Radio Broadcast (May 1928-Apr 1929)

298 RADIO BROADCAST ADVERTISER o Whatever your connection with Radio Whatever your need for instruments — whether as set builder, amateur transmitter or service and repair man — the name "WESTON" on any meter you select is the highest guarantee of long life and dependable service with the lowest possible cost of instrument upkeep. Listed herewith are but a few timely models. The complete radio line is fully described in Circular J, mailed upon request. Model 528—3-Range A. C. Voltmeter A compact little instrument with red and black mottled bakelite case —150/8/4 volts — for testing A. C. supply and tube voltages of A. C. receivers. An excellently designed and most precise little meter which will find many uses in the home and laboratory — fully as satisfactory for small testing requirements as a larger and more expensive instrument. Price $16.50. A. C. and D. C. Set Tester Model 537 A dealer's or radio serviceman's complete testing outfit. Weight, only 6i lbs. No additional tools, instruments or equipment necessary. Simple, automatic method of making connections. Meter equipment :-T wo 3j" diam. high grade Weston models. (1) — 3-range A. C. voltmeter, 150/8/4 volts. (2)— D. C. volt-milliammeter >with four voltage ranges, 600 300 60, 8 volts — (1000 ohms per volt) and two current ranges — 150/30 milliamperes. Price, $100.00. At all dealers, or write direct to: WESTON ELECTRICAL INSTRUMENT CORPORATION 604 Frelinghuysen Ave. Newark, N. J. WESTON RADIO INSTRUMENTS No. 219 Radio Broadcast Laboratory Information Sheet September, 1928 Sizes of Tap and Clearance Drills TABLE OF SIZES '"PHE table on this sheet will be found useful in constructing radio receivers and power units, when it is necessary to tap or drill holes to take a certain size machine screw. The first and second columns, headed "Screw Number" and "Threads per Inch" in each section of the table, identify the machine screw, and the third column headed "For Tap" gives the drill size if the hole is to be tapped so that the screw will thread into the hole. If the hole is to be drilled so that the machine screw passes through the hole, then the "Clearance" size drill should be used. 1 SCREW 1 NUMBER THREADS PER INCH DRILL NUMBER SCREW NUMBER THREADS PER INCH DRILL NUMBER For Tap Clearance For Tap Clearance § I 3 48 45 38 7 30 31 21 1 1 3 56 44 38 7 32 30 21 I 1 4 32 43 31 8 24, 30 30 17 1 I 4 36 42 31 8 32 29 17 I I 4 40 41 31 9 24 29 13 I I 5 30, 32 40 29 9 28 28 13 I I 5 36 38 29 9 30 27 13 I i 5 40 37 29 9 32 25 13 I I 6 30, 32 35 26 10 24 25 8 1 I 6 36 33 26 10 30 22 8 I 1 6 40 32 26 10 32 21 8 E No. 220 Radio Broadcast Laboratory Information Sheet September, 1928 The Roberts Four-Tube A. C. Receiver PARTS REQUIRED f}N LABORATORY SHEET NO. 221 is pub" lished a circuit diagram that has been requested by many readers in their letters to the Technical Information Service. It is the circuit diagram of a 4-tube Roberts receiver for a.c. operation using three 227 type a.c. tubes and one 171A type tube. The following parts are required for the construction of the receiver: Ci, C2 — 2 Tuning condensers of a size such as to cover the broadcast band with the coils used. Homemade coils made according to the specifications given below require 0.0005-mfd. condensers Cs — Neutralizing condenser, 0.00002 mfd. maximum capacity. d — Grid condenser, 0.00025 mfd. C5, C6, C: — 3 Bypass condensers, 1.0 mfd. Ci — Output condenser, 2 to 4 mfd. C9 — Bypass condenser 0.0002 mfd. Li, L2 — 2 Thirteen point spider-web coils. Li consists of 35 turns of No. 22 d.c.c. wire tapped at every five turns. L2 consists of 44 turns of the same size wire. L3, Ls, Ls — 3 Thirteen point spider-web coils. L3 is a double wound primary consisting of two parallel windings of 18 turns of No. 26 d.c.c. L4 is the same as L2. Ls consists of 12 turns of No. 22 d.c.c. mounted on a form so that its coupling to L4 may be varied. Ls — Output choke coil, 30 henries. Ri — C bias resistor, 500 ohms. R2 — C bias resistor, 2000 ohms. R3 — Grid leak, 2 megohms. R<, R5 — 2 filament resistors, 20 ohms, centertapped. S — Antenna tap switch. Ti, T2 — 2 audio transformers. 3 five-prong sockets. 1 four-prong socket. Filament transformer supplying 2.5 and 5.0 volts. No. 221 Radio Broadcast Laboratory Information Sheet September, 1928 Circuit of the Roberts FourTube A.C. Receiver