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First, a reminder to all:? Please DON'T CONNECT ANY EXTERNAL VOLTAGE TO THE QMX/QMX+ MIC/PADDLE INPUT.

For electret mics, QMX/QMX+ already has an internal 3.3V bias voltage hooked up to the mic input (ring terminal) of the connector.

And indeed, only electret mics, or other high-impedance mics should be used.? There is also an internal DC-blocking capacitor on the microphone signal.? The 'tip' and 'ring' terminals of this connector also hook directly to processor input pins (for the paddle dit/dah) in addition to the processor microphone input - so external voltages improperly applied may cause damage.

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Now, on to your problem (QMX won't turn on, per the message title):

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Looking at the schematic, I don't see how connecting 3.3V to any of the terminals of the mic input jack could cause damage such that the QMX would fail to turn on.? Unless there was some other factor, like perhaps an ESD event.? Or if the voltage applied for some reason was 6V or 9V or 12V, it indeed could have caused damage to either the processor or to the 3.3V power supply - this could happen, for example, if you were using a external supply for the 3.3V bias, and turned it from off to on while connected - some supplies can spike significantly above their set voltages when turned on, or spike well below ground potential when turned off.

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Since your QMX won't turn on, somehow either the power supply or processor was damaged - hopefully only the power supply, since it is much easier to fix/replace.? The protection diode in the 3.3V supply could have failed if there was over-voltage.? Or over-voltage could have fried some input pins on the processor, preventing it from starting up.

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A good measurement to make is the current draw while you have the PWR_ON button pressed (but don't press it too long unless you have a current-limited power supply set to about 250mA).? This can help tell what kind of fault is present.

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Then I would remove the bottom cover, and (with no power attached), and measure the resistance between GND and the three voltage busses (12V, Vcc, and Vdd).? You can find the contacts for those busses on the soldered SMPS connector pins.? (Someone just today or yesterday posted a message with a nice pdf identifying all of the voltage pins on the bottom of the QMX card).? I? suspect the resistance between Vdd and Gnd will be a low number of ohms (rather than a few K-ohms).? Such a low reading would indicate a failed component or short.? Next step would be to disassemble the top cover to get to the SMPS cards and remove them.? Then re-measure the Vdd/Gnd resistance on the main board - if it is still low, the processor is likely damaged.? If it is now high (hopefully), then the failure is on the SMPS card, likely D109.? You can use the nice procedure written in the wiki of this list to independently test the SMPS cards, or just order new ones from Hans and replace them.

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Good luck in quickly finding this, and ask if you have any addition questions.

Stan KC7XE

Without DC power you have no bias? for the diode switching so its high

loss for RX and tx.

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Allison
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Post online only,?
direct email will go to a bit bucket.

I've purchased the 50-watt amplifier kit, to be built for 80m, and bought assembled the 40m and 20m amps from Ray, VA7SK (very nice solder job, btw); on 40m it certainly makes a noticeable difference of 6dB -10dB on reports and it's much easier to reach Europe.

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Question: Can I leave the amplifier inline between the QDX and the antenna if the amplifier is off?? I'm not seeing that addressed in the assembly manual nor could I find an answer by searching the messages here.? I think so based on the schematic, but I don't trust my circuit reading skills enough to risk the QDX or the amplifier.??

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Thanks,

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Jim

K8DDV

With those settings turned on, you will be getting about 20dB lower of a signal from your mic to the audio path (compared to when the setting is turned off) - so it may be reacting with other downstream settings that require a minimum amplitude.

Crank up that 'global gain' parameter so the output of the EQ when 'on' will be similar to when 'off', and the choppiness should go away.

Yes, checked 80m also and it looks the same.

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I’ll have to look at phase predistortion, it is currently whatever default is, on I assume?

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73,

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Robert, WA2T

Hi Jim,

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L501 or C504 failure can cause these symptoms. ?Make sure to have about 2 ohms continuity on l501 at the pcb pads. ?It’s a little difficult to test C504 in circuit. ?These big tantalum’s fail though. ?You can remove it and test it on the bench.

It’s hard to diagnose the exact problems in the bias circuitry. ?There is interaction everywhere. ?But if you look around with a scope, you can often get a clue to what’s failing. ?Most often it’s one of the transistors, the coil being broken or the big cap failing.

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if you can’t find the culprit in that bias circuit, certainly send it along to me. ?

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73
Jeff Moore
W1NC

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