The QCX 5W CW Transceiver From QRP Labs

The original intention wasn’t to write a post about this, but seeing that I’ve been absent from blogging for a while, I thought there was no harm in putting up a few pictures of the kit I just assembled for a friend. He’s been having a rough time recently with his health.  As an experienced engineer and ham, not being able to put together kits like this has been very frustrating for him.  That’s the brief back story. In case you’re thinking I’m some kind of terribly altruistic being, in the interests of full disclosure, I did take quite some time before volunteering to help him out. I have a very narrow and focused mind. Some would call it INTJ, if you’re into that sort of thing. I had other projects in my head, and couldn’t extend myself to thinking of doing something else. It’s a useful kind of brain to have for meeting deadlines and things like that, but it’s not very conducive to leading a balanced life!

Anyway, enough of that. Fred (I’ll call him Fred, though that’s not his name) saw Hans Summers’ QCX transceiver kit and was really enthused about it. Knowing how keen Fred was on this little rig, yet not being in a position to assemble one, I finally realized that it wouldn’t take too much time for me to do it and, on top of that, I would get to see Hans’ latest blockbuster “in person”. Advisory – this will be by no means a review of this transceiver kit. I simply figured that if I was going to assemble it, it would make sense to post a few pictures to share with you. Think of this post as a show n’ tell.

The kit arrived in a small box only a few days after Fred had ordered it –

The board is top quality, and comes with the two SMT devices already soldered on. They are the Si5351, which is the heart of the synthesized VFO, and the FST3253, which is used for the quadrature sampling detector. Inside the red packing is the LCD module –

The assembly instructions, which are available on the QRP Labs site, are very detailed and easy to follow. They are just about the most comprehensive I’ve ever seen, on a par with those from Elecraft. The combination of step by step instructions and excellent graphics, make it easy to assemble the kit correctly. I must admit that I rather like the assembly procedures that involve building the circuit a stage at a time, and testing each stage as you go. However, it may have been that with a circuit like this, that is so heavily dependent on the micro-controller, there weren’t too many stages that could be tested independently of it. The procedure for the QCX is more streamlined, with all the parts of each type (IC’s, capacitors, resistors etc) being installed at the same time. It is quite an efficient method, as by installing multiple instances of very similar parts, you can really get into the swing of the assembly procedure for each specific type of component. My big problem, and one of the reasons I haven’t done much building recently, is that while I  used to find soldering parts something of a relaxing and “zen” experience, I now find it quite boring. I’m not sure what changed! I think I am really at the point where I ought to move more into the arena of design (or programming), but just haven’t made that leap.

Incidentally, while talking about the assembly manual, it also includes extensive details on the operation of the circuit. Although you are not required to read everything in the manual, there is a lot of information and explanation there, for the curious.

IC socket, IC’s, fixed capacitors, resistors, crystals, and diodes installed –

A few more hours parts-stuffing, and the following picture was the result, before the LCD module was installed. The only part of the assembly that was a little more involved was winding T1, the receiver input transformer, which has 4 windings. It is important that these are all wound in the same “sense”. The assembly manual contains a method for doing this that involves putting on all 4 windings at the same time, with the same piece of wire, then separating them. I found it, though foolproof, a little over-complicated for my tastes. I ended up studying the photo in the manual carefully and putting on the 4 windings separately, using a crochet hook to keep the wire snug against the core – my usual technique.

The completed board, before installing the LCD module over the micro-controller.


The left-hand 3.5mm jack was slightly off-square, and this offended my OCD sensibilities! However, the part fits very tightly in the holes, and there seemed little point in forcing anything, simply to satisfy my irrational desire for extreme symmetry 😀

Spot the two cat hairs in the next photo. I am not able to build anything here without a cat hair or three ending up on it. Some have asked how I form my resistor leads. I do it with a pair of round-nose pliers, as detailed in this post

