The AGC in the Atlas series is produced by rectification and doubling of the audio signal. This system works slower than an equivalent AGC rectified from a HF or IF signal.

Better is a double phase rectification.

[My designs have the advantage that there is nothing to be changed on a board, since the additional components can be mounted at (fig») the tracked side.]

The opamp works for the low frequency signal as a phase shifter of 180 °. Two additional 1N4148 diodes and D310, D302 act as a bridge rectifier. A diode to the non-inverting input is needed to get the same DC on both inputs of the opamp.

Preferably the diodes should be driven by a source with low impedance.

In the first design, only two diodes are driven with low impedance. Therefore the idea arose to add an inverting buffer and use a 8 pin DIL with 2 opamp.

Both AGC modifications improve the handling of strong signals. The receiver is calmer with less distortion because the AGC source is now loaded with higher impedance.

Note resistor R315 (PC-300C) or R309 (PC-300D) do not have the same value and numbering in both designs.

Atlas has never been consistent with the numbering of components at various changes to their schematics. The modifications were not always updated in their OPRERATION INSTALLATION AND MAINTENANCE. It happens that a PCB does not correspond to the publicised schematic. Even two of my PC-300C PCBs have differences in their schematic design.