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Hi all,

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I know there have been several threads on this, but I'm at a loss for how to solve this particular problem. I haven't had any real problems with TX or RX with my QMX but the SWR readings are always around 1.15 to 1.30 or so across all bands with some jumps up to almost 2 at the high end of 15, 17 and 20 meter bands. The dummy load looks perfectly flat on all bands on a nanoVNA and I've tried scraping and heat soaking all of the connects on the SWR bridge without much change to the readings.

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Is this a case where I need to dismantle and rebuild the bridge to resolve the issue? Anything else to try to diagnose the problem before taking it apart?

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Thanks in advance!

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Greg,

N1TR?

Greg, I presume you are reading the SWR in the Hardware test -> Tune SWR screen?? A few notes:

0 - The SWR meter in the QMX/QMX+ is not perfect.

1 - It is very common for the hundredths digit of the SWR reading in the QMX to rapidly change between various values in a range of four or five hundredths, and if that range nears the edge of the hundredths boundary, the tenths digit will also change between two adjacent values.? I think the reason for this is that the firmware routines that read the fwd/reflected voltages are reporting non-averaged values (or maybe very short-term averages).

2 - Within the QMX, the lines from the SWR bridge travel a ways across the board to get to the processor A/D pins, where they are read.? As they traverse the board, they inherit some RF noise, which causes some amount of reading variation, even though you are using a fixed dummy load.? This noise is most apparent on the upper frequency bands, so you may see higher variations on higher bands.

3- The QMX is not a perfect 50-ohm source, like your nanoVNA, so the bridge won't always balance perfectly with a perfect 50-ohm load.? The PA and filters for each band are designed for 50-ohms, but variations in components and in your construction technique cause variations in that impedance depending on which band filter you are using, and how constant the impedance of that filter is across its frequency range.

4- If I recall correctly, the 20, 17, and 15m bands share a single LPF.? So I would look at improving that LPF if you have poorer SWR performance on those bands, and you have already fixed T507.

5 - T507 even when fabricated and installed perfectly, has a performance range.? As frequency goes up, its efficiency goes down.? So on the top bands you can expect to see a higher internal SWR.? I don't get better than about 1.2:1 on 10m, and about 1.5 or 1.6 on 6m on my QMX+.

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You shouldn't be seeing values of 2:1 or 3:1 into a dummy load on any band.? So if you do, there is something to fix, either internal to your QMX or externally.? (In my case I had to replace the connector adapter I was using for my dummy load - it was faulty).? But if the value is at or under 1.3:1, just ignore it and enjoy using your QMX - that level of SWR is insignificant for any use of the QMX, whether it is a true reading or caused by some of the issues above.

Stan KC7XE

Hi Stan, thanks for the notes.

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Though it seems that I've mucked something else up while trying to troubleshoot this. While doing test SWR sweeps, I seem to have blown or otherwise busted something, as I now have no power output AND no receive, just the open circuit receiver noise.

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It's drawing the normal amount of idle current but it's acting like there is no antenna attached.

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Do you know if this is a symptom of blown finals? I'm not sure why the receive chain is also affected but if there's an improper short to ground from the finals perhaps it can be affected as well.

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Greg

I'm sorry to hear that.? It's never fun for things to get worse.

I don't think blown finals would have any impact on receive, unless something else blew also.

A couple of questions to hopefully help troubleshoot:

- while you were running SWR sweeps, did you have your dummy load attached?

- do you have SWR protection enabled with level set at 3?? this shouldn't matter for swr scans, they are done at the 50% voltage setting (1/4 power), but will cause a visible indication in the diagnostics screen if the SWR goes high.

- if you try to transmit (T in the diagnostic screen), what do the transmit parameters show?? (for voltage, bias, power, swr)?? and do any of the power supply values change much?

- and is your QMX perhaps in practice mode? that will disable the transmitter, so the diagnostics, SWR sweep, LFP sweep won't work

- is this a QMX, or QMX+? (just wondering because on the? QMX+ there is a JP501 jumper to the output BNC, and if it isn't securely in place, both RX and TX will be affected.

