My QMX luck is not too much better on this build than the last one. Well at least this one boots up and it actually transmits great. Got good RBN report. Receive is another story. When I first completed it, I was getting really loud noise from speaker and S-meter shows strong signal that fluctuates with Dummy load attached.?

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Here is what I have done so far. I checked T401 by pulling it out and verifying I didn't have anything crossed. I didn't.

I replaced the PCM1804 although that was with the one on the other one that the processor failed so not 100% certain it is good but should be.

9K ohms for VDD and about 50K ohms for VCC to ground.

I have attached my hardware test results for 20M as most are about the same for all bands with the areas of concern. The only other two are ADC I Q test as the displays are different but then again similar and RF Sweep.

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Any other troubleshooting suggestions would be greatly appreciated. I do have a scope, signal gen and a TinySA if any of those can aid.

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thanks and 73

Bob

WV7W

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Yikes.? You have some really bad stuff going on in the receive path.

T401 would be the likely culprit, but you checked that.? Make sure each of the leads really has the enamel cleared off.

Also check L401 - all receive signal goes through it and the related capacitors, so check those carefully.

Did the behavior change with the replaced PCM1804?? If not, it may not be the issue.? I had a PCM1804 that made horrible noise when it was cold, and was fine when it warmed up.? Replaced it and all was well.

Good luck.

Thanks Stan,

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Yes, T401 is good and good tin on all leads.

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I did a careful check around L401 and the associated capacitors C401-C404. Everything looks good there and all resistance/continuity checks of L401 are good. Again, looks like all leads are clear of enamel. I scrape off the enamel prior to soldering on all toroids.

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What I am looking for is any other tests I can run either voltage, resistance or signal flow checks with a scope, etc. to help narrow this down.?

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As for the PCM1804, I think things are better than they were with the previous one. With the original in, about half the time there would be no audio on power-up and the other half would be a loud noise, now it is just loud noise and the S-Meter to match it. I suppose this one could be bad as well but would like to know of some ways to validate whether it is good or bad by doing some checks.

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thanks,

Bob

If you have a signal generator that can put an S8-S9 signal (abt -73dBm) at the antenna jack, you can use that to trace the receive signal path with a scope.

I used my Elecraft XG3 for this, tuned to a 30m frequency.?

Tune your QMX to the same frequency.?

If you measure at the input pins 4 and 5 on the PMC1804, you should see a sine wave at 12.5kHz, about 300mV p-p.? You have to use AC coupling on your scope, since these pins are biased up.? Same for the 4th and 5th pins down the other side of the chip.? If any of these does not have your sine wave, you need to trace back (via the chips and T401 and L401) to find out why.? Note that the signal goes back to RF on the other side of the mixer chips, so it will no longer be at 12.5kHz, and it will be weaker since it is before the buffers.?

If all 4 of those pins do have a reasonably good looking sine wave at 12.5 kHz, then the problem is with the PCM1804 or with its outputs or supporting solder connections.

Just piggybacking I have the exact same symptoms on a mid-band kit I recently built as well. Following for any info.?

Thanks Stan,

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I assume I can check pin 4 or pin 5 to ground so ground clip on ground and probe pin 4 and/or 5 for 12.5KHz sine wave? I can use my TinySA and set for -73dBm and see how it goes. Will give that a try.

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Bob

Am I also correct that the Op Amps are the mixer?

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thanks,

Bob

Thanks Stan,

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All 4 pins have nice clean 12.7KHz sine waves at 630mV p-p so is there anything I can check on output of PCM1804 to ensure that is the problem and not other supporting components.

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If it really looks like PCM1804, I will pull this one again and replace with a new one.

Correct on the probing setup.

The op amps are buffers to strengthen the level of the signals, and present all 4 differential/quadrature signals to the PCM chip.? That's why you need to check all 4 - they are pairs of differential signals, offset by 90% phase.

The mixer is the prior chip - it is a multiplexer chip, with the mixing frequency driving the select pins to make it into a mixer to produce the in-phase and quadrature IF signals.

And the chip before that (via T401) is a band select chip which selects the correct bandpass filter from L401.?? If you look at the assembly document for the QDX, it has a nice description of all of the parts of the receiver/transmitter, which are almost identical to those in the QMX.?? Hans didn't repeat that info in the docs for the QMX.

