Radio Broadcast (May 1928-Apr 1929)

No. 17. February, 1929. Radio Broadcast's Service Data Sheets The Philco Electric Radio Receivers, Series 5 '"PHE Philco receivers are available in several different models but all of them use the same fundamental circuit consisting of three stages of tuned and neutralized radio-frequency amplification followed by a detector and a two-stage audio amplifier. There are six tubes in the receiver; four type 226, one type 227, and one type 171a. The receiver can be obtained in several table models or in cabinets. One model is a combination electric phonograph and radio. Technical Discussion 1. TUNING SYSTEM Four main tuning condensers are used in the receiver. They are Ci, Ci, Ca and C4. The latter three condensers have connected across them small compensating condensers, Cs, Cs, and C7, which provide a method of compensating slight differences in the circuit capacities, thus bringing all the tuned circuits into exact electrical alignment. Across the first tuning condenser, Ci, is connected the small condenser, Cs, which is called the "range control." This range-control condenser serves normally to tune the first circuit to exact resonance but it also has a second function. A small spring contact on the condenser Cs serves to ground the grid of the first tube when the condenser is rotated to the extreme left. Under such conditions the gain cf the receiver is reduced to a point which is quite satisfactory fcr average local reception. Each r.f. stage is neutralized by the Hazeltine method, the neutralizing condensers being C9, C10 and C11. 2. DETECTOR AND AUDIO SYSTEM A grid-leak-condenser-type detector is used followed by a two-stage transformer-coupled a.f. amplifier. The grid leak is Ri and the grid condenser is C12. In the plate circuit of the detector tube is connected the small by-pass condenser, C13, with a capacity of 0.001 mfd. Ti is the first audio transformer and T2 is the second-stage audio transformer. Ji is a jack into which a phonograph pick-up unit may be connected. When a phonograph pick-up unit is being used the volume control of the receiver should be turned off and the volume regulated Philco set in colored cabinet by means of the control supplied with 1he pick-up device. The power tube is a 171a and in its plate circuit are connected the output filter choke coil, Li, and the condenser Cn with a capacity of 0.5 mfd. This filler system functions to keep out of the loud speaker the d.c. currents in the plate circuit. 3. VOLUME CONTROL The volume-control resistor, R2, is connected between antenna and ground. The movable arm on this resistor is connected to one end of the primary of the first r.f. transformer. 4. FILAMENT CIRCUITS The 226— type r.f. amplifiers and the first audio amplifier tube are supplied with approximately 1.5 volts from the secondary Si on the power transformer, T, the 227-type detector tube is supplied with 2.5 volts from S2, and the 171a power tube supplied with 5 volts from S3. In order to permit an accurate hum balance to be obtained, adjustable potentiometers, R3 and R-i, are connected across secondaries Si and S2, respectively, and these resistors are adjusted at the factory to a point of minimum hum at the output. The condensers, C15 and Cm, which are connected letween each side of Si and ground, serve to bypass to ground the r.f. currents which would otherwise have to flow through the resistor, Ra. 5. PLATE CIRCUITS The detector tube is supplied with about 35 volts, the r.f. and first a.f. tubes with approximately 90 volts, and the 17lA-type power tube with 135 volts. The plate current for each 226— type tube is approximately 3.5 to 4 milliamperes. The 227-type tube draws about 1.5 milliamperes and the power tube requires about 15 milliamperes. The plate circuits of the r.f. tubes are filtered by the by-pass condensers Cn, Cis, and C19 and the resistors R5, R6, and R7. The by-pass condensers each have a capacity of 0.1 mfd. 6. GRID CIRCUITS Grid bias for the various tubes is obtained across the resistor R9. The C bias on the 226-type tube is approximately 6 volts, and approximately 28 volts is placed on the grid of the 171a power tube. There is no bias on the detector tube. 1 his C-bias resistor is by-passed by an 0.1-mfd. condenser, C20. 7. POWER SUPPLY The power supply is contained within the receiver cabinet. It consists of a power transformer, T, tapped for various line voltages and containing five secondary windings. Si, S2, and S3 supply £ lament current for the tubes in the receiver, S4 supplies filament current for the rectifier tube which is a type 280, and the secondary S5 supplies plate voltage. The filter system consists of the flter condenser C21, C22 and C23 and the choke coils L2 and Ls. The condenser C21 has a capacity of 2 mfd., C22 has a capacity of 3 mfd., and C23 has a capacity of 4 mfd. The by-pass condensers, C24 and C25, each have a capacity of 1 mfd. The power supply and filter system are designed for operation on 50 to 60-cycle a.c, but power equipment can be supplied for operation on 25 to 40-cycle power systems. The small condenser C26 is connected between one side of the a.c. line to a terminal cn the receiver marked "Loc" meaning local. For local reception satisfactory results can be obtained by connecting the "Loc" terminal to the "Ant " terminal and when this is done the power lines are used as an antenna. o c: c -J CD < 2nd A.F. C,4 Circuit diagram of receiver and power unit miHMwffliiiiMWiaM february, 1929 page 265 •