Re: Oscillation with Hycas AGC Amplifier

Richie Chambless

I thought it was the LM386, so I tried two other amps but got the same result. I swapped the Hycas with another IF amp (around 20 db gain) and it worked fine. So, I know the problem is in the Hycas. My next step was to terminate the input and output with 50 ohms and test the AGC voltage. Sure enough, it was only 1.5 volts, meaning something was oscillating internally and driving the control voltage low. I then changed R4 and R8 to 100 ohms, no luck. But when I held a 4.7k resistor across R2, the oscillation stopped! While holding this resistor in place, I put the volt meter on the AGC pin. It had around 5 volts the reduced with incoming signals. So, I soldered the 4.7k resistor in place on top of R2, but the oscillation came back! It's something about my hand capacitance that calms the instability. I think I may experiment with higher R4 and R8 values, along with a transformer impedance match in lieu of the L match. Maybe something will work!

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