This is the QCX with the LCD module installed –

Nothing much to see on the underside of the board. I did later discover that the AF gain pot, although specified on the schematic as a 5K log pot (audio taper), had been supplied as a 5K linear component. As a result, all the useful volume settings are located very close together near the anti-clockwise (low volume) end of the pot rotation. I did try, per suggestions from helpful folk in the QRP Labs group, placing a resistor equivalent to about 20 – 25% of the total value of the pot, between the wiper and ground. This should have, in theory, approximated a log response but in practice, didn’t seem to do much. I didn’t have any 5K log pots to hand that were also the same size and shape, so decided to stick with the supplied part. You get used to it –

You can see the three PA output transistors lined up side by side near the bottom left-hand corner of the board, right in front of the two yellow T37-6 toroid cores. They run in class E, which means they are very efficient and run cool. There is no need for a heatskink, though Hans does recommend caution if you are going to run it at 5W in WSPR mode, which is a 100% duty cycle mode. The rig requires at least 7V , in order to give the 5V regulator some headroom. Above that level, the only stage that benefits from higher voltage is the PA. I am getting about 3W out at 12V, and up to 16V can be applied, for up to about 5W out –

There are lots of neat things about this rig. If you want to learn more, you can visit the page for it on the QRP Labs site, and join the QRP Labs group on However, a few really handy features that I will mention are as follows –


-All the tools needed to align the rig are contained on the board (except for the small screwdriver necessary to adjust the trimcap and trimpots!) In alignment mode, the Si5351 chip generates an RF signal, injects it into the receiver, and then generates a bar-graph display on the LCD so that you can tune for the necessary peak or minimum. Brilliant – and very convenient.

-There are a number of test equipment functions available on-board, including those of a frequency counter, and RF signal generator.

-CW and WSPR beacon functionality included.


I’m not much into having QSO’s these days, for some reason. I did try calling a fellow a few hundred miles away in the middle of the day, only to receive a reply from a station a mere 1500 feet away. He was so close, he would have been able to hear my Part 15 AM station on the AM broadcast band! I’ve been running the CW beacon for a few minutes at a time, then checking the spots on the Reverse Beacon Network, and have been spotted as far as 1300 miles away (Calgary, AB) and 1500 miles (Louisburg, KS). I’m sure that in time, that distance would increase greatly. I also tried running WSPR, with the relatively high power (for WSPR) of 3W for which, I was rewarded with spots all around the US, as well as several from VK-land, and DP0GVN, the German Research Station in Antarctica. The SNR of the strongest of those spots was somewhere in the region of -5dB, suggesting that I could have been spotted by them while running considerably less than 100mW. Fantastic!

It’s a hot little receiver, including a narrow 200Hz AF filter. It’s the Hi-Per-Mite design, courtesy of David Cripe NM0S. That little design of his keeps cropping up. I have a feeling that, years from now, people will still be designing new kits with David’s little filter in the AF stages 😀

Then – disaster struck.  While idly browsing the internet, a very faint smell, reminiscent of a distant conflagration, had me wondering what my downstairs neighbor had burned while cooking. A minute or so later, it became apparent that I had left the rig on the bench with the antenna disconnected, unaware that it was still in WSPR mode. A single WSPR transmission at 3W into no antenna had committed the 3 x BS170’s in the final to the semiconductor equivalent of the rainbow bridge. When our pets go to meet their maker, we say they have crossed the rainbow bridge. I’m not sure where formerly active devices go, but my 3 little MOSFET’s sure as heck weren’t with me anymore.

I did have replacement BS170’s in the parts draw, but they were of unknown origin. This QCX transceiver was being built for another ham, and I wanted to be 100% sure he was getting a reliable repair, so ordered Fairchild devices from Mouser. The QRP Labs groups revealed that in cases like this, Q6, the bipolar transistor that switches the +ve supply to the finals can also be toast as well so, to be safe, I ordered a few of those.

In the same group, Phil G4JVF posted a photo of his QCX with a very attractive little anodized blue aluminum heatsink attached to the flat faces of the row of 3 x BS170’s. He said it was a self-adhesive heatsink for computer memories. While putting in my Mouser order, I found a little heatsink (Mouser #532-501200B00) that was intended for 14 and 16 pin DIP packages. It didn’t have a self-adhesive strip, and I didn’t have any double-sided heat conducting tape to hand, so out came the JB Weld epoxy. It was a rather lengthy process that involved  gluing each BS170 to the heatsink individually, waiting a few hours for it to partially harden, then gluing the next transistor onto the heatsink. A little piece of foam and Scotch tape helped stabilize each MOSFET while the epoxy hardened –

The cat hairs crop up everywhere…………..

