Block Diagram Of Deluxe Amateur Communication Receiver

employs a 6BQ7A cascode r-f amplifier and a 6J6 mixer-oscillator. All r-f tuned circuits are changed as the tuner is switched from channel to channel. The intermediate frequency output of the tuner is 1695 kc. A 6BA7 pentagrid mixer tube converts the first intermediate frequency to the second i-f channel, which is 455 kc. To obtain a high order of stability, the 6BA7 conversion oscillator is crystal controlled at 2150 kc.

Switch Si in the output circuit of the 6BA7 mixer stage permits the selection of one of the two mechanical filters. In this particular receiver, a 6 kc. and a 3.1 kc. filter were used for "broad" and "sharp" phone reception. Three stages of 455 kc. amplification follow the mechanical filter. The gain of these stages is adjustable, permitting the overall receiver gain to be set at a reasonable value. Too much i-f gain, will make the receiver noisy and subject to overloading effects on strong signals. Too little i-f gain will make the receiver seem insensitive. The i-f gain is controlled by potentiometer Ri and also by the value of the cathode resistors in the second and third i-f stages. In order to prevent clipping of the excellent flat-top response of the 6 kc. mechanical filter, small coupling capacitors are placed across the "top" of the i-f transformer coils to widen the "nose response" of the circuits. The "skirt" selectivity of the i-f strip is determined by the passband of the mechanical filter and not by the transformer response. To conserve filament power, 150 milliampere 6BJ6 tubes are employed in the i-f amplifier.

A 6AL5 dual diode is used as a second detector and a-v-c rectifier. Automatic volume control voltage is applied to the first two i-f stages, and a small portion of the a-v-c voltage is applied to the 6BQ7A r-f amplifier and 6BA7 mixer. The a-v-c circuit is rendered inoperative for c-w and SSB operation by segment D of switch S:. A simple delayed a-v-c system is used to allow best reception of weak signals- Potentiometer Ri sets the delay voltage applied to the cathode of the 6AL5 a-v-c diode.

A second 6AL5 serves as a noise limiter. The threshold value of limiting is set by control R;, and the noise limiter may be removed from the circuit by switch Sa. The audio output of the detector passes through volume control R< and is further amplified by one triode section of a 12AT7 followed by a 6AK6 power amplifier stage. The second section of the 12AT7 functions as the beat frequency oscillator. The frequency of the b-f-o is controlled from the panel by variable capacitor G.

The S-meter circuit consists of a single vacuum tube voltmeter using a 12AU7 dual triode in a bridge circuit. The bridge is balanced by potentiometer Rs, and the meter sensitivity is set by control Ri. Automatic volume control voltage applied to one grid of the 12AU7 upsets the balance of the bridge and causes a

"TV-TUNER"

"TV-TUNER"

Tuner Section Television

Figure 18A

CONVERTED TV-TUNER FORMS "FRONT-END" OF AMATEUR BAND RECEIVER

Several forms of replacement TV-tuners may be purchased from large radio stores. Shown above is typical "cascode tuner" to be used in this receiver. Check your tuner for circuitry before you modify it. I-f output frequency Is unimportant, since I-f components of tuner are removed. Tuner trimmers permit minor adjustments to be made to alignment of r-f stages. A—Oscillator-mixer excitation measuring point on tuner

CiA-B-C—15 ti/ifd. per section. All-Star Products Co., Defiance, Ohio. Model C3. C.— 70 fi/ifd. ceramic. Centralab 827B

Ci—15 ¡ill Bud LC-1641. {Mounted atop chassis after photographs were taken). Li-Li—See Figure 21 for coll data

Figure 18A

CONVERTED TV-TUNER FORMS "FRONT-END" OF AMATEUR BAND RECEIVER

Several forms of replacement TV-tuners may be purchased from large radio stores. Shown above is typical "cascode tuner" to be used in this receiver. Check your tuner for circuitry before you modify it. I-f output frequency Is unimportant, since I-f components of tuner are removed. Tuner trimmers permit minor adjustments to be made to alignment of r-f stages. A—Oscillator-mixer excitation measuring point on tuner

CiA-B-C—15 ti/ifd. per section. All-Star Products Co., Defiance, Ohio. Model C3. C.— 70 fi/ifd. ceramic. Centralab 827B

Ci—15 ¡ill Bud LC-1641. {Mounted atop chassis after photographs were taken). Li-Li—See Figure 21 for coll data

NOTES : 1 -* see filter instruction sheet for capacitor specifications. 2-all resistors 1-watt unless otherwise noted.

NOTES : 1 -* see filter instruction sheet for capacitor specifications. 2-all resistors 1-watt unless otherwise noted.

6ba6 Revceiver
Figure 18B

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