Third IF Amplifier


AGC Amplifier

The third IF amplifier feeds the AGC detector. No coupling capacitor is used between this amplifier and the diode in the AGC detector circuit. See AGC Circuit for details on the AGC detector.

This stage has considerable gain because it is driven hard by the preceding two amplifiers. If you experience some oscillations in the IF strip, this amplifier is the one most likely at fault.

Normally, this stage is not a problem, and it is easy to tame. You can adjust S-meter responsiveness by adjusting the output transformer slug. Use a 2K to 12K resistor across the transformer primary to tame any oscillations. If you need more, lower the resistor value at Gate 1. In the worst of cases, a 100 ohm resistor at Gate 1 always tamed the circuit. In the receivers I have built, these resistors were never necessary, tuning the output slug did the trick.

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Last Update: 01/20/2000
Web Author: David White, WN5Y