Radio and Television Today (Jan-Nov 1941)

RADIO TODAY'S SIGNAL-PATH DIAGRAM RCA Model 55X Service Notes The AC-DC super shown in the accompanying Signal-Path circuit is one of the 1942 table models. A special feature of this circuit is the twin speakers. Two PM or one PM and one EM speakers are connected in parallel across the output transformer. The converter uses a Hartley connected oscillator and electron coupling. The tuning gang is thus insulated from the receiver chassis and has the AVC voltage on the rotor. ALIGNMENT The alignment procedure is as follows. 1. Connect the high side of the test oscillator to the IF grid through a 0.01 mfd. capacitor. Tune the set to a quiet point around 1600 kc, and the oscillator to 455 kc. Connect output meter across voice coil, or electronic voltmeter to the AVC bus and turn the volume control on full. Adjust the secondary and primary trimmers of the output IF transformer for maximum output. 2. Re-connect the signal generator to the first detector grid through the 0.01, and adjust the secondary and primary of the first IF transformer for maximum output. 3. Connect a 1720 kc. signal through a 100 mmfd. capacitor to the antenna terminal and turn the tuning gang to minimum. Adjust the oscillator trimmer for maximum output. 4. Radiate a 1300 kc. sienal with a few turns of wire in a 12-inch diameter loop witli This is the code diagram to be used with all Radio Today Signal-Path diagrams. See example below. the set tuned to that frequency. Adjust the antenna trimmer on the gang condenser for peak output. SPEAKER PHASING The two speakers used with this set must be phased if the connections are changed for any reason. A test must be made to determine if the two speaker cones move in the same direction when excited with the same signal. Several tests are common and all about equally convenient. A 1%-volt dry-cell may be connected across the secondary of the output transformer and the cones watched for direction of movement when connection to the battery is made. Reverse the voice coil leads of either speaker if they do not both move in the same direction. Your audio signal generator can also be used to check the phasing of two or more speakers. Feed the lowest possible frequency into the audio system and observe the movement of the cones. It may be easier to feel the cone movement than to observe it. Zenith Conso/-Tone Circuit Uses Feedback The audio system used in several of the 1942 Zenith table models is identified with the term, Consol-Tone. The basis of the circuit is a feedback network which gives a regeneration of the bass end of the audio spectrum. The typical circuit shown is for chassis 6B01 used in models 6D614, -614W, and -629. A portion of the audio voltage in the plate circuit of the 35L6 output tube is coupled back to the cathode and suppressor grid of the 14A7 audio amplifier, which is biased by an un-bypassed cathode resistor. By checking through the circuit it will be noticed that the voltage fed back to the cathode is injihase with the grid voltase of the 14A7 and therefore, 180° out of phase with the voltage developed across the cathode resistor by the AC component of tlie 14A7 plate current. The network of resistors and large value condensers in the feedback circuit cut out the high frequencies and allow only the lows to get back. The low frequency voltages across the cathode (Continued on paye 38) ANTENNA TO GRID. 50 X 600 KC. IS X 600 TO 455 KC. 200 X 455 KC. 40 X 1 14 X 400 ^-^ I2S07 , pup .006 1 1 400 <~~> i 50L6GT 35Z5GT 20 NWW^ 1200 * 4r ± \-l3V. MAX. AVC. -n CHASSIS. COMMON WIRING. 3 VOLTS BIAS FOR GAIN TESTS. CONNECTIONS FOR l-PM AND l-EM SPEAKER