- I would do an RF sweep, SWR sweep, and LPF sweep on one of the affected bands, and see if they look as expected.? This can narrow down the source of the issue.? If you post screenshots, I'll look at them with you.

- Remember that the default SWR sweeps go across a large range of frequencies, below and above the band.? It is common on the upper bands for the right-hand part of the SWR sweep to go up to 2:1 or 3:1 or higher in an unpredictable way, even with a dummy load attached.? This is due to the filter issue I mentioned previously: the QMX filters don't maintain a perfect 50-ohm impedance far outside their bandwidth - and they also have a significant attenuation outside the bandwidth, so the fwd/reflected voltages are very small, and cause less accurate mathematical calculations of SWR.

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I'm not sure if any of that was helpful, but I hope so...

Stan KC7XE

Thanks Stan for the helpful items to check.

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Since my last post, I've done some additional diagnostics and it seems that something in the receive chain in particular has died. Transmit into a dummy load in the diagnostics screen all look great, basically the same output power that i had previously, so i can rule out an issue with the finals.

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I'll run down through your questions in order:

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- Yes, definitely had the dummy load securely attached the whole time.

- Yes, i actually run the SWR protection lower at 2

- Everything looks green with diagnostics transmit into the dummy load, power output looks great across all bands (this is a mid band unit)

- Practice mode is in fact off.

-This is a QMX

- As far as other tests go:

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? ? ? ?RF Sweep: This looks bad; everything is at least -60db across the sweep on all bands

? ? ? ?SWR Sweep: This looks ok, around 1.15 or so across all bands, basically the same as what i had seen prior to whatever broke here

? ? ? ?LPF Sweeps: looks great on all bands
? ? ? ?IQ tests: maybe a little bit of the expected shape for 60m and 40m. the rest look very bad, either flat or just noise for both channels

? ? ? ?image sweeps: some (15db) rejection on 60m and 40m, the rest have no rejection whatsoever

? ? ? ?Audio filter sweeps: also mostly noise except for 60m and 40m

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Not quite sure where to start with debugging, but seems that the receive chain upstream of the SWR bridge is where things start to go haywire given the ok transmit and SWR sweeps.

Any sage advice given the results above?

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Across the board results for 15m for reference attached:

Audio filter:

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RF filter sweep:

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Image sweep:

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SWR Sweep:

LPF sweep:

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ADC I/Q test:

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I think it's also worth noting that everything else seems to be working fine as well, no issues with control via terminal and the keyer and side tone work just fine in practice mode. I'm currently suspicious of the dreaded PCM1804 failure or a problem at or around Q508.

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Greg

N1TR?

See the wiki for a test procedure for the PCM1804; you will need an oscilloscope and a signal generator that can put out a sine wave at about 10mVpp (which is S9 +60dB or so). ?/g/QRPLabs/wiki/37111

That test can ascertain whether the receive signal path is working properly up to the point of the PCM1804 inputs.

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Another failure possibility is your trifilar transformer, if it was disturbed in any way, or the BPF toroids - they have a single input point for all bands.? Either of these could have a cold solder joint that just happened to stop working.? And there have been some failures of IC402 or IC403 (I had one), but in my case it was a side effect of an over-voltage event on Vcc.

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So those are good places to look.

Stan KC7XE

I was looking at some other threads and just checked the voltage on one side of C406, which turned out to be 4.85 volts. Sounds like this points potentially to an IC402 or IC403 failure?

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Greg

N1TR

Ahhh... getting closer to the cause.? The voltage on the top of C406 should be 1/2 Vcc, set by the resistor divider of R401 and R402.? This voltage biases the RF signal through the trifilar transformer so it sits evenly in the middle of the 0-5V span, creating an even quadrature signal into the inputs of IC403.? If the bias voltage is getting pulled up to Vcc (and the resistors are still properly in place), then that would mean either the output of IC402 or the input of IC403 is shorting the voltage up to Vcc.? It's probably more likely for an output to fail shorted up, so maybe IC402?? You can probably measure such a short to Vcc with power off to verify which device it is.