I added a little extra epoxy in between the transistors to help the thermal bonding. JB Weld is not the greatest conductor of heat, but I believe it does have some thermal conductivity –

This time, the finals should be a little more protected. The gentleman who will be receiving the rig is not planning to use it for WSPR, so I think that with this new arrangement, unless he falls asleep on top of the straight key with no antenna attached, everything will work out fine!

The board with the new heatsink attached, before fitting the LCD back on. Boy, does that slightly off-square headphone jack (on the left) bug me. It’s a tight-fitting component, so I wasn’t able to reasonably do anything about it. Let it go Dave, let it go………. 😀

A few small things (and they really are small things) –


-There are very slight key-clicks on the sidetone. The transmitted note is shaped nicely and sounds great though. It doesn’t take much to get used to the sidetone at all. Perhaps this will be fixed in a future firmware update?

-I didn’t spend enough time to pin down exactly under what circumstances this happened, but did notice a couple of times that when in CW beacon mode, the micro-controller would sound like it was keying the rig, but no RF was being generated. Rebooting the transceiver solved the problem. I’m guessing this is a firmware issue. The QCX has a in-circuit 2×3 programming header, so that you can update the firmware with an AVR programmer if and when Hans issue updates. If that’s not something you’d want to do, the micro-controller is socketed, so that a newer chip can be plugged in.

-There is some constant very low level clicking in the background. You will not notice this at all when an antenna is connected, as band noise completely covers it. Hans acknowledges this in the extremely comprehensive manual, and explains that it is a result of the simple design. I strongly suggest downloading a copy of the manual from the QRP Labs site. There is so much good information there, and it’ll give you an idea of what to expect before your kit arrives.


None of the above should stop you from getting this kit, if you have anything even approaching an interest in it. For the price ($49 at time of writing) it, like all of Hans’ kits, represents really good value for the money.

Well, that’s it for my little show n’ tell. I had a bit more fun with this rig, before shipping it off to Fred this morning. I hope you don’t mind if I don’t go into detail about all the features on this QRP transceiver. There is plenty of commentary and documentation on the QRP Labs site, as well as the QRP Labs group on I just thought you’d like to see some pictures of a neat and very affordable little rig. It’s also my way of saying, “Hey – I’m still here, and I still build things occasionally!” I’m going to be assembling an Ultimate 3S QRSS/WSPR for him next. The plan is to install it in the case supplied by QRP Labs, with the GPS unit, and an external GPS antenna.

In the meantime, I really need to learn how to write some simple code. I’ve been telling myself this for years……………



19 thoughts on “The QCX 5W CW Transceiver From QRP Labs

  1. “while I  used to find soldering parts something of a relaxing and “zen” experience, I now find it quite boring.” –Dave, your appreciation of the little rig pours out of your blog pages. Your blogging words and pictures set the benchmark that others, myself included, aspire to. Dont let soldering those pesky little parts get you down! Welcome back! — VK3HN.

    1. Paul – I just read your post about your Arduino-based Si5351 VFO and admire your fluency with code, and these types of projects. For several years, I have been teetering on the edge of doing more with micro-controllers, and especially with Arduino as a learning platform, and perhaps even a springboard to getting some familiarity with C. Often, when I have an idea I want to implement that requires a micro-controller, I go searching for the appropriate code online. Trouble is, I invariably find something close, but not exactly what I want.

      The great thing about homebrew is that we get to build exactly what we want. Often, we build things because the thing we want isn’t actually available to purchase. Well, it’s becoming more and more obvious to me that in order to build the things I want, I need to learn how to code. As a realistic goal, learning how to write for the Arduino platform, and getting enough familiarity with C so that I can modify programs written by others in C would be a good start.

      You are inspiring me to get off my proverbial rear-end and learn something new!

