QRP-Labs QCX-mini 40m CW


This page is work in progress and may be updated frequently (last update : 2021-05-11 20:23 UTC). 

Page content : 



Loving the kits from QRP-Labs, and when Hans Summer announced his new QCX-mini CW tranceiver I knew I wanted to buy and build one. And when he added it to his webshop I didn't hesitated for a moment and ordered the 40m kit with a few other nice QRP-Labs kits (when your ordering and have to pay shipment, order more). 

This little kit with a stock of 1000 was sold out in under 4 days. Showing that hamradio kitbuilding is here to stay.  
So after a litte more then a week the parcel service dropped of a package from QRP-Labs. 

Content :


The manual

As with every kit which Hans sells, the also was a very nice manual for the QCX-mini (link).  The first 2 chapters contain a Introduction with technical specification and the full Parts list of all components in the kit. Chapter 3 contains a list building general guidelines, images of the PCB layouts (track and component placement) and photo's of a finished QCX+ allowing to kit builder get a idea about what to expect. And after sub chapter 3.1 Inventory parts helping the starting kit builder to identify parts, the fun starts.

Let start building 

In the QCX-mini manual there is big difference against the QCX manual, which that Hans decided it is best to start with "Wind and install transformer T1". A detailed description about the transformer in component placement, schematic, a drawing and a photo helps the (starting) kit builder wind the transformer.   


Our cat Moppie gave me a hard time winding the T1 coil. The dancing winding wire got her interest and she got it a few times.  Pulling the coil almost out of my hand. 

But after a while so had enough of her paw being tangled in the wire so she went QSY to her designated spot in the shack. 


For removing the coating of the wires I hold the wire down on a piece of stainless steel and use  a scalpel to scrape it off. After that I tinning them.

Note : How I do this and which tools I use, I explained in this Tools and parts page.

When I install toroidal coils with more then 2 wires, I use tweezers to guide the wires one by one to the correct hole. Twist them to pairs and solder them to the PCB which is when you have tinned them a just few seconds job (instead of burning the coating a way while installing them). 


When installing the socket for the micro controller, don't forget to push it up before soldering so there is enough room for the TCXO or crystal.


Adding all the small Multi-layer Ceramic capacitors which is much less work then on the QCX or QCX+ due to the factory installed SMD components.  


I bought the TCXO option because maybe I want to do some WSPR on the holidays (note to my self : buy GPS module and assemble with 3.5mm jack compatible with QCX+ and QCX-mini)


Just like I did on the my QCX+, I didn't use the method in the manual (using cut of wires) but used headers to lift it a little above the PCB.


More and more components are getting there place.


 The ISP and UI (frontpanel) headers are installed (top left). Don't forget the instruction in the manual for allignment. the PCB holes are a little big. 


And then it was time to start on the frontpanel PCB. Hans describes in the manual that you should use a file to file the rough edges of all the PCBs. After usage you have to clean the file of the PCB dust but I can't handle the PCB dust (the particle trigger a asthmatic reaction) so I used a small piece of wood on which 120 Wet and Dry paper is glued. When the paper is full I can through away the "disposable" file. 


Following the instruction I installed the LCD on the PCB. 


And soldered the wires to connect the LCD to the PCB. 


So we can continue to the UI PCB. Installing the rotary encoder, the potentiometer and buttons.


Don't forget the install the 2 small spacers from the UI PCB. 


As you might note I didn't follow up completely on the manual. For aligning the connectors I first soldered the BNC connector to the PCB (just the middle pin), placed the 3.5mm chassis parts and put the side panels on them. Which I then screwed onto the chassis.


That way the connectors are aligned by default. 


Only the DC chassis parts needs to be held in place with a DC plug. 


All done.


When you think your done, there something important to do. 

IMPORTANT NOTE : Hans announced a "QCX-mini critical modification advisory" post in Groups.IO.   