Thanks Stan,

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I will look back through the QDX instructions. I guess I remember that when I built one of those. I will see if I get anything from that.

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I am going to order a couple more PCM1804 chips. I suppose I could have damaged this one in removing it from the other radio or it might have died when the processor did.?

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Anything is possible.

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is there any outputs signals I should look at on the PCM1804?

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thanks,

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Bob?

Here is an interesting side note. Without a signal applied, get the loud weird sounding noise, with signal applied, no noise at all.

On Mon, Nov 4, 2024 at 02:46 PM, Bob WV7W wrote:

is there anything I can check on output of PCM1804

It is a bit hard to check since all of the output is digital - I'm not sure of a good way to do it.?? You should see 'bits' coming out of pin 15 (bottom corner) and a clean clock on the sclk pin.?

You can check the other input pins (bias, etc) to see if they look good.

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Special thanks to Stan Dye. I got a new PCM1804 today and swapped it out. Used ChipQuik to remove old one and solder paste and hot air rework station to install new one. Ensured I used ESD strap while handling and very happy to report that the QMX now works as advertised.?

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Stan, I learned a ton about signal tracing around the 1804 from you. Important thing of note: Even though I had good 12KHz sine waves on all 4 pins and what looked like data coming out, the 1804 was still the issue. I could not verify that it was a good data stream. I think the real proof is the bad IQ representation in the hardware test.

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73

Bob

WV7W

Thank you for the kind words.? I'm glad I could use my newly-acquired knowledge to help someone.

Stan KC7XE

Some more on the QMX mid band high noise.

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First let me describe the issue. Turn on the QMX. Very loud static/noise. After about 10 seconds, the noise becomes intermittent, and then goes away entirely.? At that point the QMX works perfectly.? The noise is about 20 dB louder than the ambient band noise. Until today, I was just going to live with it.

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Today, I did a SOTA activation with my QMX. The noise continued for several minutes. I thought, uh-oh, I'm screwed. The noise then stopped, and all was well. I did a few on/off cycles to see what would happen. The noise would last about 10 seconds.? There is one key difference between today, and all previous uses of the QMX. The QMX was in my backpack for about 2 hours. The outdoor temperature was 45-50 degrees. This is the coldest I have used the QMX.

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Questions:

1) Is this the same noise others have been hearing ?

2) Has anyone found it to be something other than the PCM1804 ?

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73,

Steve, N2IC

I had a similar temperature-related issue with my QMX.? It would have loud 'digital' noise for several seconds, then be perfect.? And I took it out on a POTA activation, and the noise lasted longer at startup, then was fine.? But after the sun went down and it got cold outside, the noise returned and didn't go away, and I had to qrt.

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Such noise can be caused by a cold solder joint somewhere.? So at home I took the bottom cover off, and turned it on and let it warm up to proper operation.? Then I tested it a bit by using canned air to briefly spray different parts of the bottom of the circuit card, hoping to find the location of a cold solder joint by getting it cold. [Canned air is very cold when it first comes out; at work we used to use a freeze spray for this purpose].? When I sprayed the PCM1804 chip, the noise immediately returned, and it would go away quickly if I warmed it up with a hot air gun (set to a moderately low heat).? I repeated this several times, trying to reduce the area I was cooling/heating to determine the source of the error.?? After a few tests, I could cause the noise to come with a short blast of air right on the PCM1804, and it would go away again if I touched the chip with my finger for a few seconds to warm it back up.? So in this case indeed it was the PCM1804 chip.? I replaced it, and the problem completely disappeared.

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After I replaced the chip, I realized that I never tried to just reflow the solder on it, in case one of the pins had a cold solder joint.? I should have done that, so I don't really know whether it was a failed chip or a soldering issue.? Could have been either thing.

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Stan KC7XE

Thanks, Stan,

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Using the refrigerator, I have confirmed that it is heat-dependent.? At 40 degrees, it took ~10 minutes to settle down. At 70 degrees, about 20 seconds.

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I wonder if there is a batch of QMX's with improperly soldered PCM1804's ? The thought of replacing the 1804 seems daunting, but I may look into a low-cost rework station.

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73,

Steve, N2IC