Radio Broadcast (May 1928-Apr 1929)

94 RADIO BROADCAST THE LAB SET IN ITS CABINET Special coils are available for this circuit and receiver which require no alterations. Any of the coils designed for a 0.0005-mfd. condenser may be used however. The Aero LJ95 set has two coils and will be discussed as a convenient and typical set which may be modified. REVAMPING STANDARD COILS D EMOVE the hinged primary winding from the antenna coil leaving nothing but the main secondary winding connected to terminals 1 and 6. Remove the primary from the detector coil by carefully breaking the bakelite tubing on the inside and unsoldering the leads going to terminals 2, 3, 4 and 5. Tap one of the coils at the center turn vertically above terminal No. 1. Tap the other coil one third of the way up from the bottom vertically over terminal No. 2. Wind a thin strip of insulating material such as thin celluloid or varnished cambric about half an inch wide over the center of the mid-tapped coil. Pierce a small hole in the insulation and bring out the centertapped strip through it. Wind eight turns of wire (No. 28 d.c.c. is all right, but the size is electrically not at all important) on each side of the midtap for the primary. Anchor the two end turns by looping them under the vertical bakelite coil support. The baseboard and panel should be prepared in the usual manner. The template drawing, packed with the drum dials may be used as a drilling template. Because of the height of the drum dials, the condensers, Ci and C2, must be mounted on "stilts" or bushings. In this case two pieces of brass tubing cut to the proper length were used. As the condensers are mounted in this receiver, the reading of the drums increases with wavelength. If the readings are to increase with frequency the condensers should be reversed. The full floating shafts make this possible. This feature also permits bakelite or hardwood shafts to be substituted. These will be discussed under the operating details. The location of the parts is very important. In experimenting with the layout, for example, it was found that moving the choke, (L4), over between the drum dials made the set unstable. This trouble was found to be due to the greater length of the "hot" plate lead and not to coupling between the choke and coil as might have been supposed. A discussion of the important leads to watch appeared in the June, 1926, Radio Broadcast under the title "Additional Notes on the R. B. Lab Circuit." There is nothing "tricky" about the wiring. All a.c. filament leads should be twisted. Bus bar wiring was used in this set so the connections could be easily traced in the photographs. All the battery leads may be cabled if flexible wire is used. There is no objection to using "bee" line or direct point to point wiring if the leads are carefully spaced. For details on the arrangement of the leads see the photographs and Fig 3 AN UNUSUALLY EFFICIENT SET IN SMALL SPACE JUNE, 1928 The two A-battery leads on the cable are not used. If an outside C battery is to be used for the radio-frequency and first audio-frequency stages it may be added by making the following changes: Remove R3 and C7 and connect the leads going to the K terminals on the 227 sockets to the minus B or yellow cable terminal. Connect the lead going to the minus C terminal of the first audio-frequency transformer and the one connected to R2 to the black cable terminal. Connect C7 across the black and yellow cable terminals. The C battery is then connected externally by using the yellow and black leads as the positive and negative leads respectively. WHEN A B-POWER UNIT IS USED IF B-power unit which supplies 180 volts is used, a C battery with proper potential for the tube employed should be used for the power tube. Where the power device supplies 200 volts or more, an arrangement such as indicated in Fig. 1 may be used. A 30-henry choke and 2.0 or 4.0mfd. condenser should be used to keep the direct current out of the speaker. When the speaker return is connected as shown, the 2000-ohm grid biasing resistor is not in the return circuit and this insures better reproduction of the lower audio frequencies. No output device is used in the receiver. This reduces the cost of the parts when a 112-A type tube is used or a 171-A type with only 135 volts on the plate. If an external power amplifier is to 1 B Det. Int. Power B Socfcet-Powet Unit FIG. I be used the last audio-frequency transformer may be omitted. In some cases the power amplifier includes an audio stage ahead of the power tube and in this case the detector output may be connected directly to the speaker jacks on the cable terminal. The dial lights should be connected in parallel across the filament of the 171-A tube. Very little adjustment should be necessary after the receiver is completed. In adjusting the balancing circuit the "dead filament" method is not very desirable nor convenient in this case. Tune-in a carrier in the short-wave section of the broadcast band and set the detector regeneration control so the set just oscillates. Use a screw driver made from a bakelite or fiber strip to adjust the balancing condenser. Tune the radiofrequency stage first to one side of the carrier and then to the other slowly while adjusting the "equalizer."