Do apply the critical C38 mod (adding a 10uF / 25V capacitor on the input). This will prevent burning your QCX-Mini when input power is switched on and off quickly.



Close-up from the mod.



Note : The picture showing  the capacitor  lifted a little up. I only had a 10uF/50V capacitor with a size of 5.2x11mm which didn't really fit. And when I assembled it all together it prevented the UI PCB from fitting into place. It's no problem, everything works, except that the shaft of the potentiometer has a meticulous angle. It's almost not notable but when you install the knob, you may find a scratching sound from the knob to the case due to wobble. 

qcxmini_042 (picture taken after complete assembly)

Groups.io member OM David M3PMG also had that problem and fixed it by lengthen the wires of the capacitor and placing it on a other location (link to his groups.io message including picture). 

I will replace it with a smaller capacitor when they get delivered (due to covid19 hard lockdown in The Netherlands I can't go to my local electronics store).  Got the components, check out Re-doing the "QCX-mini critical modification advisory" down blow.


Let continue.  

I wounded the LPF coils (without cat Moppie pulling strings) and installed them.  Everything is ready except.


And then it was assemble time (for alignment). 


 Powering up, the band selection question came into display. Note to MiscDotGeek Ryan Flowers, the backlight of my QCX-mini lights up by default and needs to be switched of by setting it off in the menu. 


Power drawn:  just 70mA (which is perfect). 


And after the alignment which took me aprx 10 minutes, it was time to connect my QRP-Power meter and keydown to see how much watts we could get.

With 13.8V I got 5W output and with 12V just above 4W. So no need to tune the LPF this time. 


Boxing it up and it does look great. 


And then you find out that the battery icon is empty. :-(

No battery icon

What was the reason again? Taking a look at the operation manual :


Indeed forgot to put the wire between DVM and +12V (see section : 4.16 DVM and RF Power meter) 



Now it works.  Showing my 12V power supply.

Settings : 

  • 7.2 Battery On
  • 7.3 Batt. full 13,800
  • 7.4 Batt. step 0,400


Re-doing "QCX-mini critical modification advisory" 

I ordered a selection of various components at one of my regular "I need it quick and fast" component suppliers VanAllesEnMeer They delivered in  2 days which is one day more then I'm used to when you order before 15:00. But the postal services is working overtime due to covid19 and the holiday season so that's ok.

In my order i placed 6 capacitors in the capacity of 10 and 22uF with different voltages and types. 


The smaller ones specialy for the "QCX-mini critical modification advisory" because due to my first fix, the  shaft of the potentiometer has a meticulous angle. It's almost not notable but when you install the knob, you may find a scratching sound from the knob to the case due to wobble. And I know it's there so it will start to annoy me. 


Looking at it, you see that it is a little more then it should be. (yup, annoying all over the place)


So i took the 22uF/25V tantalum capacitor because Hans wrote in the manual said : "10uF should be viewed as the minimum capacitance, the value is not critical, 10uF, 22uF, 47uF, 100uF etc. would all be fine.".  And with a size of 4.4 x 7mm it was a good fit. 


Every thing fits like a custom made glove.


And the a meticulous angle of  the potentiometer shaft is gone. 


Happy OM.


Tilting feet


I really love the original case. But when you have it on a desk, the ergonomics aren't that good. So just like I did for the QCX+  I designed a set of tilt feet for the QCX-Mini, based on my QCX+ tilt feet design. These tilt feet allow a better view angle when you have the QCX-Mini on your desk or table. I couldn't use screws because the bottom of the case is to thin and there isn't much room underneath the main PCB for some nuts. So I used strong double sided tape to install them ( contact glue could also be used).


Folded away the height is the same as the original supplied feet.


The feet are in 2 versions, one with a thin strip between them (allowing more cm2 contact area to stick to the case) and 2 separate feet.


