Re: QCX 20 low output 5w at drain 1/3 w at output #problem

Ronald Taylor

Dan, many have found that removing a couple of turns from each of the toroids L1-L3 can raise the power output of the QCX20. If you are sure you have DC continuity through all of them and getting some power output and all the capacitors are right, then I’d try that step next. Before you do it, recount the turns you have in there. Many people find one extra turn because they forgot to count all the wires running through the center of the core as one turn. At any rate all you have to do is lift one wire of each toroid and unwind a couple turns and resolder it without cutting. That’s easier if you have to add a turn back on to get it right. When it’s right trim the excess wire and solder it back in place. I’ve built or fixed for others several 20 meter QCXs and have only found one get close to 5 watts output. The average I’ve seen is about 3.5 watts. And that is plenty of QRP power on 20 meters and only about 1.5 dB less than 5 watts, or about 1/4 of an S unit. 

Good luck .... Ron

On Mon, Apr 20, 2020 at 16:18 Daniel Curtin KF4AV <danieljcurtin42@...> wrote:
My QCX seems to work fine on RX. On TX I get 5 w (on the internal meter)at the drain of the BS 170s but  at the end of L1 about .3 w out. The instructions indicate the LC circuit would be suspect I do have DC continuity through the Low Pass filter. As best I can see I have the right capacitors in the right places and the right number of turns on the toroids.

I just got a digital scope (my first scope) so I tried to signal trace. I list here P-P rf voltage, the scope gives lots of other info:

DC input is about 14 V, for these readings.

On both sides of C29 40 V (including input to L3)
On the output side of L3 20 V
On the output of L2 10 V
On the output of L1 and at the BNC connector 7 V

Am I looking at the right things? Could my toroids be too poorly wound (I have not wound any in years and never any so small.)

Any help appreciated. I am having great fun with this excellent kit, and already have learned a lot.

73 Dan KF4AV

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