All orders since 2021-06-14 are REV B kits. REV B is mostly identical to REV A, with the exception of the wiring harness. REV A + B instructions are found below.

This kit is based around the circuit proposed by Jim AJ8S on the QRP-Labs discussion forum: thread here. Note: Electron Volt Limited has no affiliation with QRP-Labs, we just own a few QCX radios and hoped we could help make this mod more accessible.

The circuit has been miniaturised into the smallest size reasonably achievable (at low cost and low volume production) to make it as easy as possible to fit inside the QCX mini. Other than switching to SMD devices, the only change is the addition of a fixed 1.8kOhm resistor between the 5V input and the trimmer resistor. This limits the maximum voltage possible at the tantalum capacitors and makes the range of the trimmer slightly more useful.

The AGC kit consists of: AGC PCB REV B (fully assembled and tested), custom 20cm wiring harness with pre-fitted connector, heatshrink tubing, double sided tape, and an anti-static bag. You will need general soldering equipment and tools, plus electrical insulation tape.

This kit should be suitable for all three variants of QCX: original, plus, and mini. We do not own an original QCX so are unable to test it but the relevant part of the audio circuit is the same. To fit, simply connect to 5V, GND, audio output, and the wiper of the audio potentiometer.

Since the AGC PCB measures just 27*8mm, the PCBs are manufactured as a panel. This means that you might want to gently file the edges where the PCBs were connected together. Practically there is no need to do this as there is adequate room but in case you are particularly picky about a clean edge, you have been warned. Also note that the PVC wire insulation may shrink very slightly when soldered but this can be easily compensated for.

Note: paper manuals are not provided, PDF documents are available here:

REV A (Soldered Wires)
REV B (Pre-made Wire Harness)

Circuit Schematic

Fitting Instructions (QCX Mini) r1.2

Fitting Instructions (QCX+) r1

Circuit Schematic

Fitting Instructions (QCX Mini) r1

Fitting Instructions (QCX+) r1

Fitting Instructions (QCX Original) r1

See also Lex PH2LB's blog suggestions here.

Please have a look in the forum subcategory for help and support, please make a new topic if you cannot find what you are looking for.

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QCX Auto Gain Control Kit

  • Brand: Electron Volt®
  • Product Code: EV508000014
  • Availability: Contact Us
  • £4.80

  • Ex VAT: £4.00