Hi,
I recently completed my QCX 80 kit, followed the instructions to the letter and even measured the resistor/capacitor values prior to installing them. The kit worked first time.? Unfortunately I did not measure the inductor values prior to fitting in the PCB. Because of the s-p-r-e-a-d in characteristics with the inductor cores my QCX suffered a lower than expected output power. It still worked OK but it "niggled" at me that the PA did not meet expectations so I decided the only way to get peace-of-mind was to fix it.
Reading some of the past posts on this forum 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. So having a reasonably well stocked junk box I took a spare set of T37-2 cores and wound the number of turns specified for the QCX 80 (L1 25 turns, L2 27 turns, L3 25 turns and L4 24 turns) but this time around 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.? 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?
73,s
Des (M0AYF)
