Modding the QCX

Been busy modding the QRP Labs QCX transceiver and like to share it, so someone might get some inspirtation from it.

As i already have a KX3 with paddle, i 3d printed a bracket to mount the KXPD3 paddle to the QCX, for this had to move the headphones jack (yes i am lefthanded)

I joint 4 Li-ion cells to a 4S battery with nominal voltage of 14.8 Volt (cell type 103040, 30x40x10mm, 1200mAh, 3.7V) it fits inside the housing underneath the PCB. i put a thin sheet of insulating foam between battery and PCB keeping the batterypack firmly in place.

At the GPS connector location a made a 6Pin connector with one pin blocked so i can’t put the charging connecter in wrong, now i can balance charge the individual li-ion cells.

I replaced the power connector with a 3.5mm stereo audiojack for the headphones.

I mounted a P-channel FET witch switches the power supply on/off by plugging in/out the headphones.

The transceiver is now standalone and with 1 meter coax and a end-fed antenna using a FT82-43 transformer with 10 meter wire i now have a 5Watt 20mtr portable CW station weighing under 600 grams. As for battery life, it seems oke, but needs further testing.

some more foto’s at:


I really like what you’ve done here. However, the manual states that the risk of blowing the PA rises significantly once over 15v. Are you sure a 4s LiPo battery is a good idea? I would be more inclined to 3s in order to play it safe.
73 de OE6FEG

You are probably completly right. However the price of a few BS170 plus the fact that i have plenty lying arround makes me to go and try it anyway and see what happens. But it would be a bummer if you took big hike up a mountain and find it not working. One could try and find another form factor so a 3 cell configuration would fit the case without lost space, mAh probably would increase

Maybe throw in a Tiny SWR:

Any space left over can be put to good use in other ways.

I have built my second QCX (40m model) in a D-Link AP box.
Also using a 4-cell pack without problems so far, considering that a 4-cell pack doesn’t stay at 16.8V very long, and I didn’t bridge the “idiot” diode.
But I did put the 7805 on a large heatsink, it gets quite hot otherwise.
And I glued an IC heatsink to the three BS170’s with superglue.

In the picture : the 4 cell Li-Ion battery fits in an audio cassette case.
Using an external speaker at the time.
“Clothespeg” paddle (the history of that paddle is here ).
The antenna on the kitchen table is a portable “Patterson” loop, which got me RBN spots all over EU.

In the extra space I later added a small LF amplifier board and flat speaker (from a laptop). Here a look inside (the pushbuttons are mounted on the lid).

I used a Tiny SWR board in an external Endfed tuner (Fuchs type), so that I can use it with several different rigs (the TinySWR is bottom left in the picture).

More pics and details on request.

73, Luc - ON7DQ


Since the QCX is a monoband transceiver i am using dedicated antenna specifically tuned for it. personaly i don’t see much benifit in a (tiny) swr meter.

I like what you have going on here. I have spent the past couple of months doing little things trying to get an almost all self-contained QCX. I removed the BNC and direct wired the QRPguys Mini no tune half wave. I have also hard wired an iambic key and mounted to the case. It’s not pretty, but my current light weight kit runs about 1lb 7oz including logbook, head phones and 1800mah 3s battery… I am not quite brave enough to start playing with batteries :).


I don’t have it but willing to brew one. So far I got some success in the PA stage with 20W RF (KX3 owners :broken_heart:)
E-PA 20W

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Very nice looking paddle. Love it!

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Receiver Test

That sounds really clean, how does the transmitted audio compare? Also, are those audio filters I see on either side of the uC?

More info from Guido PE1NNZ

I will get to the Tx step in a few days, would shot a video clip then.
For your information about the cost of SDR home brewing:

  1. 1999, “DSP-10, 2m Transceiver, by W7PUA”
    The DSP evaluation board from Analog Devices, 21xx EZ Lite, landed in VK with a tag $1200 Aud, shipping included.
  2. 2010, “PICASTAR, HF with IF DSP, by G3XJP”
    The pair, Codec & DSP, ADSP2181 and AD1885, from China, for $25 Aud.
  3. 2020, “QCX-SDR, by PE1NNZ, an Atmega 328P, less than $2 Aud !!!, we are paying a hefty price for those bloodless invaders.

Cheers and 73 from Pascal

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Sorry, missing that bit.
The capacitors and inductors are for DC filtering, over killed from my junk box.

QCX breadboard


I just uploaded a short video on how to update the firmware with an Arduino:

               73 de OE6FEG

Here you are my QCX+ 20 band, works great, some videos:

RX QCX+ vs IC-756

Adjust PLL (reference frecuency)

Increase PWR throught LPF turns

Hi Jon

Very nice radio, unfortunately this class E RF amplifier is a “ghetto class”. You can improve the RF output by using this calculator

Class E spreadsheet

Mine, is not really a mod, is a brew from scratch

Brewing a QCX-SDR

Cheers and stay safe
Pascal, vk2ihl

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Nice tool Pascal thanks for the link! Something to play with during Lockdown perhaps :-s

Nice job!

73 de Paul G4MD