Dear group,
I have built a QCX 30m (#1229). Building went straight forward. I could peak the input filter nicely with the trimmer in the mid position. However output power was a bit low: 2W with 13.6V after the diode. The LPF already had all toroids with 1 winding less than per manual. So I decided to remove one more winding. This did not had any effect at all ... ! Ian, G4GIR, noted me on the fact that a 30m QCX might as well work on 40m. It appeared the power output was 3W on 40m. The power actually almost linearly increases when decreasing the frequency.
So I decided to rebuild the LPF on a separate piggyback PCB using high quality silver mica capacitors.
Toroids are wound with 0.4mm wire, 1 winding less than indicated in the manual. Caps are Cornell Dubilier silver mica (~�4 each ...). This resulted in a schoolbook filter behavior as measured with a miniVNA, see attached file.
After building the piggyback PCB onto the QCX, I ended up with 3W output on 30m. That is a clear improvement. However, now I have also 1 W more on 40m => 4W out, so I still have some unexplained attenuation on 30m compared to 40m.
Having ruled out the LPF now, does anyone have a clue what component could be responsible for this behavior ? I can only think of C29 or C30 (500V ceramic disc replacements are on the way). L4 is already rewound with 0.5mm wire so I really expect no issue there ...
Are more builders seeing this behavior ?
Any suggestions welcome.
(P.S. you can easily QSY out of band by programming a preset with the desired frequency in menu 1.x, and then recall the preset and write it to the VFO)
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Regards/73,
Enno, PF5X
Enno Korma, PF5X