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Too bad - I was hoping that L504 or L514 was broken, since they are so easy to replace.

To test L501, the 330uH choke, I would first test it for proper continuity through the circuit card by probing the devices it connects to.? That would be the top end of D502 and C504.? Those are the large-ish diode and capacitor along the sides of L501.? The top end of the diode is toward the edge of the circuit card, and it is the stripe on the capacitor.? This should show continuity (a few tenths of an ohm or so), and I would touch the coil while measuring, and apply a little pressure to move it around a bit to see if there is an intermittent connection.? Likewise with power on and in transmit mode, the voltage to ground from either of those points should be about the same.? If you look at the voltage on those points with an oscilloscope, you will see some saw-tooth-like variations at the frequency of the switching, around the DC value, with the capacitor side being somewhat more filtered.

I hope that shows something.? You can also check for continuity across L504 and L514 in case something is hidden there.

Stan

I noticed when I have EQ turned on, that I get choppy audio.? This happens when "Use EQ" is set to yes.? It persists while I am using the terminal or when I am out of the terminal or even when usb is disconnected completely, and after a reboot of the transceiver.? I am not sure if it is setting related or just an issue with the EQ functionality.

Jerry,

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When I look at the uBitx LPF, its structure is the same as that of a "shunt first" input Chebyshev LPF.? Does that mean it shunts the high frequencies to ground instead of a "series first" filter?? That would make sense to me as the capacitor in parallel with the input would tend to short high frequencies.? Or do I have that backward?

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There are some differences between the model and the HFSignals schematic.? ?C261 and C262 in the HFSignals schematic are "zero" values and not installed.? I also note significant distortion on the output waveform (across R1).? There is no LPF, so this may be the actual waveform.

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I attached the model with C261 and C262 feedback removed.? I also added the LPF arrangement for the v6, where all bands go through the 10m filter.? I did not change any of the other values upstream of the IRF510.? (the v6 has different feedback values in the emitter of Q90).

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Thank you for your knowledge sharing.

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73

Evan

AC9TU

Hans,

Adam,

I would not do that as the low-pass filter is designed to attenuate the harmonics of 20-meters so the second and third harmonics of 40-meters will exceed regulatory requirements.

Gary

W9TD

Hello,

I uploaded the charts of the simulation to

The simulation now creates all charts at once and is easily customizable.

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Now I can start with my experiments to modify the QMX20-10 to QMX17-6

I am aware of the fact that this simulation is not reality. But they give me the direction.

My first impressions:

- The BPFs are very broad?

- The 10m BPF could also work on 6m

- Very experimental: using the 15/17m LPF for 20m

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Vy 73 Peter?

I'm one of those people posting about 20.? I re-ran all three calibrations for 20 one at a time and have been able to make QSOs

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I'm also having issues with sensitivity on 12 m and 10 m.? I didn't know if it was band conditions or something else. I noticed dB levels being significantly low in between cycles.?

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Robert,

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did you test also other bands? Looking at your 40 m spectrum the signal outside the SSB chanel is only 15 ... 20 dB below the peak in the channel (around 7.152). Did you check the spectrum with Phase predistortion ON and OFF? Is there a difference?

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73 Ludwig

Since I loaded the 1.01.003 fw the sensitivity on 10 seems to have dropped considerably. A loud signal on my 705 is barely readable on the QMX.

Not sure why.

Evan,

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My guess is that this thread is looking at RF amps in general, and how to evaluate the voltages and currents that might blow the finals.

Curious, on revisiting that uBitx amp, it looks like a current mode topology, which would expect the LPF to short harmonics to ground.

I don't think the LPF's on the uBitx do that.

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The post you pointed to has a bad link to the LTSpice schematic pdf, that was corrected a few posts later in the thead:

The schematic for the uBitx is here:

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Jerry

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On Mon, Mar 24, 2025 at 10:56 AM, Evan Hand wrote:

I rather like the Musk approach to innovating.? The big, establishment space company, United Launce Alliance, tries to get it right the first time, spends twice as much as SpaceX, but takes forever and then ... blows it.? Multiple times, actually.? Musk's approach is let's build it, launch it, and see what breaks.? Fix that, launch again, it goes further before something breaks, fix it, launch again, rinse and repeat until you're successful.? ?He's made space affordable, brought internet and texting to places that would never have it, etc, etc, etc.? I'm sure the engineers and programmers who work at Elon Musk's companies have a whole lot more fun than those working at Boeing.

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No offense to my friends across the pond, but if you want lots and lots of innovation, you know where to find it...? ? :-)

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Best,

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Jim

K8DDV

Hi Adam,?

Im not sure. I’d have to check and see. I’m done with it for now. I tried to use it today and for some reason 10 meter sensitivity is not as good as it was previously. Not sure why. I could hear a station 30 miles north of me on a 705 connected to discone. On The QMX connected to the long wire I could barely hear that same station.?
I’ve ordered the mic kit from QRP GUYS and will try it when that arrives. I’m getting a lot of noise on the audio and it doesn’t sound right. I’m running the mic gain at 50 and compression at 5.?
It actually sounded decent when it was on 40 meters. Not so much now. It’s just a cheap Baofeng mic. I noticed that if I turn on the mic equalizer it’s completely unintelligible.

Also noticed during testing at 20 meters the readings are not even close to what they should look like. I don’t remember the setting at the moment but it was bad. I’ve seen others that have the same issue on 20 posting on here.

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Don.

Hello, I was trying to connect a electret mic to qmx, I applied external 3.3volts to mic input and it died, won’t turn on , doesn’t draw any current. Any help would be appreciated. Thank you.

It should not matter. The reverse voltage is less than either value, and the forward current is very short in duration. The 1n4148 is the best choice because of the junction's switching speed. I would go with the least expensive, most available device.

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73

Evan

AC9TU

On Mon, Mar 24, 2025 at 02:49 PM, Drew Riedle wrote:

I use the ICOM HM-219 hand mic

I also built an adapter for mine.? In addition I reworked one of the Retevis mics.? I'll see which works/sounds best, they are kinda comparable in size.

73, Don N2VGU

What settings does the 60-15 have for 40m under the terminal band config?? ?I never use 11m and wonder if I can just fill that slot with the 40m config??

As others have reported, I have lost half of the previous output power on my QMX+ after running the calibration.?

I am beginning to suspect that I lost one or more of the transistors in the final power amplifier.? I will do some more testing to verify.? I ordered some replacement BS170s just in case.

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The calibration routine appears to stress the finals and sort out the weak from the strong MOSFETs.

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--

Steve KA0SM

Cross-posting here in the hopes of Hans seeing it: /g/QRPLabs/message/141596

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Thanks.

I have four QDX's and I've decided to add a 1N4148 across L14 as many other have done here to add some protection against accidental high SWR.??

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Allison and others.... To be clear, I brought up this old thread to understand the discussion about the RD16HHF1 being a "16v max" device. I have no interest in the original question from another poster in 2021 in trying to put different finals in the QCX 50w amp, or get 50w out of the RD16HHF1.

? I'm exploring a small amplifiers for POTA/SOTA etc, and trying to learn along the way. That research is causing me to want to understand the datasheet limits, so I can understand max DC voltage for transistors used in the 10-30 watt amplifier range.?

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Jerry's comments are also of interest to my exploration of reducing QDX output power and finals.?

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Just wanted to be clear that I'm not trying to do anything magical! ?

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-Nate?

N8BTR