A quite large proportion of problems were caused by the fact that a percentage of the boards are affected by a short between the drains of Q103 and Q104 MOSFETs on the 5V buck converter power supply board. It’s a manufacturing fault but the transistors are close together; for the next batch I will certainly move them a bit further apart! I believe the proportion of affected boards is somewhere around 10-30%. However once the problem was known and understood, we did start checking all boards for this fault before shipment, and fixing them where necessary (which is a sub-1 minute operation, if you have a hot air SMD rework tool).
Other than that - it IS a detailed and compact assembly; which of course is partly why it is such an attractive device for SOTA (the enclosure being only 95 x 63 x 25mm). As in any design in which there are multiple power rails (QMX has 3.3V, 5V and 12V) there are certain short circuits which the device won’t forgive you for. There’s a lot to be said for a thorough visual inspection with a jeweller’s loupe, and starting off at a lower supply voltage and current limited supply.
Other than the QMX intro video and the 52 QMX tips video, I plan next to do a video showing how I first power up and check out a QMX. Then some details on the technical design. I made a play-list for QMX videos here https://www.youtube.com/playlist?list=PL_SLL2GpedUNcA07XzWN0vAbNM2qULHtw
Andy, I totally agree with you on the very good manuals, explanations and comments from Hans. Perhaps you are right about the lack of experience of some? But it’s a kit, so a good way to learn. QRPLabs offers good progression in their kits. I started with the “big” QCX+, followed by the more integrated QCXmini, ready for the next step with QMX.
This QMX uses buck converters with a very smart system for adjusting PMW according to your operating frequency. Theoretically I love this solution, making a very beautiful example of co-design between power management and RF EMI (I keep it for my students !!!).
My question was more about the context of using QMX in the field. Feedback is welcome.
You are right, my fault. A harmonic is „the integer multiple of the fundamental frequency“, and 1 is of course an integer .
The rest of my post holds, though.
A general point as for measuring unwanted emissions (a bit off topic in here): One problem with using a spectrum analyzer is that depending on the bandwidth used for the measurement, one may miss unwanted products
a) close to the fundamental (e.g. if ripple from dc/dc converter with a low frequency of say 150 kHz mixes with 14 MHz signal) or
b) those that are rather narrow.
The amplitude measured and shown for a harmonics may then be lower than it actually is.
And since broad sweeps (e.g. 7 - 50 MHz) take some time, many people try to reduce the resolution of the sampling, hence I am often sceptical when people report the absence of unwanted emissions from DC/DC converters.
I used my QMX the last days on two summits. The only problem on my first activation was my poor working cw brain decoder . Problems with the volume control at home were absent on the summits.
In May Hannes DL9SCO (Palm Radio Keys founder) visited me here in SW Turkey and we did a little “SOTA activation” (though not SOTA, because Turkey is not part of SOTA); we made a video of it you can see here https://youtu.be/mwslo4P6IBs?si=6c8pteC5CTfSpYNM
QMX worked absolutely great; as Hannes and I took turns operating it, we did have to keep turning the keying speed back and forth between 25wpm and 16wpm :-/ But I’ve been practicing hard the last couple of weeks, improving my skill. The only trouble was the horribly large transmit/receive switchover clicks (BANGS) that you can hear in the video. But this was already resolved in a firmware update back in June; it’s very quiet on full break-in now, and fast. I measured 15ms latency which is comparable with well respected Elecraft rigs and a lot faster than many other commercial transceivers.
Bear in mind for anyone nervous of building it, we do offer an assembled version too see QMX 5-band 5W Digi transceiver though we do have waiting list we’re working through. Even including the $50 assembly fee it’s still only $165 all in which is still less than half the price of a Mountain Topper, AND the QMX transceiver has vastly higher performance and features, even now, before all the forthcoming firmware updates will only make it even better.
Not that difficult a build if you do indeed follow the instructions VERY carefully.
However, mine smoked its Tantalum 5V smoothing capacitor despite not having any other build faults, and I don’t think we have an explanation for that yet. I have it running fine on a temporarily rigged linear 5V supply, which I will replace once Hans has replacement PSU boards available.
Mine is also down a bit on output on 20M, so I probably need to check the turns on the LPF toroids, on the other hand it could just be the toroid and capacitor tolerances.
Apart from the one problem, I must say I’m very impressed with the design and packaging - there are some very clever features in there with more to come.
If you want more 20m output power, try squeezing the turns of L513 together, so they cover say 180-degrees of the circumference of the toroid. That should do it. On the other hand - more power puts more stress on the PA transistors, and provides really tiny benefit in S-points… so striving for maximum smoke isn’t always a good thing.
I was a bit wary of buying an early(ish) release of this kit but decided to take the plunge a few weeks ago. It’s a relatively easy but fairly cramped build - a bit more complicated that the QCX-mini kits but only because of the tight fit. I was careful to check for the fault that has been reported on the little pre-assembled switch-mode board but mine was thankfully ok. I chose to build for 12V and initially powered it up on a 9V pp3, kidding myself that no damage could possibly happen at that voltage…
It runs nicely on a 3S LiPo or 3x18650’s. The built-in diagnostics suggest that a supply of 12.4V or less is OK so I run the pack down to that level before using with the radio… Output is between 4 and 5 watts on all bands so I’m a happy camper
I finished my QMX very late this past Friday night. I barely had time to load the firmware and quickly check to see if it even received or transmitted. I tossed it into my SOTA pack with the KX2 and went out early the next morning. After activating the summit with the KX2 I fired up the QMX on 40m and started calling CQ for several minutes with no response…Well no wonder. I forgot to connect the antenna!!! After ACUALLY connecting it to my home brew resonate trapped endfed 40m, 30m, 20m and 17m antenna It’s very first 2 contacts were S2S’s on 40m from New Mexico to South Dakota and Arizona!! I also brought it yesterday to the next activation. I was thinking the RX sounded a bit dead but when I contacted EA7GV on 20m with it I was sold. I love it. I can’t wait until the 20-10m version comes out. I will for sure have that rig too.
Thanks Hans.
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. Problems with the volume control at home were absent on the summits.
