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QCX Mini (30m 1.07 FW) good power out, but harmonics in output - LPF issues??

 

Hello all,?

? ? ?I just completed my 30m QCX mini. I followed the alignment instructions and the tuning video on youtube and I am seeing ~4.8W @ 12V, and ~5.7W @ 13.8V. This was achieved by removing 2 turns from L3 and adjusting the windings of L1, L2, and L3. However, I am hearing a very harmonic-rich signal in my IC-746, so it seems like the LPF isn't working? I have verified that the enamel has baked off of the windings at the solder pads with my DMM, and I've inspected with a loupe and haven't found any obvious issues. I'm using the QRP Labs dummy load, and the Icom has the attenuator engaged and the RF gain is turned down; I have used this setup for testing other rigs in the past.? I can definitely hear multiple frequencies when I key down - it sounds very much like a dial tone.? I'm thinking of replacing the capacitors in the LPF section, although I have no real reason to suspect them other than that they are in the LPF. I've looked for anyone else that has had this issue but found nothing, so I am sure it's something I've done or failed to do.?

73 de Jeff NZ5G
 

I assume you do not have access to an oscilloscope?? Without a means of looking at the signal being produced it may be difficult to troubleshoot without guessing.? Sounds like you double (triple) checked the soldering.? Have you checked component values and proper placement?


Glory to God in the highest, and on earth peace, good will toward men. Luke 2:14
 

I am hearing a very harmonic-rich signal in my IC-746,
Jeff,

How are you monitoring? At least the TX and RX need to be connected to dummy loads.
Even like this signals can be strong and give misleading results.
The RX can be overloaded, signals bypassing the LPF may be received.
I find it is not easy to be sure.

73 Alan G4ZFQ
 

Jeff, before you do that ¡­. The sound you are talking about is probably because your 746 is too close to the Mini. It happens at my QTH too. You need to have someone else listen to you or listen to yourself on a WebSDR receiver. You will probably find that you have a very clear clean sounding signal. If you know someone with a spectrum analyzer see if you can use that. You will most likely find that the signal harmonics are well below the allowed limits. Good luck ¡­. Ron

On Tue, Dec 21, 2021 at 09:13 Jeffrey Krehbiel <jeffrey.krehbiel@...> wrote:
Hello all,?

? ? ?I just completed my 30m QCX mini. I followed the alignment instructions and the tuning video on youtube and I am seeing ~4.8W @ 12V, and ~5.7W @ 13.8V. This was achieved by removing 2 turns from L3 and adjusting the windings of L1, L2, and L3. However, I am hearing a very harmonic-rich signal in my IC-746, so it seems like the LPF isn't working? I have verified that the enamel has baked off of the windings at the solder pads with my DMM, and I've inspected with a loupe and haven't found any obvious issues. I'm using the QRP Labs dummy load, and the Icom has the attenuator engaged and the RF gain is turned down; I have used this setup for testing other rigs in the past.? I can definitely hear multiple frequencies when I key down - it sounds very much like a dial tone.? I'm thinking of replacing the capacitors in the LPF section, although I have no real reason to suspect them other than that they are in the LPF. I've looked for anyone else that has had this issue but found nothing, so I am sure it's something I've done or failed to do.?

73 de Jeff NZ5G

 

Thank you for all the quick replies!?

I've got dummy loads on both rigs, but the one on the IC-746 is internal to the MFJ-969 tuner... I think it was picking up a lot of extra hash, because I dug out my FT-817 and put an Elecraft dummy load on it, and the QCX sounds perfect! Now to get it in the case and on the air....? I've tested other QRP rigs like this, but this is the first time I've used that tuner and rig to listen... even my 817 sounded a bit rough on the bench in the 746.? Probably something loose or oxidized in there somewhere.

TNX es VY 73!!
Jeff NZ5G
 

Ron, I never thought of a WebSDR.... guess I was too focused on the bench LOL

73 de Jeff NZ5G
 

If you don't use a dummy load and a proper sampling attenuator connected to the local receiving instrument, you may be picking up "bleed" instead of the actual output signal.

Sometimes this will be a mashup of the output signal and signals from other parts of the radio in random phases.?In other cases (like I had happen earlier this week) you will be picking up the output signal from the dummy load, but in series with any noise voltage appearing between the ground side of the equipment under test and the ground of the receiver.

In my case, this was adding a nasty 60hz amplitude modulation to what was actually a very clean CW signal! I wasted quite a bit of time looking for the problem before connecting the receiver to a proper attenuated sampling point on the dummy load.

I've been bit by this before, but I always seem to think I can get away with running one quick test without a proper cabling setup!

It would be great if all dummy loads included a sampling tap!

M (AE0GL)
 

Yes? - I've been there and done that and got it entirely wrong. Last Christmas Eve, I came by an unbuilt QCX kit from about 2017. I built it up on Christmas Eve evening and had it working by midnight. Then, listening on my Kiwi SDR , and transmitting on a dummy load, I could see sprogies all over the place at harmonic intervals and elsewhere too. I was simply overwhelming the receiver and creating mirages.? I'm sure that if you built the LPF to the specs provided and have been able to tune it for good power output, you have done it right and your transmitter will be fine.

If you really do think something is amiss, get someone to check the harmonics? a couple of miles away when you are transmitting on an aerial. Or you could use one of those old fashioned wave meter things we used to build in the 1980s. They were a sort of crystal set with a variable capacitor, a suitable coil and a small meter. You could scan for the fundamental, adjust the meter sensitivity for full scale and then slowly open the vanes of the variable cap and look for peaks at the harmonics.?
 

This is the kind of meter I was talking about. Cheap and easy to build.
 

Jeff,

What you are hearing on the IC746 is the harmonics of the SI5351
which apparently is in the same room if not the same bench.

Does the 756 also hear it when the QCX is in RX mode.
Without a spectrum analyzer (TinySA will do) your searching in the
dark with your eyes closed.

Also not it the filter meets spec (minimum) and is -43dbc the signal
heard (harmonics)?will still be around -7dbm (+ or minus) and easily
heard on the pro.? Without actually measuring? you do not know.

Allison
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