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Principles of operation

3.4.3 Variable frequency Oscillator

The vfo uses fixed capacitance and variable inductance to tune the range of 2.5 to 2.7 MHz The series combination of capacitor C308 and diode CR301 is connected in parallel with Capacitor C303. The diode switches C308 into or out of the circuit, depending upon the polarity of a bias voltage Impressed across the diode junction. When USB emission is selected, the bias is positive and C308 is switched into the circuit. Ihe capacitor then is adjusted to shift the vfo frequency by an amount equal to the frequency separation of bfo crystals Y16 and Y17. This allows the selection of either sideband Without upsetting tuning or dial calibration.

3.4.4 High Frequency Crystal Oscillator

The high frequency crystal oscillator V13A, is crystal controlled by 1 of 14 crystals selected by BAND switch S2. Output from the high frequency crystal oscillator is fed to the transmitter second mixer and to the crystal oscillator cathode follower. The cathode follower provides isolation and impedance match between the crystal oscillator and the receiver first mixer cathode. The output frequency of this oscillator is always 3.155 MHz higher than the lower edge of the desired band. This high frequency injection signal is the crystal fundamental frequency for all desired signals below l2 MHz. For operating frequencies higher than 12 MHz the crystal frequency is doubled in the circuit of the oscillator. Instructions for calculating crystal frequencies for the desired Bands are given in section 2.

3.4.5.Crystal Calibrator

The l00-KHz crystal calibrator V12A, is the pentode section of a type 6U8A tube. Its output Is coupled to


antenna coil T3 the calibrator may be trimmed to zero beat with WWV by adjustment of capacitor C76.


Audio output voltage from the second microphone amplifier V1B is coupled to VOX GAIN Control R39 A portion of this voltage is Amplified by VOX amplifier V14B and fed to the VOX rectifier, which is one of the diodes of V14. The positive dc output of the VOX rectifier is applied to the grid of VOX relay amplifier V4B, causing it to conduct current and actuate VOX relay K2. Contacts of K2 switch the receiver antenna lead, the other relay coils, and bias voltage. Relays K3 and K4 switch the metering circuits from receive to transmit, the low plate voltages from receive to transmit tubes, and the AVC and ALC leads.

The anti VOX circuit provides n threshold voltage to prevent loudspeaker output (picked up bv the microphone circuits) from tripping the KWM-2/2A into transmit function. Some of the receiver output audio voltage is connected through C235 to ANTI-VOX GAIN control It45. Signal from the slider of this potentiometer is rectified by the anti VOX rectifier, which is the other diode of V14. Negative dc output voltage from the anti VOX rectifier, connected to the grid of V4B, provides the necessary anti V()X threshold. ANTI-V()X GAIN control R45 adjusts the value of the anti VOX voltage threshold so that loudspeaker output will not produce enough positive dc output from the VOX rectifier to exceed the negative dc output from the anti VOX rectifier and cause V413 to actuate However, speech energy into the microphone will cause the positive V()X voltage to overcome the negative anti VOX voltage and produce the desired action of K2

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