Radio Broadcast (May 1928-Apr 1929)

200 RADIO BROADCAST AUGUST, 1928 FIG. I. THE TOP VIEW Because the Rice amplifier has both the stator and rotor plates of the tuning condenser above ground potential — neither is grounded — some hand capacity may be evidenced. The easiest method of preventing such trouble is to use insulated shafts. Fortunately the Hammarlund condensers used by Mr. Knowles have removable shafts. It is a simple matter to put in a hard rubber (or other insulating material) shaft in place of the metal ones supplied by the manufacturer. PANEL LAYOUT A PICTURE wiring diagram and panel layout sheet may be obtained by writing to Radio Broadcast, Garden City, N. Y. The parts list on this page gives the apparatus actually used in the receiver. Similar parts may be used, of course. The only special apparatus is the coils, which may be built at home if desired. WHAT BUILDERS THINK OF THE LAB CIRCUIT using the 25" and 18" Western cones, switching the cones. If this last set had fit my console I surely wou d have passed the old set on. I was able to bring in Toronto with no interference from kdka or wht — and I am close to wht. koa, wsmb, — and others came in strong, clear, and without distortion. Not so on the earlier model. I could not get Toronto. You see I had two complete outfits — speakers, power units, aerials, and grounds. I tried to make a fair and intelligent test of the two sets. With the later set I actually tried to get interference on the lower waves on the locals. It couldn't be done. The letter below is from Alfred V. Waller who lives in Halifax, Nova Scotia: Just a line to let you know that I have been an R. B. Lab. Circuit fan since June, 1926, and can say there is no outfit to come near it in performance, or in any way, for that matter. I've tried everything up to 8 tube supers, but the R. B. Lab. tops them all. On the speaker I've had over and over on good nights kfi, kgo, cnrv, cze and every other 10 kc. from 200 to 545. LIST of parts Ci, C2 — 2 Hammarlund ML-23 condensers C3 — 1 Hammarlund MC-15 midget condenser C4 — 1 Hammarlund equalizer C5 — 1 Tobe 1.0-mfd. bypass condenser C6 — 1 Aero vox 0.0002 5-mfd. grid condenser L2, L3 — 1 pair Aero coils, type RB 8 L4— 1 Hammarlund choke, RFC-85 Ri — 1 Electrad volume control, type P i R2 — 1 Electrad grid resistor, 500 ) ohms R3 — 1 Frost deluxe resistor, i ohm R6 — 1 Durham grid leak, 2.0 megohms, with mounting SWi — 1 Yaxley s.p.d.t. switch No. 30 SW2 — 1 Yaxley s.p.s.t. switch No. 10 Ti, T2 — 2 Silver-Marshall audio transformers, No. 240 T3 — 1 General Radio output transformer, type 367 1 Westinghouse Micarta panel, 7" x 2 1" x a 1 Yaxley cable and plug, No. 669 5 Eby 4-prong sockets 2 Eby binding posts 2 National single drum dials 2 Coils of Celatsite hook-up wire 2 lengths of \" bakelite rod. The additional parts necessary to make the set operative are three 201A or 1 12A type tubes and a 171 type for the last audio stage, a loud speaker, and A, B and C batteries or power supply units. The receiver will give best results with an outdoor antenna, 75 or 100 feet long. S1" NCE the first appearance of articles on this ircuit in Radio Broadcast, many readers have written of their experiences. The first of the letters quoted below is typical. It comes from a man of about forty who sells locomotives — we cannot, we regret, publish his name. For your personal information, I take the occasion to write you that your last edition of the "Lab Receiver," as shown in April issue, far excells anything I have ever experienced in radio receivers. I had a set built according to earlier issues, mounted in a handsome console with illuminated dials (Hammarlund), and coupled to a Western Electric 25" cone. Till recently I thought 1 had > perfection. However, I built a set for a friend as shown in the April Radio Broadcast, using Thordarson R-200 transformers and bypassing only the first B lead with a 2 mfd. condenser and audio choke. I used 3 ii2a's and a 171A. Both sets have the r.f. and detector stages in 6" x 10" copper boxes. This last set was more selective, sensitive, and gave far better tone quality. I put up two grounds and aerials and ran both on the same station, © © FIG. 2. THE CIRCUIT DIAGRAM The detail diagram at the lower left shows the proper connections to the cable plug