Radio Broadcast (May 1929-Apr 1930)

RADIO BROADCAST. " Constructional details other than those suggested in the pictures, and the diagram are not really necessary. "The regular circuit tests are made with a test cable-plug made from the X base of an old '99-type tube and a cable with phone tips on the other end. The cable used should have leads sufficiently heavy so that there will not be an appreciable voltage drop when testing a.c. tubes. The phone tips are attached to the four tip jacks on the right of the panel, starting with the grid on the left, the filament leads in the middle, and the plate on the right. The tube base is plugged into the socket that it is desired to test, using a suitable adapter if necessary, and the tube is placed in the tester. The two switches below the a.c. voltmeter and the rheostat should be in the off position. "Placing a shorted phone plug into the phone jack on the extreme left of the panel will cause the d.c. voltmeter to read the grid circuit. The 500-volt range should be used when testing the grid circuit of a power tube. This test will not give a true voltage reading because of the resistance in the grid lead and will indicate continuity only. The next jack will read the plate voltage on a 500-volt scale. If the set is d.c. operated the third jack will read the filament voltage on a 50-volt scale. Pushing the button on the left below the a.c. voltmeter will short the 50-volt resistance and the voltage can be read on the 5-volt scale. If the polarity is reversed it can be changed by turning the switch above the button for the 5-volt range. The fourth jack reads the plate current. The switch below the plate-current jack is for the shunts for the milliameter, 1 being for the 8 mA. scale, 2 for 25 mA., and 3 for 100 mA. If the set is a.c. operated, the filament voltage is tested by turning the a.c. switch, being the one to the right below the a.c. meter, to the 'on' position, making sure that the rheostat is in the 'off' position. " If it is desired to use the meters independently the tip jack to the extreme left is the plus lead. The second tip jack is for the milliameter lead. The switch below the plate current jack controls the milliameter shunts. "The third tip jack is for the d.c. voltmeter. The switch to the extreme left and above the plus lead controls the voltmeter resistances, 1 for 5 volts, 2 for 50 volts, and 3 for 500 volts. The fourth tip jack is for the 3-0-3 ammeter in the upper right corner of the panel. This meter is used for testing chargers, etc., itbeing connected so that if the plus lead is connected to the plus of the battery the meter will read discharge when the set is turned on and charge when the charger is turned on. The ammeter is used whenever there is any question about the polarity of the charger or its charging ability. The two middle tip jacks of the four on the right of the panel are for the a.c. meter. The a.c. switch should be on and the rheostat off. "A toy transformer with adjustable voltages is used for filament supply. The one used gave from 2.5 to 25 volts in steps Front panel view of the home-made test set described by Mr. Taylor. of 2.5 volts. The plate voltage is supplied by a tapped resistor across the 110-volt leads. The switch below the socket controls the filament voltages, 1 for 2.5 volts, 2 for 5 volts, 3 for 7.5 volts, and 4 for 10 volts. The rheostat is connected in the circuit to give closer regulation of voltage. 110 V o Tube Test Complete iviring diagram of Mr. Taylor's test set. The Beynon Radio Shop insure the neat appearance of their servicemen by providing the coats shown above. "The testing of a tube is accomplished by attaching the cord to a convenient light socket and placing the tube in the socket. The a.c. switch should be in the 'on' position and the filament voltage adjusted by means of the voltage switch and the rheostat. The phone plug should be placed in the plate current jack. The meter should now read about 1.6 mA. for an a tube. Pushing the button below the a.c. switch will short the grid resistance, and the plate current should now be about 5 mA. The voltage on the plate is about 80 volts. It is also possible to test 280 and 281-type tubes in the same way. " A coil is connected in the grid and plate circuits in the proper relation so that the tube will oscillate when making the tube test. It may be necessary to reverse the plate voltage leads to make the tube oscillate. The desired frequency is obtained with a 0.0005-mfd. variable condenser that can be adjusted with a screwdriver. Theoscillator is tuned to about the middle of the broadcast band and may be used in place of a broadcasting station whenever needed for set adjustment. The condenser is mounted inside the case and may be varied if necessary with a screwdriver through a small hole in the back. "The rejuvenation of tubes is accomplished by placing the tube to be rejuvenated in the socket and the adjustment of the filament voltage to the baking value, 4 volts being correct for the '99 and '20type tubes, and 7 volts for the A-type tubes. The phone plug is placed in the phone jack nearest the socket which disconnects the voltmeter and applies the flashing potential of 12 volts for the '99and '20-type tubes. The other jack disconnects the voltmeter and applies the flash voltage of 17.5 for the A-type tubes. "The fine voltage may be checked by turning the voltage switch to the 5-volt tap and reading the meter. By testing the line voltage at various times a reading can be obtained which will represent 110 volts. Any variation above or below will indicate a similar change in line voltage proportional to the voltage being read on the meter. "A duplicate phone plug was made from a piece of \" brass rod threaded on one end to take a rubber binding post top and the opposite end turned to the shape of a phone plug. To facilitate construction the wiring diagram is drawn as the instruments appear from the back of the panel, but the operating instructions are given as the instruments appear from the front. 3 16 • • OCTOBER 1929 •