Alignment Fm

For FM alignment a frequency modulated generator (60 to 400 cycle modulation, 400 kc sweep) and an oscilloscope are required. Connect the modulation soiree on the signal generator into the horizontal amplifier of the oscilloscope. It may be necessary to connect a phase shifting network in this line between the signal generator modulating source and the oscilloscope horizontal amplifier in order to get the correct pattern on the oscilloscope. Connect the tuner output to the vertical amplifier input of the oscilloscope.

Introduce a 10.7 mc (400 kc sweep) signal into the grid of the first 6AU6 limiter tube (Pin # 1) through a .01 ufd. condenser. Make the ground connection of the generator to the center post of the 6AU6 socket with as short a lead as possible. Remove the last 6AG5 IF amplifier tube to avoid the possibility of stray signals coming through the IF system and confusing the discriminator alignment procedure. Adjust the signal generator sweep and signal amplitude, and the oscilloscope for a pattern like the discriminator pattern shown in Fig. 3. Adjust the top adjustment on the 04194 discriminator coil for maximum vertical amplitude on the oscilloscope pattern and adjust the bottom adjustment on this coil for best symmetry of the pattern about the center. Repeat these two adjustments until no further improvement can be made. This completes the adjustment of the discriminator coil. Replace IF tube.

For alignment of the IF amplifier the same oscilloscope setup is retained except the input to the vertical amplifier. Feed the input to the vertical amplifier with audio taken from the first limiter grid return. This point is identified as point X on the circuit diagram of Fig. 3. Connection should be made to this point through a 1 megohm isolating resistor as shown by dotted line in Fig. 3.

Introduce a 10.7 mc (400 kc sweep) signal into the signal grid of the 6BE6 (Pin # 7) through a .01 ufd. condenser. Make the ground connection of the signal generator to the center post of the 6BE6 socket with as short a lead as possible. Adjust signal generator and oscilloscope to obtain a pattern like the IF pattern shown in Fig. 3. Adjust top and bottom adjustments on the four 04193 IF coils for maximum amplitude and symmetry of the pattern, keeping the signal level from the generator as low as possible throughout the adjustment. If the pattern tends to become double peaked or badly unsymmetrical during adjustment, the trouble is probably due to incorrect placement of some of the connecting leads in the test setup. Corrections should be made to eliminate the trouble and the adjustments repeated.

For the high frequency adjustments the same oscilloscope setup may be retained. In connecting the signal generator to the antenna terminals, the signal generator is not connected to chassis as in the previous connections. Connect two 150 ohm resistors to the two antenna terminals on the tuner. Connect the other ends of these two resistors to the two generator output terminals. If the setting of the dial pointer has been previously checked during AM alignment, it is not necessary to recheck it at this point. Set the signal generator and the tuner to 106 mc and adjust the oscillator trimmer (identified in Fig. 1) to bring the pattern to center on the oscilloscope. In case this is possible with two different positions of the oscillator trimmer, use the position of least capacity. Adjust the RF and Antenna trimmers (identified in Fig. 1) for the greatest amplitude of the pattern keeping the generator output as low as possible during the process of adjustment.


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