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Section IV
AN-08-10-112 Paragraph 24-27

The crystal filter may he ssitched in or out of the circuit by the crystal “Out-In” switch I67 which is actuated from the front panel. The filter band width is adjusted hy the neutralizing capacitor. See figure 5. This calpacitor consists of two wires and two lugs extending from the neutralizing coil. See Figure 11. For adjustment see Paragraph 34e(10).

25. SECOND DETECTOR.

Tube V'T-233 also functions as the second detector. A relatively high level signal is supplied by the third i-f amplifier to the diodes of this tube. One diode functions as the signal linear detector while the other diode is capacity coupled and provides high level delayed a-v-c control bias.

26. OUTPUT.

.a. DESCRlPTION.—The high level signal diode supplies audio output for driving output Tube VT-152 without additional audio amplification. The design choice of three i-f amplifier stages and high level detection results in a number of operating advantages. The high level detection is relatively free from distortion due to avoiding the characteristic curvature at the lower end of the diode curve. The direct drive of the output tube from the diode detector simplifies the dynamotor ripple filtering and eliminates possible microphonics resulting from high audio amplification. The high diode level further provides relatively high bias

 

voltage insuring an unusually flax automatic volume control charateristic with the desirvd time delay. The dual volume control 110 comprises the 350,000-ohm audio control potentiometer and the 20,000-ohm bias control potentiometer. The audio volume control functions only with switch 169 in the a-v-c position and in this position it permits the desired adjustment of the audio level to the output tube and load. See Figure 6. The bias volume control also varies to some extent the r-f and i-f tube bias. For manual volume control with switch 169 in the m-v-c position the bias volume control becomes the active control operating on the cathode bias of the first and second r-f and the first and second i-f .Implifier tubes. These potentio-meters provide a smooth variation or sensitivity.

b. CONSTANT INTERNAL RECEIVER NOISE.—

The flat gain characteristics of the r-f and detector coils insures a uniform noise level over the bands. This is obtained by a combination of inductive and capacitive coupling in the coils.

27. DYNAMOTOR DM-28.(*).

The dynamotor and associated r-f filter circuits are assembled in one unit (Ref. 400) which supplies all of the high voltage direct current required for the operation of the receiver. In addition a maximum of 20 milliamperes at approximately 200 volts d-c is available at contacts 2 and 5 on the connector plug at the rear of the receiver for the operation of external accessory equipment.

Figure 5

Figure 5 Crystal Filter Circuits

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