Hello everyone,
I’m preparing for the construction of the kit and I read various pists about the difficulty of the kit. I’m used to building kits and this is not the first but reading about so many problems warns me. What can you tell me who have already made it? 73
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Build 2 and repaired 12 (11 for others). It’s not good for a first build but not to difficult if you are used to build kits. Yes, the most of the threads about QMX(+) building is about problems. But the most of builders without problems don’t write about there success.
73 Ludwig
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You definitely benefit from a good quality 60W+ soldering iron, a decent pair of loupes, and good lighting. I made several errors during my build that I had to correct and yet it works great. Be conscious of the polarity AND orientation of the 1N4007 diodes and the orientation of the 47uH inductors otherwise the low pass filters won’t fit. Take your time, double and triple checking as you go, and you’ll do great. Amazing value for such a versatile radio.
73, Jared, N7MAW
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do NOT use 13.4V. Limit is 12V…
do NOT use a 2.5mm connector, even if it fits. You need 2.1mm
ask me how I know… 
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How do you know? (You said to ask 
)
I chickened out and had Hans’ team assemble mine. I have the IQ of a bubble in an Malteaser and the soldering skills of an octopus that’s just arrived home from Asda carrying heavy shopping.
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Thanks guys for the comforting answers. I built three trusdx, much simpler, and at least with inductors I have good practice. I have both glasses with magnifying lenses and 60w soldering irons with various types of tips. Unfortunately I don’t have a hot air gun, I don’t want to get to need it, you got me right.
I bought the connector together with the kit. I still have to decide on the supply voltage even if I lean towards 12v since I have 3s packs, and I will use a srep down reducer if necessary, hoping that it does not introduce noise. What can you tell me about it?
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I’m curious
Hi Luca,
I built a QMX with no problems. I’d say read the manual, and follow the instructions carefully - it is very well thought out.
I built for 12v as it is more versatile. It will work on anything between 6 and 12 volts. The 9v version will only work between 6 and 9 volts.
With the experience you have, I’m sure you will be fine.
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I found using my phone to photograph the boards and then scroll an enlarged view was easier than using a loupe or microscope. Inspect very carefully. Do it again after soldering coils and capacitors looking for solder splashes or beads.
Take particular care with the little power boards - both mine had broken smd end attachments diode on one and capacitor on the other. Possibly careless handling when cleaning up the board edges. Failure here will destroy the CPU!
The Wiki is a great resource.
I have made many ssb QSOs with mine - still using the Beta firmware 003.
For real expertise use the QRP Labs forum
73,
Rod
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Thanks again for the encouragement and advice. In my kit constructions I also check the values of components such as capacitors, resistors and inductances before soldering. I notice a high rate of broken components on the pcb during shipping and this is very strange.
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I’ve got to the point where having a good low-power binocular microscope makes a difference. I had one significant problem with my QMX build. I didn’t manage to get the power connector properly soldered first time, and so power didn’t get through to where it was needed. Simple to fix, once found…
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Hihi !! Well, since you asked, the 2.5mm caused a multitude of very short on/off transients, which is the likely cause of my unit not powering up at all anymore. I did check all the diodes; at this point i think it’s likely a fried IC. (had a bunch of help from groups.io). I have since ordered another unit, i will not make that mistake again, and will also build a voltage regulation unit to go from my 13.4V batteries to 12V.
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I think I made the same mistake with the trusdx using a male connector that had too generous a diameter inside. As far as I can think the construction of qmx is complicated by two factors, the first is that there are high chances of finding problems due to soldering that is not well executed, So using 60/40 tin and flux is mandatory. According to Aspett, given the possibility of finding PCBs with broken components, a visual inspection is mandatory before starting soldering. I would add that checking the conformity of the components we are going to weld is a good thing. If after all these checks something goes wrong, Murphy’s law comes into play.
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I build 2 for me, one low bands, and one high bands. It took me about 8-10 hours to read the manual and build my first kit. The second one was probably 4 hours. So, there you have it.
Don’t stress too much, just take your time, be gentle, and don’t expect 5W at every band. If you get more than 2-3W you are good to go.
I made changes to the toriod windings for some band filters and could squeeze one more watt here and there, but every build has his own power output.
Happy kit building!
73 Alberto EA3GNU
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Been there, done that - the QMX needs a reliable power source or the DC-DC converters get confused
Rick
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Some additional recommendations 
- Clean the coating off the ends of the enamelled wire for the toroids very well. Trying to burn it off with heat may work but it also leaves contaminants behind
- Add plenty of thermal compound around both sides of the PA transistors. I find this makes the PA much less likely to overheat.
I plan to make a 3d printed case with built in (ex vape) battery to reduce weight and make the power connection safer
Rick
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For SOTA, I’ve been using a 550mAh 3S LiPO battery pack and it works great (no step down). A fully charged 3S is 12.6V. The manual warns about exceeding 12.0V. Many have blown their finals, so use a 3S LiPO at your own risk.
73, Jared, N7MAW
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It is precisely this limit of 12 volts not to be exceeded that It puzzles me. It is possible that Hans designed with a limit of 12 volts knowing that in circulation there are 3s packs that at full voltage give 12.6 but quickly go down at 11.7volts. However, if you don’t want to take any risks, you have to use a Buck voltage reducer with an efficiency of more than 90% And low noise generation.
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I have a diode permanently in series with the battery connector. Off-load this gives 12.1V and after a few minutes on receive only this drops to 12.0V. For testing and other work where the output power is not significant I use only 2 cells and the QMX still seems to work OK.
73,
Rod
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In my trusdx equipped with No. 3 bs170, there is no possibility of applying dissipation, but I have never feared for them by adjusting the efficiency well.Now qrpguys has made a kind of support to dissipate heat. I imagine that even in the qmx we need to work on the fine-tuning of efficiency.