The Q of the IF output stage in the KISS transceiver is lowered by a 3.9-kohm resistor across the output tank circuit. Without this de-Qing measure, the stage tends to oscillate, causing instability that shows up as an inability to adjust the AGC loop pots.

The oscilloscope trace (right) shows 9 MHz signals both before and after amplification by the MC1350P amplifier chip. The waveform is clean, with no trace of instability at any setting of the 9-MHz tank circuit trimmer capacitor.

The schematics showing the IF amplifier and AGC subsystem also show the audio output stage.

Originally, the receiver IF strip was configured with no post-filter amplification. Subsequently, a single 2N2222 amplifier was added; it also serves as a transmitter-mixer pre-amplifier.

Click here to see the AGC circuit, as captured and simulated in Multi-Sim (apologies for poor image capture). The "spare" op-amp in the quad LM324 chip is used as a one-shot to reset the AGC when going from Transmit to Receive modes. This eliminates annoying popping in the audio output, as well as momentary pegging of the S meter.

Click here to view the full-wave output from the AGC circuit. Click to see the actual signals as shown on the oscilloscope.

These waveforms reflect the signals applied to the AGC's full-wave rectifier. During test, the AGC amplifier string is excited by an adjustable sinewave signal applied from an audio signal generator.

Initially, an LM386 device was chosen for the audio amplifier, but this was later revised to use an LM380. The LM380 provides greater audio output (over two watts into an 8-ohm load).

The photo (right) shows the IF amplifier under test. The balanced product detector transistors have yet to be installed on the board.

Click here to view a simulated waveform that shows the action of the AGC rectifier. The upper trace is the AGC recifier output with a constant 3-V 1-kHz applied sinewave. The lower trace shows the voltage at the AGC RC circuit.

The photo (right) shows the AGC and LM380 (an ECG740A equivalent is socketed) hardware under test prior to installation in the module.

Here's a photo of the IF board with the balanced detector transistors and output circuit installed.


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