QCX Mini Part 2

I thought I’d better start part 2 of the QCX Mini thread, as there are probably a few more tweaks in the pipeline as more people complete their build and start activating with it. My build is coming along nicely:

I decided not to install the reverse polarity diode, as it will make installing the 10uf capacitor in parallel with the voltage regulator easier. I always use powerpoles, so reverse polarity is very unlikely. I have a 10uf electrolytic at home but it’s 63v, so a little large. I’ll have a look in Graz on Monday and see if I can’t find something smaller. I’ll post another picture when I get it installed.
73 de OE6FEG


Any reports of folk applying the SSB mod yet? Presume that’s fine still

Good news: my QCX Mini (17m version) works!
Only found time to make a few CQ calls, and check RBN, but all looks good:

I built the LPF first, before mounting C30 or L4.
Measured and adjusted the LPF with the help of my nanoVNA.
Then continued construction as in the manual.
I get 4.5W out at 12V DC without further adjusting, and I have installed the idiot diode.
Since Hans mentioned that the output at L4/C30 is not necessarily 50 Ohm, I may get a few mW more if I tune the coils a little more (especially L3).

I also managed to mount a 1A fuse in line with the idiot diode, and still had room for a 22µF/16V tantalum capacitor.

Bad news: this thing is TOO MUCH FUN to write anything more now, I just want to play with it first! I will make a full report on my blog with pictures later. I promise! (I think) :wink:



When I say leaving out the diode will make adding the capacitor easier, what I mean is ‘neater’, as one leg of the capacitor can go in the hole the diode would have occupied. There’s no huge benefit in leaving out the diode, it’s just my personal obsessive choice. Clearly there is no great difficulty involved, whichever route you choose to go down.
73 Matt

That 1A fuse sounds like a good idea. I must see if I can find one in the UK. Does it have a technical name other than “1A fuse”? :slight_smile:


I don’t know where I got that fuse, it came from my junk box. You can find them in old ADSL modems, Cable set-top boxes, SAT TV tuners, etc …

To give you an idea of what to look for, here a picture of some of the small fuses I have. The one up front, that looks like a resistor is the one I used in the QCX Mini. The thicker round one is the model that I put in my two QCX “Classic” rigs.
The one with “1000” on it might as well be a 10A fuse, does anyone know how to identify them properly without … well … blowing them ? (hi)

If you can’t find any, the best alternative is to make a dedicated DC cable for your mini, with an inline fuse. It will be easier to replace as well :wink:

73, Luc ON7DQ


They’re Polyfuses aren’t they?

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Just thinking aloud… If you can measure its resistance, you could compare the reading with that of a fuse of known rating. They would ideally be of a similar physical size. That would give you some idea…

If it’s a polyfuse it will reset itself sometime after blowing.

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The thick round ones are definitely ordinary fuses, I have opened one once.
Not sure of the resistor-like one, it might indeed be a polyfuse.


Today I made some QSO’s to test my 17m QCX Mini on the air.
Here the stations I worked, with the RST I got:
0803 RW3PZ (559)
0943 UT2LA/QRP (559)
1304 TA1PB (599) ,YES that’s Turkey … but not Hans Summers , hi.
I couldn’t catch anything more on 17m, so I put my 17m rig on 14060.50, and started calling CQ.
Guess what … I got called by YT2T/QRP and we had a nice 2xQRP QSO, and my RST was 569 !

The 17m QCX Mini was not retuned in any way, but of course I switched to a matched antenna for 20m (a monoband groundplane, on 17m I use an endfed of 33m length). On 20m I get abt. 3.5 W output. Reception may also be degraded some, but it surely receives well enough for a couple of QSO’s, even with another QRP station.
Harmonic suppression is still ok with the 17m LPF.
So for emergency work, you get a second band for free :wink:
Great fun !

Just one thought: since the L4/C30 network is not tuned for 20m, efficiency is probably lower, and some more power is dissipated in the BS170’s … so use at your own risk.


