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820D-2 I k\\ AM Trai»xmiUcr

The entirely new 820D-2 1-kW AM Transmitter offers many new and innovative features to improve AM performance and reliability. By utilizing effective cost-control methods. Collins is now able to offer a new transmitter, of superior design, at a lower price than ever before. In addition to a lower price. the new transmitter performs up to specifications that used to apply to FM broadcasting only.

An all new cabinet design places every component within easy reach for maintenance. The modulator and finał rf tubes are at shoulder height. making removal as trouble free as possible.

By using straightforward design concepts, Collins has been able to build a 1-kW AM transmitter that will operate many trouble-free hours, at a very Iow cost. In addition, maintenance costs have been reduced by the use of standard components and conservative ratings.

8201) 2 AM Traiisiniltcr

Exciter. The exciter for the 820D-2 AM Transmitter consists of a dual oscillator to develop the necessary input to the rf driver. A two-position switch enables the operator to select the oscillator that is to be used on the air. At any point in time, the other oscillator may be used, providing a ready standby in the event of failure. The frequency of both oscillators may be adjusted from the front panel. Since quartz crystals are most stable at frequencies above the broadcast band, Collins operates them in that rangę, and then divides with an integrated Circuit multivibrator to derive the station's frequency.

RF Driver. The rf driver is completely solid State, utilizing one 2N5039 transistor operating in Class C. To achieve the high gain that is necessary to drive the PA, the transistor Circuit employs a common emitter configuration, driving a matching network consisting of a tuned secondary rf transformer.

Power Amplifier. The power amplifier is designed to deliver 1100 watts nominał output into a 5012 load. Two long-life 5-500A pentodes are operated in parallel Class C, and are modulated in a conventional manner by a transformer-coupled modulator. Bridge neutralization is used to reduce rf intermodulation products. Power cutback to either 500 or 250 watts is possible by reducing piąte voltage. The power output of the 820D-2 is controlled automatically to within 2-1/2%.

coupled by an inductor. Nodes two and three are top coupled with an inductor which serves as the fixed adjustment for loading. Coupling circuits provide a 90° phase delay between nodes. Q distribution is such as to provide a symmetrical passband response for reduction of audio distortion at the higher modulation frequencies. Harmonie attenuation exceeds FCC requirements.

Audio Driver. Two push pull driver stages amplify audio to drive the modulator. The relatively Iow voltage required by the modulator eliminates the necessity of stepping up the audio signal by means of an interstage transformer. The finał stage of the audio driver is a regulated 290 Vdc, ensuring ample collector swing capability. Both driver stages operate Class A, common emitter, to achieve high gain.

820D 2 AM Transmitter

Output Network. The output network is a bandpass filter consisting of three nodes. The first node is tuned by a vacuum variable capacitor. Nodes one and two are bottom

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