Radio broadcast .. (1922-30)

388 RADIO BROADCAST FEBRUARY, 1927 FIG. 5 Note how the bypass condensers have been placed touching both the coil socket and tube socket. Some of the condensers manufactured since this receiver was first described, have mounting lugs that will not exactly match those in the chassis and shield pan. This can be corrected by clipping out whatever parts of the mounting lugs get in the way of the fastening screws. This photograph also shows how a condenser damaged in transit should be adjusted so as to obtain perfect interleaving of the rotor and stator plates leaved with the stator plates, and carefully examined at each side to see that they interleave centrally. They should then be turned ap- proximately all the way out so that stators and rotors overlap but half an inch along their peri- phery, and should be re-examined. It is vitally essential that at this point the rotors interleave absolutely centrally with the stator plates. If they don't, a small wrench should be procured— such as will be found with all of the later B type condensers—and the lock nuts on the brass rods, to which the stator plates are soldered, loosened up, as these lock nuts serve to hold the brass stator bars tightly in position in their mounting holes in the bakelite insulating strips on-each end plate. When these nuts are loosened up, the entire stator plate section at one side may be shifted either forward or backward and thus moved to a position where the rotor plates interleave abso- lutely centrally with the stator plates on this right-hand side. With the two nuts on the rear end of the stator rod left loose, the two nuts at the front end should be tightened up on either side of the bakelite strip until they are quite tight against it, and hold the stator rod definitely in position. If first one nut is tightened half a turn, and then the other tightened likewise, it will be found that the stator plate section can be permanently locked in a position where the rotors will interleave centrally with it. The two nuts on the rear of the tie bar locking on either side of the rear bakelite insulating strip should then be moved up half a turn at a time until they are tight against the rear bakelite insulator, so as not to impose a "thrust" or "pull"between the two insulator strips. This operation is really extremely simple and can be easily ac- complished since the condensers are specifically designed to permit of such adjustment by the user should the occasion require. Once the adjustment has been made, let's say, on the right-hand or low side of the condenser, the ad- justment may afterwards be made on'the left side stator rod so that the rotor plates interleave centrally with the stators when the plate sec- tions are entirely engaged. As previously remarked, an adjustment of this nature should seldom be necessary, but it is al- ways an excellent idea, after the receiver has been assembled and the link motion put in place, simply to check over each condenser with the small wrench provided, and make absolutely sure that undue strain has not been imposed on the condensers either in transit or in assem- bly. Several builders of the "Shielded Six" have noticed that the length of the condenser shafts may be easily varied. This is a feature of the con- densers recommended for the set, and the shafts may be pushed either forward or back- ward in the condensers to any desired position. This is accomplished very simply by loosening the set screws in the lock collars on the shafts, pushing the shafts into the desired position and then performing the operation illustrated in Figs. 3 and 4. The rotor plate section lock collar should have its screw tightened up on the shaft in the position desired by the builder. Then, holding the condenser in the left hand, as shown in the photographs, with the thumb upon the rear end of the shaft, and the second and third fingers of the hand pushing the lock collar on the front of the condenser toward the thumb, the set screws in this collar should be locked up FIG. 6 The details of the plate spacing adjustment. Notice the small wrench, on one of the lock nuts which determine the position of the stator plate assembly tightly. This will compress the small spring washers between the front lock collar and the frame, and will eliminate entirely any play which might creep in were the hand pressure not exerted. Several photographs (Figs. 5 and 6) accompanying this article indicate how this adjustment may be made. The ganging of the condensers in the receiver is extremely simple and has been outlined in the previous article. A photograph (Fig. 7) in- dicates exactly how the builder would view them to arrive at the proper adjustment of each one before the link motion should be locked to the three condenser shafts by means of the set screws in the link motion collars. Another feature which may come to the builder's attention is the fact that the front edge of each shield pan will have to be pressed forward a fraction of an inch to allow for the bottom mounting studs on the front end plate of each condenser. This is an intentional part of the design, the shields and condensers having been so made that, if necessary, a screw can be put through an ordinary mounting panel into the upper mounting stud of the condenser, which, if tightened up against the front panel, would thus serve to clamp the front edge of the shield pan between the bottom mounting lug, and the front panel. Should the builder endeavor to operate the "Shielded Six" with a standard B power-supply device (battery eliminator), the connection of a 5-to 15-microfarad condenser across the high-voltage output of the battery eliminator, or possibly across the detector output, will be necessary should there be any tendency for the combination of power-supply device and receiver to howl or produce a "motor-boating" sound in operation. Such noises would be due to the high internal resistance of the eliminator, which, being common to the plate circuits of all the receiver tubes, would serve to couple these plate circuits together and cause audio oscilla- tion. A second caution concerns the purchase' of an eliminator with a sufficiently high power output to operate the receiver properly, which requires approximately 25 to 35 milliamperes when a 171 tube is used, with 180 volts plate potential. FIG. 7 Attaching the link motion to the condensers. In attaching the link, the air-gap between stator and rotor plate edges should not be as great as is shown here, it being exaggerated for photographing pur- poses. The gap should be the tiniest one visible to the eye, and should be the same for all three condensers