QCX Mini -> uSDX Mod

Hans Summers (G0UPL) has released a true Artwork Masterpiece, the QCX Mini. Hans was friendly enough, to support a non commercial party like us, and made the uSDX Mod as easy as possible.

If you don´t have a QCX Mini -> Buy One !!!

This Site is about the Modifications needed, to turn the QCX Mini into a uSDX (with AM/FM/CW/SSB and DSP Filters )

But let me warn you: This modification is nothing for beginners, because of very tiny parts involved, and the Danger of accidentially damaging something else on the (SMT) pre-populated Board of the QCX Mini. There is some nasty 0603 Size SMT Part soldering involved.

The basic Modification was negotiated between Hans (G0UPL) and Guido (PE1NNZ). Hans has come up with the Daughterboard solution (which is provided with every kit -> Break-Away PCB inside the LCD PCB) and added some 0 Ohm resistors for easiy disconnection of un-needed parts of the circuit.

Guido did the first modification in Hardware, and provided me with the steps required to modify it. While Guido did the mod, beginning with a fully built and functional QCX Mini, I found it easier to start with a new Kit. This Instruction will be for building (Monoband) uSDX from a brand new QCX Mini Kit.

What Parts are needed for this mod ?

  • Following (SMT) Parts in Size 0603 (0805 can be used with mimized space for soldering)
  • 2x Resistor 1k – Tolerance 1%
  • 7x Resistor 10k – Tolerance 1%
  • 2x Resistor 82k – Tolerance 1%
  • 3x Capacitor 100nF
  • 2x Capacitor 10nF
  • 2x Capacitor 1nF

If you leave out T1, additionally those parts will be needed:

  • 2x Resistor 1k (THT – not SMT)
  • 1x Capacitor 10nF SMT, Size 0805

For Serial resonance Class E Modification additional parts are required. See Table in the related section below.

Our suggestion would be to start with populating the tiny uSDX Daughterboard. See it as a test run, if you can deal with those tiny parts. This way you´ll create no Damage. While trying to solder it, you can always decide to build the original QCX Mini.

There will be some optional steps involved along the way. Depending of what you want to end up with, you will need to make some choices.

Let´s start with populating the Daughterboard

Picture shows designators of the parts on daughterboard. See the Circle Cutout in the lower left as orientation (to make room for the electrolytic capacitor on the mainboard)
Start with 1k resistors on Pos. R253 and R254
Continue with 10k Resistors on Pos: R204, R249, R250, R251, R252, R256, R257
Solder 100nF Capacitors on Position C239, C240, C258 (Shown in Picture: Capacitors Size 0805 in Comparison to 0603 Resistors)
Finish Daughterboard by soldering 10nF Capacitors on Position C242 and C252

If everything is alright, you should be able to measure the following with your Multimeter:

  • GND-DVM=6.6K
  • DVM-AREF=6.6K
  • DAH-AVCC=10K
  • GND-AUD1=7.6K
  • AUD1-AREF=7.6K
  • GND-AUD2=7.6K
  • AUD2-AREF=7.6K
  • GND-I=High Ohmic
  • GND-Q=High Ohmic
  • DAH-DVM=High Ohmic
  • GND-SIDETONE=High Ohmic

Now you need to decide if you want to continue or not….. Last chance to bail out without any damage….

You didn´t quit ? Great !

Next Step is removing some (tiny SMT) Parts from the Main PCB:

Remove the 0 Ohm Resistors R244, R245, R246 and R247 (Top Side of PCB) :


Remove Resistors R53, R54, R56, R58 an R59 (BOTTOM Side of PCB) :


Next step is a little more challenging, because we need to exchange some SMT Parts.

Remove R7 and R10, Replace them with 82k Resistors:

R7 and R10 removed
R7 and R10 replaced by 82k

Remove C4 and C7, Replace them with 1nF Capacitors:

C4 and C7 removed
C4 and C7 replaced by 1nF caps

Next Step ist to remove C32, unsolder C31 and install it in Position of former C32 :

C32 and C31 removed, Former C31 waiting to be soldered in Position C32
Result of this Step: C31 empty, and former C31 now in Position C32

Then we start with the (simplified) build according to QCX Mini Assembly Manual

If the Link doesn´t work, because it was replaced by a new revision, use this link, scroll Down and get yourself the latest Version of the QCX Mini Assembly Manual. Please read it, as there are many steps, that are important for the end-result. If you ignore them, you might end up with something that cannot be undone !

From here on I will only comment the Deviations from the QCX Assembly Manual:

Changes for uSDX Mod:

Step 3.6: Leave out this 47 nF cap. It is not needed for uSDX)

Step 3.11 can be skipped, not needed for uSDX

Step 3.15 and 3.16 can be skipped, not needed for uSDX

Additional changes need to be applied, opposite to Manual:

Skip Step 3.2 (Transformer T1)

Skip Step 3.10 and 3.18 (Caps for Input Band Pass Filter)

Instead: replace it by 2x 1K Resistors and 1x 10nF Capacitor

Install 2x 1k Resistors from Top Side (on T1 Footprint)
Solder in 10nF SMT Capacitor on the Bottom Side (on T1 Footprint)

Skip Steps 3.7 / 3.8 / 3.9 (Class E related Capacitors)

Skip Step 3.24 / 3.25 / 3.26 (Inductors for parallel resonance class E)

Instead we´re going to install a serial resonance Class E Network:

Class E Network will be changed. L4 will be a choke with high inductance. Use different Toroid FT37-43 for Best Results and wind on 22 Turns. This will result in about 200µH. This is the same for all Bands !

The other components will be changed into this Pattern with Band dependend Values.

  • C30 is Band dependend
  • C29 is fix
  • L3 is Band dependend
  • C in Pos L2 means: We place this capacitor INSTEAD of L2
  • C25 and C28 are Band dependend
  • L1 is Band dependend
  • C//L1 means: solder a capacitor across pins of L1 (parallel to L1)
C30 (pF)68043030020040
C in Pos L2 (pF)18601240940660470
L3 (µH)1,851,230,970,640,37
L3: Turns T37-2 (RED)2117151210
L3: Turns T37-6 (YELLOW)2420171411
C25 (pF)180012001000680470
C28 (pF)180012001000680470
L1 (µH)1,020,620,450,320,22
L1: Turns T37-2 (RED)15121087
L1: Turns T37-6 (YELLOW)181412108
C//L1 (pF)620390300220150
Table with Band dependend Values. Gives you also amount of required Turns for different Toroids.

Capacitor values that cannot be bought, are usually generated by paralleling 2 Capacitors. I personally use 0805 SMT NP0 Capacitors, rated for 100V and solder one on the Top Side, and the other on the Bottom Side. For Example: To get 1860pF -> place 1500pF parallel to 360pF. Be creative…..

In any case, we need to finish the Build with Installation of the uSDX Daughterboard:

Blue show direct connections Daughterboard to Mainboard, Red shows wires to connect Mainboard to Daughterboard.
Blue shows direct connections: Daughterboard to Mainboard, Red shows wires to connect Mainboard to Daughterboard.
In my case, I used „break away“ jumper headers. This also ensures enough Distance to the Mainboard. Otherwise you can use cut-aways from parts. But be careful, not to short anything !
Daughterboard soldered in Place
Connect the Wires according to Layout above. I to I, Q to Q and DAH to DAH

Now, you´re ready to program the uSDX Firmware.

The Process is the same as for updating the firmware of the QCX.

See this Document for the Details.

This is the uSDX Firmware for QCX Mini WITH 25MHz TXCO

This is the uSDX Firmware for QCX Mini with 27MHz Crystal

And the end result should look like this:

Note: VOL Pot is not in use ! It´s just in there to cover the hole in the housing 😉

Thanks to Hans (G0UPL) for making this possible

Thanks to Guido (PE1NNZ) for his Software and figuring out the Details of the Modification.

73 Manuel; DL2MAN

EDIT / Changelog:

10 May 2021:

Option 1 was removed, as there was no internal way to adjust Input Bandpass Filter of Original QCX with uSDX Software. In my personal Opinion, for the uSDX there is no real benefit of T1 and Input BPF, so why encourage you to build it ?

For Multiband, QCX Mod is not really suitable, because it cannot be built onboard. You would need to skip all the values from table, and instead bridge L1, L2 and L3 (to pass RF out to Antenna Jack unfiltered) and provide EXTERNAL Class E and LPF Board for every Band. This will not be covered here, as it would lead way too far….. Since we are now using serial resonance class E, this external LPF is no longer working. If you want the uSDX all the way (Multiband) I suggest to build a sandwich.