Stan.

Ok, having taken everything apart, the band selection logic getting to IC402 looked good and no obvious short to Vcc through the chip with power off.

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However IC403 DOES show continuity between pin 16 Vcc and pin 9 to the trifiliar based on the schematic.

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So, I guess now the question is why did IC403 fail, and, subsequently what all components do I need to replace to bring the receiver back to life.

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This is my first time debugging the QMX to this extent, so any further advice would be appreciated! Everything so far has been extremely helpful.

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Thanks,

Greg

N1TR?

I think this is some kind of bizarre internal failure of IC402.?

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The voltage on c406 depends on the band selection. It's is 4.8v on 60m and 40m, then zero / short to ground on all other bands. It seems that this all happens internal to ic402.?

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I not sure what other points to check before attempting to replace ic402.

I should also note that the 3.3v logical signal inputs for band selection seem to come in just fine from the microcontroller to ic402 and I can measure them going high/low on both pins on ic402 as I cycle through the band selections.

Going to continue this in a more appropriately named thread for better visibility.

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Good diagnosis.

I think you just need to replace IC402.

Do you know what QMX board rev you have? (label is just above the square white qrp-labs logo)

IC402 used to blow often on the rev1 boards due to some strange tx leakage into the rx path.? This was resolved in later board revs by adding D516 to the design.? So if you have a rev1 board, or a rev2 board without the D516 fix, it would be best to update it with a D516.? But if you have a rev 3 or rev 4 board, perhaps something in tx may have slipped by if you had an abnormal condition, or if your D516 or Q508 failed for some reason.? So I would also look at them carefully.? If your Q508 (tx/rx path switch) is failed/shorted source to drain, upon transmit you will kill IC402 again.

Stan

On Wed, Mar 19, 2025 at 04:31 PM, N1TR wrote:

The voltage on c406 depends on the band selection.

Greg, I had a QMX for repair on my workbench some days ago. I saw a similar behaviour. Both IC402 and IC403 were damaged. I desoldered T401 from IC402 and saw values at T401 different to the status when connected but away from 2.5 V. So both IC were not ok. After replacing both the RX was ok.

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I think the damage was caused by a broken Q508. The full signal from the TX was going to IC402 and IC403. Please check voltages at Q508 before changing IC402 and IC403.

RX: Gate at 3.3 V and Drain at 0.x V

TX: Gate at 0 V and Drain at "12 V" (voltage close to the supply voltage).

I don't know why Q508 was broken.

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73 Ludwig

Thanks for the feedback, Stan!

I have a Rev 4 board, built from a kit, and i've confirmed that D516 is in place and seems to be behaving like a diode :)

I did also confirm that Q508 shows very high resistance? / no continuity from source to drain, that's a good tip.?

At this point i don't think Q508 is the culprit here, but i'm still not sure what killed IC402....

Greg

Ludwig,

Thanks so much for the insight!? I did just now take a closer look at Q508. The voltages are as you describe, with Gate at 3.3V and drain at low / zero voltage during RX, and Gat at zero, Drain at nearly supply voltage which was 9v in my case, the actual voltage measured was 8.8 volts or so, which i think is likely fine?

That being said, i still don't know what killed IC402 and potentially IC403.

My plan is to order multiple replacement ICs and double check IC403 bias voltage as you describe with IC402 removed.?

Do you (or Stan)? have a recommendation for sourcing the replacement chips? Directly from TI or Digikey / another reseller?

Thanks again,
Greg

I got my replacement chips from Digikey.? The chips are inexpensive, and Digikey has a $7 shipping fee which arrives to me in less than a week.? You can order tons of parts and supplies for that same minimum shipping fee, so think of everything else you may want to add to the order...

Stan