 Tube Sockets Sw — Yaxley No. 10 Antenna Switch One Yaxley No. 63 Triple-Pole Switch Ri — Carter MW-3000 3000-Ohm Potentiometer R2 — Carter MW-1000 1000-Ohm Potentiometer R3 — Carter MP-20 20-Ohm Potentiometer Pm, R6 — Carter Type L 50,000-Ohm " HiOhm" Carter Cathode Tube Adapter Re — Durham 2-Megohm Leak Rt — Frost FT-64 Mid-Tap Resistor C4 — Carter 0.0001 5-Mfd. Grid Condenser, with Clips Cs — Carter 0.002-Mfd. Condenser C6 — Carter No. 210 i-Mfd. Bypass Condenser Jones No. BM 410 Ten-Wire Cable Cortlandt Panel 7" x 24" x T%" Drilled and Engraved Resistor Mounting Strip if" x 75" x TV Drilled Baseboard I2"x23"xf", Plywood Preferred Two Rolls of "Braidite" Wire, Two Colors One Fahnestock Clip and Assorted Screws Total $1 The following additional equipment is partly necessary to operate the receiver. A choice in several instances may be made by the constructor. Four c-327 Tubes Three cx-326 Tubes Ti — SM 325 Filament Transformer One S-M 660-210 Power Pack or — One S-M 660 171 Power Pack One Fritts Cabinet 7" x 24" x 12" 35 .00 8 1 00 •25 .50 1 60 1 •25 1 ■25 75 4 .00 1 AO .50 50 40 50 1 25 3 25 7 50 50 75 60 7» 18.25 $24 00 9 00 8 00 83 50. 66 50 23 50 HOW The coil L4, L5, and L5, while Li is to the right THE PARTS ARE LAID OUT il units to the left comprise L2, L3,