The radio is a Ver. 1 QMX plus that in general performs exactly like I expect.? It does not have the GPS option installed, and there is no battery in the clock battery slot.? The radio is used for CW only.? As the title suggests, I have an approximate 1000 tone on only 12M and 6M bands.? It is internal, in that I can remove antenna and it's still there.? It does not change in level or tone while tuning around the band.? It appears to de-sense the receiver a bit, as a signal must be pretty strong for me to be able to copy it on 12M.? There is normally a 'beat' frequency between the received signal and the tone when I can copy someone.?

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I have run the I/Q sweeps in Putty, using a raspberry pi computer.? The tone is present with or without the computer attached.? ?The I/Q sweeps look very nice on the lower bands, and begin to deteriorate as the frequency goes up.? They are still showing the two sine waves, but are a bit distorted( not a perfect line of x's ).? This deterioration is also noted on 10M and 15M ( to some extent ) and the sweeps for 12M and 6M are not suspiciously worse than others.? 10M works flawlessly, as do the other bands.? The remaining filter sweeps look good, with some tweaking needed possibly, but totally functional as is. ?I have a set of QMX HB and LB radios to compare symptoms, and the HB QMX does not have the 'tone' on 12M when swapped into the system.

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I am curious what path to take troubleshooting this issue.? I did not find anything in the archives that addressed this strange behavior.

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Thanks & 73

Dick
W4PID

This is indeed curious.? The QMX/QMX+ does have some birdies at specific frequencies, but these are only heard when tuning to the exact frequency.

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Your problem sounds like there is a self-oscillation present somewhere in the receiver chain for those two bands.? Check the band configuration screen for which LPF and BPF each of these bands uses, then inspect the windings and solder connections closely for the related toroids, and verify you put the correct capacitors in place.

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The I/Q plots sound fine - they do naturally degrade somewhat on the higher bands.? But as long as they are 90 degrees phase shifted, and both are of the approximate same magnitude and no significant DC offset, then they are good.? They also should have a reasonably large amplitude, e.g. +/- 30K or so, rather than some small numbers on the left axis.? Small numbers would indicate an attenuated input signal.? Different DC offsets indicate some other problem.

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I had a QMX+ (rev 2) I recently repaired that had a similar symptom. ?A tone was present on some bands but not others. ?There was also a bit more noise than usual. ?This is all with a dummy load attached. ?The A/D sweeps all looked good. ? I replaced the PCM1804 A/D converter chip and it completely resolved the issue. ?

I’m not saying this is your problem. ?But it’s an idea to try.

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--

73
Jeff Moore
W1NC

I have superficially similar behavior on 6m (QMX+ rev1).

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But I have not gotten around to troubleshooting it since I don't use 6m too often.

Replacing the PCM1804 is beyond what I would normally consider as a try-and-see experiment but it may come to that.

Please share what you find out.

Thanks.

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73, Mike KK7ER

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I have tried the dummy load with no change in symptoms.

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Mike, your FT8 plot at least gave me a hint of how to 'see' the tone, HIHI.? I had to install WSJT-X on my little Raspberry and set it up ( piece of cake ) and now I can see mine, and it looks a great deal like yours.? Only on two bands HIHI.? Actually with the high end test using WSJT-X software, I can detect a tone on 10M as well.? But it's way down in the noise.? I may do some quick soldering around the filter parts, etc.? but the traces all look good on the bands in question so a bit of heat might rule out a bad connection.

I think the 1408 is the issue.? I am going to order a small quantity to see if I can change it myself.? On this radio it's pretty much out in the open, so should not be an impossibility.? There have been several notes on the soldering process to change out the 1408 so I am going to give it a try.?

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I will report back when i find the issue.? It may take a while as I have to get components here, then find time to concentrate on the fix for a bit.? I am *very* slow on stuff like this.? It took me 7 months to build the QMX+ HIHI.

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73

Dick

W4PID

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Same problem here.....? QMX+

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6 meters FT8 waterfall

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Phil.? G4JVF

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make that an 1804 ?

I can hear the approx 1khz tone on all bands of my QMX+ v1.

Loudest on 12M and 6M, but to varying degree on all others.

Barely perceptible on 160M, but there.