      1. Dave, you are right that homebrewing is time consuming but affords the ultimate personalisation — building ‘exactly what we want’. I also find that true of Arduino coding where other’s scripts typically do what I want but with features or quirks I don’t want. It’s worth the effort to pick it up Dave, and it’s not that hard. Start with an Arduino on a breadboard and something small and work from there. Hang an LCD off it for visibility into your code and trace every thing into a Serial window besides the IDE. I find it engrossing. Good luck. And keep melting solder! 73 de VK3HN.

  2. Dear Dave,

    Live can be seen as a railroad track with switches and sometimes long rides. We like to control the switches our-self but often other pull the handles on them putting us on tracks we didn’t expect which can be pleasant or annoying or even horrifying. But sometimes a switch puts you on the long rides which can be interesting and relaxing at the beginning but when the landscape doesn’t change enough you may wander off and lose interest or inspiration.

    For being a human and a DIY and kit builder my self, I have changed tracks by my choice or by the actions of other. And I have rode the long tracks and find my self sometimes pulling for a stop to re-think things over and control the switch my self. Changing interests isn’t a thing to apologize for, it’s called finding new inspiration.

    But that aside, It warms my hart to read that you embrace the ham spirit and help a fellow ham in a difficult time. Even if the helping hand is on a long track from which you can predict the landscape. That means you control the switches.

    73 de Lex PH2LB

  3. Great read as usual Dave. I am a huge fan of Hans’ kits. I have been playing with and modifying his Ultimate3 kit for the past year –

    I’ve told Hans that the U3 is some of the best fun I’ve had in Ham Radio ever.

    The U3 uses a similar BS170 final as the QCX kit. I recommend keeping a bag of these cheap devices around 🙂

    After my Bitx projects are finished, I am thinking of jumping in with this kit as well. Certainly reading your review of it has it on the shortlist for me!


    1. Glad to know you’ve been having fun with your U3S Patrick. I’m looking forward to getting started on a U3S build for my ham friend in the next few weeks. Looking at the schematic, the simplicity of the hardware is very appealing. The RF chain is just the output of the si5351 straight into a BS170, the LPF and then the antenna! I love that simplicity, and the thought of the great distances than can be spanned with it. It reminds me of the simple CW transmitter plans I used to look at a kid. The fact that a single transistor could send a signal over great distances was very beguiling.

      PS – I am stocked up on BS170’s, and already have a few earmarked for other projects too.

  4. As per usual, superb write up Dave, thanks for the heads up on this newish kit, worth a punt me thinks,
    I did however note that your levels of excitement over building/soldering seem to of taken a hit, as for why, only you know, but as previously mentioned earlier, your write ups, builds, designs have set the benchmark for other to follow…. I sincerely hope that you aren’t on the downhill spiral,
    As we brits say, “keep your chin up eh”…..
    Keep well..
    Kind regards

    1. “Chin up lad” (spoken in a broad Yorkshire accent!)

      Thank you for the kind words Lynd. I do have a few ideas up my sleeve for the direction I want to go in, so there might just be an upswing in the future!

    1. Not sure how credible MBTI is, but I regularly test as INTJ, though most of those categories are borderline for me. I just squeak by, which says something about me, though I’m not sure what!

  5. I have one to build for 20. When I get to it, I am going to wire the pots off board to the case along with the antenna connector and headphone jack. My idea is to put it in a die cast aluminum box with the controls on top. Or, I could board mount the pots and use shaft couplers to bring them up to the top of the box. You did a nice job on the torrids too.

    1. John – make sure to join the QRP Labs groups on if you haven’t already done so. A number of folk experienced low power output on the 20M version, and solved the issue by replacing C25 and C26 in the output low pass filter (both 390pF) with higher quality C0G/NPO parts. It seems that the ones which came with the kit aren’t up to snuff. Have fun building it!

  6. Hi Dave, I have a look at your blog from time to time. Thanks for sharing the build process. My kit is waiting back home to be built. Congratulations on the reception by DP0GVN. I’m the current operator of the station until February and glad that so many stations can be received here in WSPR.
    73 from the South! Matthias, DH5CW

    1. Great to hear from you Matthias. It was a thrill to be spotted by your station in Antarctica. Enjoy building yours when you get back home. Stay safe (and warm) – and safe travels back home!

      73 for now,


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