Radio today (Sept 1935-Dec 1936)

Centralab comes to the rescue with its New Selector Switch • When you fall asleep, dead to the world from fatigue, trying to dope out how in blazes you're going to get all those dizzy connections right on that new analyzer . . . take it easy . . . CENTRALAB'S new SELECTOR SWITCH KIT, with thousands of combinations will ALWAYS solve your problem. See your jobber today. SWITCHES FOR ANALYZERS I OUTPUT METERS • R F OSCILLATORS • A F OSCILLATORS • TUBE CHECKERS • DECADE BOXES CAPACITANCE t RESISTANCE • HAM EQUIPMENT • PA WORK • VOLT-OHM-AMMETER • RADIO RECEIVER REPLACEMENTS Centsalab Milwaukee, Wis. BRITISH CENTRALAB, Ltd. Canterbury Road, Kilburn London, N.W. 6, England FRENCH CENTRALAB CO. 118 Avenue Ledru-Rollin Paris XI, France Fixed Resistors Volume Controls Wave Change Switches Sound Projection Controls SERVICING NEW CIRCUITS {From page 42) During the interval of heating up tube filaments or cathodes, the large condensers in the power pack are charged to peak voltage of the transformer secondary, and since there is no current drain, there is no voltage drop in the filter and the speaker field. Hence the same peak voltage is applied to all the smaller condensers, such as tubulars, as well as other equipment. These peak voltages are apt to raise havoc with small condensers not intended for such surges. Overload protection Aerovox engineers have developed wet electrolytics for given regulation voltage. Properly applied, these units The regulating wet electrolytic effectively keeps the voltage from rising above the regulating voltage. prevent voltage rise above a given point, and consequently protect other condensers and components. It is highly important that such regulating condensers be connected across the output of the filter, and not immediately after the rectifier. In the latter position the regulating condenser would cause an enormous peak current at the start, which would have to pass through the rectifier. The regulator condenser can be placed after the first choke or after the second one, if there are that many. KADETTE 87 SERIES * Inspection of circuit diagram shows the model 87 to be a 7-tube superhet covering the broadcast and short wave bands. Set has a 448 kc. intermediate frequency. Antenna wavetrap is adjusted also at this frequency for minimum response. Broadcast band is aligned at 1,400 kc. Alignment of oscillator conden ser at 600 kc, if necessary, is done by bending condenser plates. The three trimmers on the bottom of chassis are, reading from the end of the chassis toward the center, BC oscillator, SW detector, SW oscillator. There is no trimmer for BC detector. SW oscillator alignment is done at 15.5 megacycles — SW detector at 12 megacycles. WESTINGHOUSE MODEL WR-311 • Model WR-311 is a 6-tube allwave superhet. Schematic diagram is shown on opposite page with chassis layout and alignment frequencies. Coil resistances in ohms are placed adjacent to coils. Note the new method of biasing the tubes — cathodes are grounded. In the I.F. and R.F. circuits a bias is obtained through one section of the 6H6 tube. The cathode is connected to the center tap of the power transformer and is negative with respect to the chassis or amplifier cathodes. The plate is connected to the AVC or grid return circuit — and a current flows through the diode load circuit to ground. The current through the diode load produces a negative bias on the amplifier tubes. This keeps a negative bias on the tubes even when no signal is tuned in. After a station is tuned in, the diode detector produces a bias for the AVC control — this is greater than the no signal bias, and since it is more negative, current ceases to flow through the other section of the diode and the set performs like any other set with AVC. Coupled to the wave-change switch is a switch that alters the frequency response of the audio system — it is shown in the diagram just below the diode detector and load resistance. Note antenna wave trap is inductively coupled to the antenna coils. It is adjusted for minimum output when an I.F. signal is fed into the antenna of the set. PERSONAL APPEARANCE * That first impressions are often the most lasting ones should be remembered by all servicemen, particularly in this day of "gyp" artists. A clean, neat, efficient appearance and courteous greeting will break down more sales resistance than a line bathed in honey and oil. A (To page 47) 44 Radio Today