QCX-series transceivers (QCX, QCX+ and QCX-mini) do not have built-in Automatic Gain Control (AGC).

This small 19 x 16mm ready-assembled AGC module is based on the proposal by Jim AJ8S in this thread on the QRP Labs groups.io forum:
https://groups.io/g/QRPLabs/topic/a_simple_qcx_agc/80821592

The shape of the AGC module is specifically designed to fit snugly into the QCX-mini, where it should sit on top of IC8 and IC9, a few mm above the main PCB. The perimeter of the AGC board is designed to fit between the surrounding through-hole components on the QCX-mini. On the QCX and QCX+ there is much more space available so installation position is less critical.

It’s not the most perfect performant AGC circuit in the world. But it works very well for a very small number of components and avoids any modifications to the QCX apart from making the four (or five) connections. The circuit provides a very useful degree of AGC and will protect you from sudden very loud signals.

Additionally a few additional components have been added such that a control signal can switch on/off the AGC. In QCX firmware version 1.08 and above, there is a menu item in the “Other” menu, which allows you to switch AGC On/Off. This requires a connection from LCD Pin D6 to the “AGC” control input of the AGC module. If you prefer to keep the AGC always on, there are a pair of pads on the PCB that you can short with a solder blob, a small wire, or an 0603 zero-ohm resistor.

The AGC module can be installed in ANY of the QCX-series transceivers. The installation manual includes instructions for each model and transceiver PCB revision. 

On a QCX+ Rev 4 (and above) PCB, there are 5 holes on the main QCX+ PCB that line up exactly with the 5 holes on the AGC module. Installation is therefore exceptionally easy using 5 component lead off-cuts to make the connection between the two boards, and no other changes. 

On a QCX-mini Rev 3 (and above) PCB, there are 5 holes on the main QCX-mini PCB that line up exactly with the 5 holes on the AGC module. Installation is therefore exceptionally easy using 5 component lead off-cuts to make the connection between the two boards, and no other changes.

On other QCX-series models, all of which essentially share the same schematic, the principles of circuit operation are the same, but you will need to install wires to make the connections. 

Documentation

The manual includes assembly instructions, operating instructions, design details, performance details

Manual (document version 1.01 published 15-Feb-2022)

Photographs

Varions Installation photographs