I made the design publicly available so you print your own but licensed it Attribution-NonCommercial-NoDerivs 3.0 Unported (CC BY-NC-ND 3.0).
Meaning : 

  • Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
  • NonCommercial — You may not use the material for commercial purposes.
  • NoDerivatives — If you remix, transform, or build upon the material, you may not distribute the modified material.

It's not to be childish but it now happened a few times that a 3D design I made was sold by other people just because is was "Open source, so I can do anything I want with it" without any credits or even a token of appreciation

The STL files of the QCX-mini tilt feet can be download from this page  on Thingiverse (if Thingiverse doesn't give a 404) or you can download this Zipfile with the parts from this website. Or when you use TinkerCad download it by clicking on the link below.  


When you want it to be used Commercial, please contact me.   


A Electron Volt QCX Auto Gain Control Kit

In the QRP-Labs groups.io there is a thread about a AGC circuit proposed by Jim AJ8S. Electron Volt Limited made a very nice kit from this design and sells that a reasonable price in there webshop.

For my QCX+ and QCX-mini I ordered two of those kits and within a few days the postman dropped of a envelop from the UK containing two ESD sealbags containing the kits. I printed the manuals, put them into a binder with a schematic of the QCX+ and QCX-mini and the PCB from the QRP-Labs manuals. So when I had time to install them, I had everything in place.

On my birthday I took a afternoon of to take a seat in the shack and build in these kits. Our shack-cat Moppie wasn't interested today and stayed on the bed.


Let's get started. 



This is the place where we going to place the AGC PCB.  Please note that I have installed the TXCO on header pints which raises it aprx 2mm. 


I already soldered the wires to the AGC when I installed one in my QCX+. Wrapped it in shrink tube but left a small place open to access the trimmer.


I pulled the wires through the holes from AVCC, AREF, DVM, AUDIO2 (Just below IC2 on the drawing) for reasons I will tell you later. 



When the wires where pulled through I left them long enough to allow the AGC be placed above the TXCO.


The wires where soldered on the bottom to the  following pins/pads : 

  • Black - ground -  sleeve  audio jack 
  • Red - +5V - to pin 7 of IC2 
  • Blue - audio in - to hole "AF Out" (see top silkscreen)
  • Purple - mute output - to C22 pin connected to pin 3 of Controls header (it has a better soldering angle).

See picture below.


When all wires where soldered, and double checked for pieces of wire, solder blobs etc,  it was time for the smoke test. 

On the image below you can see why I pulled the wires through the holes, they now a bit longer allowing to keep the AGC external for adjustment. 


Then it was adjustment time. To bad the wasn't a lot of activity on 40m this afternoon, but there where enough stations to do a initial adjustment. 

The way I adjusted it (with connected antenna): 

  1. Turn volume control to 1/3
  2. Turn the trimmer full CW, notice the background noise.
  3. Turn CCW until background noise drops.
  4. Turn slowly CW until the background noise is aprx as was at 1.
  5. Seek week morse station and when needed adjust for good readable (because we adjusted to background noise there shouldn't be much to adjust).
  6. Seek strong morse station and adjust CCW until acceptable. When needed repeat 4..6 until you find the correct AGC setting for you. 

The picture above is showing the end position (which is the same as on my QCX+).

Following the manual I added tape to the bottom to hold the wires in place, for when i install the case.  


The same for on the top. 


Then it was time for installing the case again and again a smoke test (no-smoke = good).


So far the installation of the AGC on my QCX-mini, also checkout the installation on my QCX+

My thoughts on this kit :

  • Pro : A very nice addition to the QCX line because which will probably safe some eardrums during a contest or when a local QRO stations is coming back to you.
  • Pro : The price is very reasonable.
  • Pro : The kit is well documented. 
  • Pro : Easy to apply (unless you uncap-able of soldering and reading manuals / schematics).
  • Pro : looks easy to adjust, time will tell if I did it ok.
  • Con : nothing I can think of. 

It's nice to see that there are more and more people building small (and effort-able) kits for the hamradio community.