Re: QCX-20 ... low output power (~1.5 W) #qcx #20m


Ronald Taylor
 

Rob, I went back to some old notes and see that I did exactly what you have done in turns removal on a couple of QCX20s. 2T off of L1 and L2 and 3 turns off of L3 for a total of L1=14T, L2= 15T and L3=13T. In all cases, my spectrum analyzer showed that I was still within FCC guidelines for harmonic suppression. Certainly your results could vary, but that is what I saw on mine. Hope that helps.The shift in power from 300 mW to full output and back really sounds like a continuity issue in the lowpass filter somewhere. Or also possible is L4 continuity. All of these have the dreaded possibility of not enough enamel removed as you alluded to. Hope you find that… 73 … Ron


On Sat, May 30, 2020 at 7:19 AM Rob French (KC4UPR) <kc4upr@...> wrote:
So, I finally got around to working on this.  I appear to have solved my overall power output problem, but I do have a couple of questions:

(1) Did I take off TOO many windings?
(2) NOW what's going on?

Re: (1)... I proceeded by taking 1 winding at a time off from L1, L2, and L3, in sequence, checking power after removing 1 winding from 1 toroid.  Using a fully charged 14.4V LiPo battery (so ~16V to start), when I got to 2 windings removed each from L1 through L3, I had ~3.5W.  So I removed one more winding from L3, and am now at about 4.5W using the ~16V supply.  I figure I'll leave it at that, BUT, my question is, have I removed too many windings, especially from L3?  Am I going to end up generating a bunch of spurs/harmonics?  (I do not have a way to check for this.)

Re: (2)... The other thing that's happening is that it seems like about 30-50% of the time when I key the transmitter, it only shows ~300mW output!  Sometimes when this happens, it seems like if I immediately re-key it, it will pop back up to 4.5W.  It seems like the first time I key it, it is about 300mW, but it also happens later, and seems to be random... but almost always, if I see this happen, and I quickly let up on the key and then re-key it, it pops back up to 4.5W.

I have verified whenever this happens that I have good RF power measured across the drains of the BS170s.  After that it gets a little trickier.  Trying to use the built-in RF watt meter, I have touched various pads from the top of C29 (works fine, good power), all the way out to the output pin of the BNC jack, and I never seem to get any good power reading, even when I am getting good output on my external watt meter!  So I am assuming I'm having contact issues between the probe and the solder joints, probably flux.  This is obviously making troubleshooting difficult... any suggestions would be appreciated.

Regardless of my measurement issue, any ideas as to what could be causing this varying power issue?  I will say that it's pretty binary... power output is either ~4.5W, or it's ~300mW.  "Seems" like maybe a solder joint issue on the toroids, and maybe just the vibration of re-keying is causing a good connection to be made??? But when I soldered those coils back in, I held the soldering iron on the joint for quite a while too ensure the enamel burnt off, and I also have tried to reheat each joint on the coils to ensure a good connection.

Are there any capacitor issues that could be causing this?

What about anything with the keying circuit and/or L4?

Thanks!
-Rob KC4UPR

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