Radio broadcast .. (1922-30)

A Good Four-Tube Receiver 905 FIG. 5 The panel layout. Where other parts are to be used, it will be necessary to provide other mounting holes. The center holes then may remain as shown is the case, it indicates that the wiring in this circuit is correct and the second tube should be inserted in the next socket. A click will be heard as it goes in and if its grid terminal is touched, a click or squeal will also be heard. The 2-megohm grid leak should be placed in the clips of the grid condenser and the detector tube inserted in its socket. Then with the detector condenser set at about 50, the tickler should be rotated and as it goes from o to 100, at some point along its scale a "plunk" should be heard. If, after this plunk has been heard the grid condenser is touched with the finger, a thud or squeal should result if the detector is oscillating. If the tickler is then set at zero, nothing but a click or squeal should be heard as the grid condenser is touched. If the thud is not heard at all, it is due to failure of the detector tube to oscillate. This may be corrected by reversing the leads to the rotor or tickler coil of the vario-coupler. Now connect a ground to the ground binding post and an antenna to the small tap located on the stator winding of the coupler which also goes to one side of the neutralizing condenser, which will be connected later. This leaves out entirely the r. f. amplifier tube and gives a straight regenerative detector and two stages of audio amplification. For test purposes, the tickler may now be moved up beyond the point where a plunk is heard and the detector tuning condenser rotated until a "tweet" or squeal is noticed. This indicates a station and if the tickler is then set just below the oscillating point the signal will be heard with its true modulation. ADJUSTING THE NEUTRALIZER AFTER these tests have been made, the •'*• neutralizing condenser should be connected to the grid terminal of the first tube socket and to the tap on the stator winding of the vario-coupler. The neutralizing condenser consists of nothing more than two pieces of bus bar of equal length and so arranged that when soldered to the tap and grid terminal referred to and running in the same direction their ends will fail to meet by approximately j inch. Both ends are inserted in a length of spaghetti which will, when run from the tap up to the grid terminal, serve to hold these two pieces of bus bar firmly in position. Before they are finally soldered in place, a small 5-inch length of brass tubing should be placed over the spaghetti and left entirely unconnected so far as the balance of the wiring goes. This completes the neutralizing condenser. An antenna should now be connected to the antenna binding post of the set and the first or r. f. tube inserted in its socket. The rheostat should be approximately threequarters on. The tickler should be set at zero and the tuning condenser at a position where a station was previously heard. The 5-inch brass tube should be pushed up toward one end of the spaghetti tube on which it slides and the r. f. tuning condenser varied around approximately the same setting as that of the detector condenser, when the station should be heard again. Stations should now be tuned-in over the entire range of the receiver, with the tickler at zero. If the r. f. amplifier oscillates, the small piece of brass tubing should be slid down the spaghetti J inch at a time until all tendency toward oscillation is eliminated. If it cannot be eliminated by moving this tubing along, this indicates that the primary of the r. f. transformer is improperly connected and the leads to it should be reversed. This primary is the small 1 5-turn coil located inside one end of the vario-coupler stator coil.