Sunday, December 3, 2017

QCX - 30M Transceiver

Recently, I built a QRPLabs QCX, the 30M version.  I have stopped recommending anything to anyone, but I hope to build both the 40M and the 20M versions in the near future.  I have collected a few hints, in no particular order:


*  There is a impressive video of a QCX build here by IZ7VHF.  Worth a look if you are considering the QCX.

*   The local big-box office supply store wanted more than the list price of the QCX to print a color manual - even just B/W was more than Hans charges for the kit plus shipping.  I put the manual on an old iPad and propped it up near my work area.  I only printed the inventory pages, the section concerned with winding of T1 and the Cheat Sheet near the end of the manual.  I had the Cheat Sheet laminated.

*   If you use something like an iPad or PC to display the manual, all the illustrations can be handily expanded.

*   I used a pair of binocular magnifying glasses.  Also very handy.

*   I would strongly advise a complete inventory of all parts (also see below).  I had one extra capacitor.

*   I would also strongly advise organizing - at an absolute minimum - the capacitors and resistors by value.  Muffin tins would be nice (and would only take moments to wash and slip back into the kitchen).  If you are fortunate enough to have a workplace that will remain undisturbed, at least draw circles for each value on a piece of cardboard and stick them in that.

*   All of my placement errors were with the resistors.  I strongly advise the hint above, and re-checking the color codes against each other and the inventory listings.  Use a VOM to resolve problems with color resolution.  Slight color (colour?) blindness runs in my family.

*   With my tired, old eyes, I found it hard to count the turns on T1.  I ended up taking a photo with my phone, transferring it to my PC and enlarging it on-screen.  I had one turn too many on the S3 winding.

*   When you cut the small connectors apart, I would strongly advise covering the connector and tool with a cloth.  My first cut ricocheted off the wall, then the bookcase then my chest.  Miraculously, I found it in the work area after an extensive search of the floor.  Of course, use eye protection.

*   Take Hans' suggestion and orient all the parts either the same way or at least in such a fashion that the values can be read later.  Remember that this project will be very crowded at the end.

*   The page breaks in the manual are occasionally unfortunate.  Be extra careful when the instructions and the illustration are on different pages.  As mentioned earlier, the illustrations in the Manual I downloaded to my iPad can be enlarged when clicked.

*   From my experience with SMT projects, I now have a large, cheap aluminum foil cookie sheet with a properly sized piece of white cotton cloth taped inside.  Very handy.  If you ground it, even better.


First impressions - the receiver is HOT!  I am impressed.  The QCX seems to hear almost everything my K3 does.  

Power output was low - measured externally at about 1.4W.  

I spent some time reading the QRPLabs e-mail reflector and then followed Hans Summers' recommendations for mods to the QCX.


Here is a listing of the modifications I made.

Before and after each change, I made a series of measurements.  First was a 5 second key-down followed by a 15 second rest, then repeated twice more.  Then a 30 second key-down, followed by a 1 minute rest , then repeated twice more.  I am reporting averages of these measurements.  Power was measured independently of the QCX with a 20W Elecraft 50 Ohm dummy load.  I also recorded the supply potential.

I did the mods in two stages, mostly because the changes to L1, L2 and L3 were a little tedious.

Mod #1        -   12.55V supply
    Before:      5s      1.55W
                30s      1.69W
    ** Removed 1 turn from L1, L2, and L3
    After:       5s      1.36W
                30s      1.38W
    ** Re-alignment
    After        5s      1.56W
                30s      1.52W 

Mods #2&3&4   -   12.38V supply
    Before       5s      1.4W
                30s      1.5W
    **Install 1N5819 Schottky diode
    After        5s      1.5W
                30s      1.6W
    **Install 10K resistor from Q5 drain to +12V
    After        5s      1.4W
                30s      1.6W
    **Replace MPS 2907 with MPS751
    After        5s      1.5W
                30s      1.6W

    After        5s      2.0W
                30s      2.0W

The image below shows the diode and resistor addition to the bottom of the PCB.

I think (but do not know) that the removal of one turn from the low-pass filter did not do much.  I think the addition of the 10K Ohm resistor and the change to the MPS751 did a little good.

At this writing, I am satisfied enough that I will await more information from the designer before diving into the circuit board again.  

As it stands, 2W seems to do well on 30M given reasonable propagation.  The Reverse Beacon Network seems to hear me just fine.

More to come, I hope.

cln - Nick



  1. Thanks for the advice and info. I have just received my 20m kit, but I am now wondering whether to build it as a 30m rig instead.
    Decisions, decisions!
    73 de David vk2dmh

  2. I have since completed more mods. See:

    for details.