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Conar Coletronics Service Inc. Training Kit 3R Передатчик
Training Kit 3R
25 Watt CW Transmitter


Circuit Description Your Transmitter uses a high perveance beam tetrode in a grid-plate power Oscillator circuit. Output power for the three bands is taken from the plate circuit and coupled to the Antenna by a pi-network tank circuit. A dual-silicon rectifier, full-wave voltage doubler power supply, furnishes approximately 320V dc for the plate of the 6DQ6B. A combination voltage divider-bleeder resistor furnishes lower voltage for the screen grid of the tube at the same time it acts as a fixed load on the Power Supply to improve voltage regulation and discharge the filter capacitors when the Transmitter is turned off. On 80 meters, fundamental frequency crystals (3.7 mc-3.75 me) must be used. On 40 meters, either fundamental crystals (7.15 mc-7.2 me) or 80-meter crystals (3.575 mc-3.6 me) may be used. Using 80-meter crystals on 40 meters will produce less output power than 40-meter crystals because the Oscillator must operate as a frequency doubler in the plate circuit, a less efficient manner of operation than operating at the fundamental frequency. On 15 meters it is necessary to operate the plate circuit as a frequency multiplier since fundamental frequency crystals in the 21.1 mc-21.25 me range are very expensive in addition to being hard to find. Therefore, crystals in the range of 10.55 me to 10.625 me are recommended for 15-meter operation. As an alternative, 40-meter crystals in the range of 7.034 me to 7.083 me may be used. The plate circuit must be tuned to the third harmonic of the crystal frequency when 40-meter crystals are used for 15-meter operation, a less desirable method of operation than using crystals in the 10.55 me range, since frequency tripling results in lower efficiency and less output power. Table 1 summarizes the crystal selection for the three bands. To provide stabilization of the crystal frequency and to protect the crystal from excessive crystal current, a positive temperature coefficient resistor in the form of a pilot lamp is used in the crystal circuit This variable resistance lamp virtually eliminates any chirp frequency shift of the output signal when the Transmitter is keyed. At the same time it protects the crystal from excessive currents if the Transmitter should happen to be operated out of resonance accidentally. The large out-of-resonance crystal currents, which could fracture the crystal, will burn out the lamp before they build up to a level sufficient to damage the crystal. A cathode bias resistor is used in the Transmitter to limit the plate current to a safe level as the plate tank is tuned. If the Transmitter did not have this protection, the plate current would drive the plate current meter off scale and possibly damage the tube. The pi-network tank circuit provides a good load match into unbalanced resistive Antenna loads of 50 to 600 ohms impedance, the most widely used Antenna impedances. In addition, the large output capacitor helps suppress harmonics from the Transmitter. Adequate harmonic suppression is required by the FCC so that you will not interfere with other radio services by radiating harmonic signals.

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06 Август 2017

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