Radio Broadcast (Nov 1926-Apr 1927)

472 RADIO BROADCAST MARCH, 1927 Power. Compact FIG. 4 terminal on the filament side of the Thordarson Power Compact to the minus-C binding post (the fifth binding post from the rear of the baseboard). Then connect the twisted leads from the filament terminals of the power tube to the two terminals marked " F" on the power compact, connecting one wire, of course, to each terminal. Inasmuch as the filament of this tube is heated by alternating current, we do not have to worry about polarity. The connections are completed by inserting the leads from the cone speaker in the Imp jacks and then placing the clips of the trickle charger on the proper terminals of the storage battery — the red lead going to the positive terminal. The set is now ready for test. Insert the cables from the power compact and trickle charger in their respective sockets of the automatic relay. Connect the relay cable to the a. c. source. Before turning the combination rheostat and ■switch to light the tubes in the set, make sure that the controls of the Clarostats are screwed all the way out, by turning them in a counterclockwise direction. Then turn the rheostat to the "on" position and adjust the Clarostats. Caution! Never cut out all the resistance of these units by turning the controls as far as they will go in a clockwise .direction, as you are apt to paralyze the tubes. Adjust the Clarostats on a weak signal for maximum volume and clarity. A milliammeter, with a scale of o to 50 mA., will be found valuable in achieving quality of reproduction. This measuring instrument is to the radio man what the stethoscope is to the physician; it enables us to see the functions of certain parts not otherwise visible to the eye. Temporarily connect the milliammeter in the minus-B lead from the power unit to the receiver, remembering that the plus terminal of the meter connects to the receiver. Tune-in a powerful local signal and observe the deflections of the indicator needle. The needle will flucutuate No'eS! Receiver with the music: the ,ouder the signal, the greater the deflection. The normal movement of the needle is slightly downward toward the lower end of the scale as loud signals are received. If the needle swings violently upward it is evidence of too high grid bias, and distortion is being introduced which will mar reproduction. Insufficient grid bias will cause excessive downward fluctuation of the needle. To increase the bias, use a lower value of resistor Power Tube Filament than the 2000-ohm size specified. Increasing the value of the resistor decreases the bias. If any difficulty is experienced in equalizing the radio-frequency stages, the procedure can be simplified by the addition of two Sangamo Series A 0.25-mfd. bypass condensers in the following manner: Remove the right-hand screw holding the detector circuit tuning condensers to shield section 1 C. Place a spacing washer under one terminal of the bypass condenser, and fasten it to the shield by replacing the condenser mounting screw. This screw provides one connection to the bypass condenser as the shield is grounded to the negative part of the A battery. Connect the remaining terminal of the bypass condenser to the B plus terminal of the second radio-frequency auto-couple coil. The other bypass condenser is mounted by making a small hole in shield section 2C directly back of the detector-tube socket. This condenser is wired in the circuit in the same manner as the previous one. To 2nd. A F -C hindng post No. b To filament terminal of 171 tube socket To Lamp Socket FIG. 5