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Principles of operation

are neutralized to ensure stability. When rf driving voltage to the pa becomes great enough that positive peaks drive the pa grids positive, the grids begin to draw current and the signal is detected. This produces an audio envelope. The audio is rectified by ALC rectifier VI7A, which is connected to produce a negative dc voltage. The voltage is filtered by Cl59, Cl60, R118, and R1I9 (which also determine the ALC time constants) and is used to control the gain of V4A and V7. This system allows a high average level of modulation without driving the pa tubes well into the grid current region, which would result in increased distortion..

3.3 RECEIVER CIRCUITS

3.3.1 RF Circuits

Signal input from the antenna is connected through relay contacts to the tuned input circuit, T3. The signal is applied from T3 to the grid of the receiver-transmitter rf amplifier, V7. Amplified signal from V7 is applied from the tuned circuit, consisting of L10 and bandswitch selected capacitors, to the grid of the receiver first mixer V13B.

3.3.2 Receiver Mixers

The input rf signal is fed to the grid of V13B, and the high-frequency oscillator injection signal is fed to the cathode of V13B. The difference product of the first mixer is applied from the plate of the tube to variable if transformer T2. output of T2 in the range of 2.955 to 3.155 MHz is applied to the grid of second receiver mixer V17B, across Parallel-tuned - trap circuit Z5. This trap circuit minimizes a spurious response that would otherwise result from harmonics of the high-frequency crystal oscillator. When signal input is applied to the grid of VI7B and vfo injection signal is applied to the cathode of VI7B, the 455-kHz difference product is fed from V17B plate to rnechanical filter FLI.

3.3.3 IF Circuits

The output from FL.I is applied to the grid of first if amplifier V1B. Ihe if signal is amplified by V1B and V3B and applied through T5 to

   

AVC rectifier V15A and to the grid of product detector VI5B. Beat-frequency oscillator signal is applied to the cathode of VI5B, and product of mixing is the detected audio signal output of the AVC rectifier circuit is applied to the two receiver if amplifiers and through contacts of relay K4 to the receiver-transmit rf amplifier. This AVC voltage controls gain of the receiver and prevents overloading.

3.3.4 AF Circuits

Output from the product detector is applied through AF GAIN control R92 to the grid first af amplifier V16A. Amplified audio output of V16A is coupled to the grid of af output amplifier V16B, which produces the power to operate a speaker, headphones, or phone patch.

3.4 OSCILLATORS

The transceiver contains the tone oscillator the beat-frequency oscillator, the variable frequency oscillator, the high-frequency crystal oscillator, and the crystal calibrator.

3.4.1 Tone Oscillator

The tone oscillator operates when the EMISION switch is in LOCK, TUNE, or CW position. It is a phase-shift oscillator operating at approximately 1750 Hz. Its output is fed to ti transmitter audio circuits for CW operation. some of the output from the oscillator is applied to the receiver audio circuits for side tone monitoring in CW operation. Due to the 1750-Hz tone applied to the balanced modulator during CW operation, the actual transmitted CW signal will be 1750 Hz above the KWM-2 2A dial reading.

3.4.2 Beat-Frequency Oscillator

The bfo is crystal controlled at either 453.650 or 456.350 kHz, depending upon whether Yl6 or Y17 is selected by EMISSION switch section S9H. The unused crystal is shorted out by this switch section. These crystal frequencies are matched to the passband of mechanical filter FL1 so that the carrier frequency is placed approximately 20 dB down on the skirts of the filter response. This 20-dB carrier attenuation is in addition to the 30-dB suppression provided by the balanced modulator.

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