Radio Broadcast (Nov 1926-Apr 1927)

586 RADIO BROADCAST APRIL, 1927 ANTENNA PRIMARY -SECONDARY AUTO-COUPLE COIL FIG. 4 off at the 30th, 52nd, and 73rd turns, corresponding with the first, second, and third position of the tap switch. The fourth tap switch point includes the entire 95 turns of the loading coil in the antenna circuit. Details of the layout, together with coil specifications, are given in Figs. 4 and 5 respectively. HOW TO NEUTRALIZE THE RECEIVER CIRCUITS that are represented to readers as of the non-blooper type due to some scheme of neutralization employed, do not always turn out that way. This is largely due to the indifference of the builder to his job when the time comes to neutralize his receiver. A receiver which is perfectly neutralized is better than one in which neutralization is not complete because, in the latter, oscillation is likely to occur at some point in the broadcast band and at that point reception is spoiled if not completely eliminated. To neutralize a receiver does not require a great deal of technique and, in the end, the time spent on this important point is more than compensated by the superior results obtained. Where each tube of the "Hi-Q" has been supplied with its own filament ballast, the job is made quite easy. Simply remove the ballast in the first r.f. tube circuit from its clips (after a station has been tuned-in), and, with a stick sharpened to a thin edge, adjust the neutralizing condenser until the signal is reduced to a minimum, or entirely eliminated. It is important to note that while the process goes on it is necessary to re-tune the condenser of the first stage as the neutralizing action usually de-tunes the first condenser circuit slightly. If the stick of wood doesn't hold out long enough to complete the job, take an ordinary screw driver and wrap the shaft with insulating tape to within a half-inch of the blade. Then if the tape-covered part of this tool inadvertently touches the can walls, no blown-out tubes will result, which would be the case if the screw driver were not insulated. After the first r.f. stage has been neutralized, the ballast may be replaced in its clips and the same process duplicated for the second r.f. stage. A circuit diagram of the receiver incorporating all of the changes outlined in this article is shown in Fig. 6. The following parts were used in making the changes described. Other makes can of course be employed, if the electrical characteristics of the unit concerned are similar. 1 4-Point Carter Tap Switch, No. 404 1 Loading Coil (See Winding Specifica cations, Fig. 5) 2 Bypass Condensers, Sangamo, 0.1-Mfd 1 Amperite and Mounting, Type No. 4V (For Use with 3-Volt Tubes) 3 Amperite Cartridges, Type No. 4V 1 Amperite, Type No. 120 ... 1 Carter Double-Circuit Jack, No. 104 Total . 1. 00 1.60 1. 10 3.30 1. 10 1. 00 $10.10 Previous articles on the building of the "HiQ" receiver have appeared in Radio Broadcast for January and March of this year. r^~ 95 Turns No. 25 S.S.C. wire,space wound. 4 (Space between turns 95 T.jequals half the diameter of No. 25 wire.or the full diameter of No.32 S.S.C. wire. 1 1^ \fr~\ Coil is cemented ~' to strip with collodion . Supports made from Vcopper tube. FIG. 5 _ Neutralizing Condensers^ ^— Shield Cans-^ & \ ' V ~T '2 meg. ~tr pH-[ i 0.00025 mfd-Pjp FIG. 6