QCX QRP Portable Transceiver Build

This compact “QRP” or low power radio kit from QRP Labs in UK was too good to resist.  At $49.00 USD, it packs a lot of functionality into a small package.  The radio is based around an IQ phasing receiver for CW (morse) and an Class E efficient transmitter.

The transceiver includes built in alignment software and hardware, a WSPR beaconing mode, CW decode and internal memory keyer.  See the QRP LAbs site for the details.

The software alignment is most impressive as is the well thought out menu structure.

The radio looks ideal for lightweight SOTA operations.  As the kit is supplied without a case, some thought is needed to package it efficiently.  This plastic enclosure from Jaycar fits quite nicely with a little space to fit a 3 cell Lipo pack.  There is still some work to do around fitting the two pushbuttons, volume and tuning encoder.

A shaft extender will be used from the encoder shaft through the top panel.  The volume control and two push buttons will be top panel mounted and cabled back to the board via a connector.  I had to cut down the supplied board stand-offs to about 8mm long with a Dremel so that the display was the right height to sit behind the top panel.

Nice fit of QCX and battery

Top view without controls

 

 

 

 

 

 

 

 

 

RHS panel with BNC and paddle socket

LHS with earphone socket

 

 

 

 

 

 

A shaft extender for the rotary encoder was turned up on the lathe.  I could have probably bought one, but this one is small and light.  A bush from an old pot might be placed on the top lid of the case for a bit more support of the shaft.  It doesn’t feel to bad though.

Shaft extender

The controls have now been fitted without problems.  It was nice that there are connector pads for off board controls.

The radio looks a little off balance with placement of controls now, probably not helped by the extra space above the display.  The DC screw terminal connector had to be removed as it interfered with the side panel of the enclosure.   A barrel type 2.1mm DC bulkhead socket is fitted to the side panel for power in.

A bezel of some sort is needed for the display to protect it and improve the appearance.  For internal batteries, the 3 cell Lipo fitted ok, although I have settled on using 3 x 18500 round cells instead.  These cells take up a bit less space and are more robust.

Controls Fitted

 

It is still work in progress.

The RF power output had a tendency to immediately go to about 1.5 watts and then slowly rise to just under 3 watts from cold.  A quick re-ptt saw the power go about 3 watts.  If the rig was rested for 10 seconds or more, the power would go to 1.5 watts and slowly rise again.

As suspected, it turned out to be capacitors in the low pass filter.   One by one the capacitors were replaced and the RF output behaviour checked.

There was no change when the outer 2 x 180pF (C27+C28) caps were changed, but a noticeable change when one of the inner 390pF (C25) was changed and then perfect when the other 390pF (C26) was changed.
I changed each cap with a 250V ATC ceramic chip cap.

The output power is now very stable and the supply current has dropped.  Output is 3.5W with 13.8V supply.  Some squeezing of the turns may improve this.

I also experimented with the 30pF capacitor from the drains of the finals to ground.  The efficiency went up slightly without it.  I put a 30pF trimmer in its place and found best efficiency at almost no mesh of the trimmer.

1500mAh 18500 cells

This photo shows the 3 x 1500 mAh 18500 lithium cells fitted.  A balance connector is connected to the cells.

After running a WSPR test, the power output and current consumption became erratic.  Sometimes it would draw 1000mA and transmit only half a Watt and other PTTs it would draw 450mA and put out 4 Watts.  Q6 was also getting very hot.

Q6 was replaced and it completely fixed the problem.  Q6 is on the limits of its ratings.  It looks like Q6 has been replaced with an MPS751 is later versions of the kit.

 

I had noticed that the first PTT after power up resulted in a high current draw and low power output. This behaviour was completely repeatable after every power up.  Others on the forum (Olgierd SQ3SWF) also had some issues with RF output changing from PTT to PTT as I had previously experienced.  Olgierd had noticed some RF or possibly oscillation on the gate of Q5 when the radio was transmitting low power out and consuming high current.  I put the scope on my QCX and noticed that also, on my first PTT high current, low power out condition there was some RF on the gate of Q5.  It seemed that Q5 was shunting RF into the front end in this condition.

A 4k7 resistor was placed from +12V to the drain of Q5 and in my case the first PTT issue was completely resolved.  About 30 power cycles were tried with no recurrence of the issue.

4k7 resistor on back of PCB

See photo for placement of the resistor.  Also note the SMD low pass resistors that were replaced.

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5 Responses to QCX QRP Portable Transceiver Build

  1. David says:

    Hi Glenn, this is great to see you have the QCX already and are working on the enclosure. Looks like I will not be receiving mine until late October. There are a few here that I know of that are awaiting their kits too so this is a very popular kit. Looking forward to assembling it and getting on air.

    73
    David
    VK2JDR

  2. Paul Taylor says:

    Well done Glenn. You might be the first VK to build this new kit. Keen to get your impressions on how it sounds on the shack bench and on a summit. A great design. 73 Paul VK3HN.

  3. vk3yy says:

    Hi Paul, not sure on who else has one except for Andrew, VK3JBL. So far very impressed with the software functionality.

  4. Andy says:

    nice job!
    i am waiting my kit. while I think about placing it in the case. Your information was interesting to me.
    73! RW9RN

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