It is not a 100% constant tone, there are small almost regular interruptions, as if digital and the breaks in the tone perhaps related to interrupt cycles(?). Beyond my knowledge base here…

Anyone else gone down the PCM1804 route as Jeff mentions and got a cure??

My radio appears to be working fine in all other respects.

I have built three QMX (mini) and not had any problems at all.

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Rowan.

G0KLH.

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Maybe a missing / broken / not connected C around the PCM1804? There are 10 capacitors.

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73 Ludwig

Is this an issue related only to Version 1 of the board???

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Ludwig, I was very careful inspecting the board, but it's perfectly possible that is the situation.? I will reinspect and make sure I can locate all 10 caps.??

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Thanks & 73
Dick
W4PID

Nevermind.? ( Thanks, Gilda )?

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I see Jeff had one that was Version 2.??

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Ill go back to my cave and ponder while the parts get here.

73
Dick
W4PID

On Fri, Jan 10, 2025 at 05:19 AM, Richard Hattaway wrote:

Ludwig, I was very careful inspecting the board, but it's perfectly possible that is the situation.? I will reinspect and make sure I can locate all 10 caps.?

Richard, some days ago I read a post about a not connected R or C. Visual it was ok. But electrical there was no contact. You have easy access to all C, IC407, VDD and VCC from the top side of the PCB to check continuity.

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73 Ludwig

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Any update on this? Did you find anything? My QMX+ has this tone clearly present on the upper bands, strongest on 6M. The PCM1804 was replaced by Jeff, W1NC. The tone is very noticeable with a dummy load attached. Band noise makes it much less noticeable when I connect the antenna.

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The tone is actually at 750Hz and does not change pitch if I move the filter center up/down.? Using an audio spectrum analyzer, I looked at the op amp outputs (inputs to the ADC)? and the tone is not there.

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There was also a weaker tone that? sounded like digital keying. I tracked that to the GPS module. I put an inductor in series with the supply (VDD and VBAT) to the GPS board and knocked that down. This signal COULD be seen on the op-amp outputs.?

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Right now the tone is not a big problem but I would like to fix it if I can.? I have inspected the caps and they look OK but I am not sure how to see if they are actually connected properly.

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Tony

AD0VC

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I forgot to mention a couple of things:

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1. As others have noted, disconnecting the USB cable stops the tone. There is a delay of 5 (ish) seconds when reconnecting the USB before the tone starts.

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2. The place to look for the tone in the audio spectrum is around 12 KHz +/- 750 hz. The tone from the GPS was visible there, bouncing up and down. With the

GPS removed that goes away. Nothing else is visible there while the 750 Hz tone remains present at the DAC output.

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Since the USB just connects to the processor, the only paths I see are via the power connection to the ADC (or the DAC??)? or in the firmware creating the samples being sent to the DAC.

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Again, my PCM1804 has been replaced.

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Tony

AD0VC

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Hi Tony,?

To answer your question, no progress here.? I had another issue ( or the same issue with more symptoms HIHI ) with my QMX+ in that as I was doing some torroid tuning, something went 'snap' and the RX quit completely.? The snap noise I believe came from the headphones.? There is no evidence of anything really bad on the board, no smoke, no warm chips etc.? The TX chain is still fine, Putty traces look good on the TX side, but RX is dead.? My QMX HB and LB have been carrying the load here, I'd love to get the QMX+ running but lots of other issues have kept that from happening.??

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Your tone is at 750 which may make it a totally different issue than some of the others mentioned in this thread.? I loaded WSJT-X software to see the tone, and it was squarely on 1000 Hz, and had some residuals up the band every 1000 Hz.? I will post here again as I make progress.

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73

Dick
W4PID??

I also forgot to add, the tone is there if I have the computer attached via USB or not.? My computer is a raspberry pi.??

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73

Dick
W4PID

On Wed, Jan 29, 2025 at 07:56 AM, Richard Hattaway wrote:

the tone is there if I have the computer attached via USB or not

Same here, Dick.

I even took the rig outside just to be sure I was not picking up something in the house.

The new PCM1804 just arrived from Digikey.

And my rev 1 QCX Mini taught me how to replace SMD audio op amps of similar size.

Hopefully, this one will go more smoothly than that (where I ended up lifting a PCB trace in the process)!

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73, Mike KK7ER

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