Radio and television today (Jan-Nov 1941)

RE-SETTING PUSH BUTTONS {Continued from page 41) board" mechanism is a combination of electrical and mechanical units. To set-up the buttons, see Fig. 5 and proceed as follows. 1. Remove the tuning knob by pulling it out from the panel and pull out the set-up knob on the same shaft as far as it will go. Rock the knob slightly to engage the gears. Turn the knob clockwise as far as it will go. During this operation, the dial pointer will move to the right and stop. The set-up knob is rotated about % of a turn beyond this point. 2. Push the button that is to be reset, being sure it is all the way in. Using the set-up knob, manually tune the desired station for best reception. This button is set as soon as another button is pressed. Proceed to set the remaining buttons by pressing each down and tuning the desired station. 3. To release the last button pressed, push the set-up knob on the tuning shaft back into the cabinet as far as it will go and then pull it out again, being sure to rock the knob to mesh the gears. 4. To lock the mechanism, turn the set-up knob counter clockwise until a definite stop is reached after the dial pointer moves to the left end of the scale. The set-up knob is pushed back in and the tuning knob replaced. This completes the set-up procedure. KEY AND DISC ELECTRIC TYPE The electric tuning mechanisms which travel to one end of the dial scale and then return to the desired station are usually arranged like the system of Fig. 6. Set up as follows. 1. Warm-up the set, put the range selector on standard broadcast, and the control knob to "Electric". 2. Press the first button to be set, and wait until the pointer stops. 3. Turn the control knob to "Manual". 4. Put the set-up pin into the adjustment strip above the selector disc corresponding to the particular push-button. Be sure pin is well SWITCH ON CONDENSER GANG REVERSES MOTOR UPON REACHING! EITHER EXTREMITY down into the notch. 5. Tune the receiver carefully to the desired station. 6. Remove the set-up pin. 7. Turn control to "Electric". This button is now properly adjusted and the others may be set-up by holding the correct disc with the pin while tuning the station manually. The newer types of inductance and capacity trimming units for push-button receivers are simple to re-adjust. The mica trimmers or iron cores are adjusted for best reception on the particular station within their tuning range. The oscillator trimmer is adjusted first, and then the detector and RF trimmers are peaked. Tube Failures in AC/DC Battery Portables Burned-out filaments in the tubes used with portables are one of the chief complaints in these sets. The ACj/DC, battery type in particular give trouble because of the usual surges in voltages common with half-wave rectifiers. The four, five, or six tubes are connected in series with a voltage dropping resistor across the output of the rectifier. A filter capacitor of 50 to 100 mfd. is connected across the series group of filaments. If one of the tube filaments burns out, the filter condenser charges to approximately 150 volts. If a good tube is then installed, the 150 volts across 4 or 5 1.5 volt tubes will quickly burn-out one or more filaments. Even of the line power is off, the condenser may have sufficient charge to blow the tubes. CONDENSER FAILURE In some circuits, the high capacity condenser is of 10 or 25 volt rating in which case it will fail if one of the tubes burns out and allows the full rectified voltage to build up across it. If this condenser is out, it should be replaced with one capable of withstanding the full rectified voltage. Since the power tube is located at the positive end of the string to obtain bias, the plate and screen current from this tube would have to flow back through the filaments of the other tubes if a bleeder resistor to ground were not used. 1000 ohms is the usual value. The best type of circuit will have a bleeder resistor across the high capacity condenser which will prevent a tube from being blown due to a high charge on the condenser. New tubes put in while the set is turned on the line power may not be protected however. In any of the combination battery portables, the filament circuit should be studied so that you will know exactly what happens when tubes are removed or burned out. The set should be disconnected from the line and the filter capacitor shorted before any tubes are replaced. X REVERSING SWITCH MOTOR -GLAK ON CONDENSER GANG Push-button Tone Control on Zenith Models The Radiorgan tone control on many Zenith models is shown in the accompanying diagram. A 6J5G is used as a diode second detector. A separate triode is used as a audio driver. The six push-buttons give 65 different combinations of tone. Closing switch A gives low bass. Switch B is "treble," C is for "voice," D closes is "normal," E is "alto," and F for "bass." The switches control the shunting across the volume control and the feedback from the plate circuit of the audio driver. KEY USED FOR -ADJUSTING OISC1 TO STATION I KEY ADJUSTMENT SLOT pX PUSH BUTTON ! STATION SELECTOR MOTOR CIRCUIT COMPLETED AT ALL TIMES EXCEPT WHEN INSULATED SEGMENT IS AT BOTTOM. INSULATED SEGMENT UNIT ENCLOSED IN DOTTED LINES IS DUPLICATED 8 TIMES 8 UNITS IN PARALLEL FOR 8 STATIONS Fig. 6 — Notched disc electric type has separate disc for each push-button. To set-up, push button and wait until motor stops; drop key into notch, tune station manually, remove key from notch, continue with other buttons. RCA Model 15BP Changes Better reproduction of the high frequencies will result if the RF by-pass capacitor across the first AF output is changed from 390 mmfd. to 100 mmfd. Cases of hum in this model can be reduced by putting a shield around the 1H5GT and securely grounding, or by adding a filter network to the plate circuit of that tube. Add a 100M ohm resistor in series with 1H5GT plate supply and by-pass the plate end of the new resistor to ground through 0.1 mfd. Dial cord slippage may be eliminated by taking an extra turn around the drive shaft without lengthening the cord. 42 RADIO TODAY