I see the lower than expected power is quite a common problem and the fix (also described in the build manual) is to remove a couple of turns from each of the inductors.? I have no doubt this works but I did not want to risk damaging the PCB with re-soldering a number of times during the cut-n-try operation.
Des,
Hans has suggested that all that is needed is to re-solder the coils onto the board, not get the wire through the hole again.
I've not done this with a QCX but often only one end need be removed, cutting the wire is not necessary until enough turns have been removed. One could even rewind a turn. The inductance of the unwound wire will not be significant at at HF.
I used the QCX as a signal source with a kitchen table lash-up to measure resonance and therfore the value of each inductor with sufficient accuracy so that they matched the values quoted in the build manual.
After the last round of posts about this I started trying methods of using the QRP Labs VFO to do this.
I attach one of the working schematics. Resonance indicated by peak reading. And an idea of a crude test rig.
I was stumped however because many will not have a junk box with 2% components in it.
I was wondering if the actual LPF capacitors could be used, they are already good spec, would any variation be compensated while testing?
Your idea of using the QCX VFO sounds good.
There are various web pages like which save brain damage from the maths.
73 Alan G4ZFQ
The result was an instant increase in output power from
the much less than 2 Watts I started with to 5 Watts as measured on the QCX. Now my power output-v-supply Voltage curve is close to that shown on page 129 of the manual plus I only had to desolder/replace the inductors ounce. So very happy :-)
While measuring the inductors, removing turns and re-checking the values a number of times the thought struck me that since the QCX already has an in-built signal source it would only take a small number of additional components (and software) to make some sort "peak detector" such that the resonance of each PA stage inductor can be checked while building. This (in conjunction with some simple math and a pocket calculator) would permit the measurement of the inductor values prior to fitting in the kit and also add a usefull bit of test gear to boot. The only "critical" component is a capacitor of reasonable close tollerance to resonate the inductors with (I used 820pF +/- 2%? for resonances around 3 to 3.5 MHz) but the value of the capacitor or the frequency are not that critical so long as both are known in order to work out the value of inductance. It might add a few 10's of cents to the cost of the kit but perhaps well worth it. So, what do you think?