My QCX is just about ready for the smoke test:

The LCD screen needs to be soldered as flush as possible on the underside:

I don’t think I’ll have any problems, but in hindsight, I would have cut the wires flush with the board before soldering. They are held in place by the solder joints on the upperside, so no need to solder first. The connection would still be perfect. One of the pins on the ISP connector comes quite close to the far end of the LCD, but some insulation tape will take care of that. They have some micro miniature electrolytic capacitors in stock in Graz, so I can pick one up tomorrow and begin alignment.
73 Matt
73 de OE6FEG


Hi all QCX Mini builders.
I’m still adding stuff (results and measurements), but most of my building tips are on my blog:

I hope it may be of use to someone.
Feel free to comment or report any errors that you see.



Just finished my QCX today:

After fiddling with the toroids a bit I get 4.5 watts out at 12v and a healthy 7 watts out at 15v. Alignment was a breeze: no need to take windings off T1. Can’t wait to get out on the hill and try it out for real. In the end I kept the idiot diode, as the cap would have to lay flat in any case; no need for more pictures of that.

          73 & MX de OE6FEG

AMS1117-5 has a maximum breakdown voltage at 15V.
I made a bad choice to stock those regulators for my QRP home brewing. It would be better to use AZ1117C-5, rated at 18V breakdown, a larger margin of reliability in case of voltage surge (intermittent connection from battery). I wouldn’t use a rig with ASM regulator for SOTA.
A drama video clip on YouTube AMS1117S Are they any good.
On my bench, it is ok with 13.8V loading a 33R resistor (150mA)

I stop using analog regulator as a direct conversion to 5V, instead I am using switch mode converter from battery to 7V than an AZ1117C to 5 or 3.3V.
Cheers, 73 Pascal

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The weather this morning turned out to be good enough for a short activation of the Hochglaserer (OE/ST-482). I took both my QCX 80m and new QCX Mini for 40m:

Unfortunately, my 80m endfed did not like running past so many trees and I couldn’t get a satisfactory tune on 80. It was all down to the untested QCX Mini:

Conditions were pretty appalling today and several stations did not make it into the log due to QSB, sorry if you missed out. I’m not used to the keyer in the QCX Mini, so there was also a bit more QLF than normal. Generally though, I was very happy with the QCX and am looking forward to better conditions. The snow is melting at the moment, so many mountains should be accessible in the New Year:

There was even a fire salamander out and about:

     73 de OE6FEG

I completed my Mini today and gave it a quick test in the garden. It’s the 30m version and range seems less than my 20m QCX+ which can usually contact a few RBN spotters on the east coast of Canada and the US but the coverage in Western Europe seems good. :slight_smile: I will have a fiddle with the windings and see if I can tweak it a bit.

It is tiny. :slight_smile:


I’m still waiting with baited breath for my replacement BSS123 (Q5) to arrive from Mouser so I can replace the one that appears to have been defective on my board. Should be here Tuesday.

Now I have to go back and re-work L2 that I had dorked with trying to figure out my low power issues. Since Q5 is the last transistor before the LPF I’m sure that had something to do with it. :slight_smile:


Hi Richard, I hope you get your transistor soon, this toy is soooo much fun!

Yesterday I had my first DX contact, with W3NP, Dave in WV, who is also an SKCC member like me.
I contacted him twice, first on 17m, and then again on 20m, with only 3.5W and a groundplane antenna.
WOW, just WOW!

In the mean time I found out that the sensitivity on 20m is roughly 5dB less than that on 17m, so good enough for QSO’s, but you could also say that the BPF in the QCX is not very good :wink:

I added more details on my blog, about RBN reports and the QSO’s I made so far (see link in a previous post above). Still to add: some spectrum analyzer measurements.

73, and Merry Christmas to all !


I came down to find an email saying my QCX mini has been shipped. :slight_smile: