QCX Mini -> uSDX Mod – DL2MAN´s HAM Radio Page

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:

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

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:

I´m providing 2 Options from here on:

OPTION 1: Stay in QCX Style, Parallel Resonance Class E Circuit with Input Transformer

OPTION 2: uSDX Style, DL2MAN Serial Resonance Mod, No Input Transformer.

Option 1 is safe to work. More then 10000 sold QCX´es work that way. That can´t be wrong.

Option 2 is a short cut. No complicated T1. It usually has higher Efficiency then Option 1, needs only 3 Toroids in Total (including L4) Might be a little more sensitive to Broadcast interference.

General changes for uSDX Mod: (Option 1 and Option 2)

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

The Rest is according to Manual for OPTON 1

In case you want OPTION 2: 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)

Band	80m	60m	40m	30m	20m
C30 (pF)	680	430	300	200	40
C in Pos L2 (pF)	1860	1240	940	660	470
L3 (µH)	1,85	1,23	0,97	0,64	0,37
L3: Turns T37-2 (RED)	21	17	15	12	10
L3: Turns T37-6 (YELLOW)	24	20	17	14	11
C25 (pF)	1800	1200	1000	680	470
C28 (pF)	1800	1200	1000	680	470
L1 (µH)	1,02	0,62	0,45	0,32	0,22
L1: Turns T37-2 (RED)	15	12	10	8	7
L1: Turns T37-6 (YELLOW)	18	14	12	10	8
C//L1 (pF)	620	390	300	220	150

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…..

For Multiband, 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. This will not be covered here, as it would lead way too far…..