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Someone Please Help ME!

The QMX Mid Band model I assembled on Saturday also cannot resolve the loud RX noise.

I was able to receive the CW signal sent by placing the QCX-mini next to it. I was happy that the transmitting output was 5W.

Transmission with FT8CN was also perfect. But I couldn't go out to POTA on Sunday because only the reception was useless.

?At first I suspected the noise was coming from a mobile battery or something, but that wasn't the case. I tried hard to find a soldering mistake, but I don't know what else to do.

Is PCM1804 the problem? It's a gloomy Monday...

Mitsu JK1PDR

The most common receive/audio path issues from kit assembly, that are independent of transmit, are:

1) T401 - the trifilar transformer.? Make sure you matched the pairs properly, and all of the solder connections are good - sometimes the insulation doesn't get completely burned from the wires.

2) L401a/b - the receive filter transformers.? Same checks as above.

3) In some cases the PCM1804 chip has failed, cause unknown, but perhaps due to handling/ESD during assembly.? See /g/QRPLabs/wiki/37111

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It may be helpful to post screenshots of the "RF filter sweep", the "Image sweep", and the "Test ADC I/Q" tests, which may help narrow down the problem.? They are done on a per-band basis, just do one band that you are having the problem on.? Compare them to the Operations manual examples.

Stan KC7XE

Thank you, STAN
My QMX symptoms are:
 < loud noise and (nearly) no real signals when receiving >
seem to be very similar.
I'm starting to suspect PCM1804.

I also carefully checked L401a/b and T401 along with the surrounding capacitors. But I'll check it again.

and I would like to post images using PuTTY soon.

Thank you very much.
Mitsu JK1PDR

If you have an oscilloscope, you can verify by measuring at the inputs of the PCM1804.? See this post I just made for another op:

Stan KC7XE

The good news is that the LPF sweep looks great, and so does the diagnostics page.? So hopefully your transmitter section is all OK.

The bad new is that all of the receive related scans are bad, and generally show 'noise'.? This includes the RF scan, the IQ scan, and the image rejection.? Since these are all bad, it could really be anywhere along the receive path: L401, T401 (trifilar), and the PCM1804.

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I would start by examining carefully the trifilar toroid, and make sure you paired the wires correctly, and that they are all soldered in well.? Reheat the joints to make sure the enamel burned completely off the wires.? Same for L401A (it does 20M).? If you are very confident in those, it may be the PCM1804.? The only way I know to measure for sure is using an oscilloscope and signal generator as mentioned last time.? Or you could just replace the PCM1804 and see if the problem goes away (but that is quite a fine-pitched chip to replace, so take care and practice on something else first, or get someone else to do it for you).

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Stan

Good luck.? Let me know what you find.

On mine, L401a at first gave me an issue, on the hole near the edge of the board where the starting wire for both coils connect.? My L401a wire didn't get connected well there, and I ended up wrapping it around the L401b wire and soldering it to the pad to fix it.

Stan KC7XE

Just be careful not to forget to remove the CW key.

Mitsu JK1PDR

Dear Stan, Please give me some advice.
I tried sequentially inputting signals from 7Mhz to 21Mhz to My QMX using Tiny SA. I was able to receive everything.
So I think replacing PCM1804 should be prioritized rather than rewinding T401 or L401a/b. Is this idea wrong?

I want to minimize damage to the PCB.

Today, I asked a store in Akihabara-town in Japan to order PCM1804. There was no shipping charge, but it was little expensive.

Mitsu JK1PDR

What output signal level did you have your tinySA set to?

I think you are probably correct in replacing the PCM1804.? Just do so very carefully - there are lots of other parts very nearby.? I did mine OK, but I had to practice first by removing/replacing a similar-sized surface mount device on an old retired disk drive.? The practice taught me what would and wouldn't work with my particular equipment.

Good luck.

Stan KC7XE

The output level of TinySA is -60dBm.

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Stan, did you use heat resistant tape and cream solder when you replaced the PCM1804?

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I am trying to make the exhaust port of the heat gun thinner to remove and attach the broken ArduinoNANO chip.

The solder will melt in about 15 seconds. It seems like it can be done somehow. However, I think there is a possibility of failure.

The PCM1804 is scheduled to arrive in a week, so I will continue training until then.

Mitsu JK1PDR

I have an inexpensive hot air station, and I used a medium-sized nozzle, about half the size of the chip.

I did not have chip quick or solder paste, so I used more rudimentary means.

For removal, I first put flux along all the pins, and used copper desoldering braid to remove as much of the solder as would easily come off.? Then I heated it with the hot air gun, with the air flow turned down to a medium value, moving the air over just the chip and its pins.? It took a while, but through my magnifying glasses I could see the solder liquify, and I just 'scooted' the chip off with a set of tweezers.

I then cleaned the pads with more solder wick and flux, followed by alcohol to clean it up.? Then I put more flux along the rows of pads, carefully placed and aligned the new chip.? Using a very fine soldering tip and very fine solder (this is important!), I carefully tacked one corner pin to hold it in place, and then soldered the rest of the pins with a minimum of solder.? A few solder bridges formed between pins, but they were easily 'dragged away' after I had soldered the whole thing.? The hardest part was aligning the chip, and keeping it in place while I tacked the first pin.? The fine solder is a must with such tiny pins - I used 63/37 solder .015" (.38mm).

I understand using solder paste may be easier, but I didn't have any and don't know the technique.? I just learned off youtube videos.

Hi Mitsu.

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i like what you have done with your QMX build.

could you give a description of it?

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thanks.

Neil VK3ZAG

Thank you Stan.
Your explanation has given me a lot of hints and things to be careful about.

It's very helpful. I may encounter some unexpected troubles in my work, but I'll deal with them carefully.

Mitsu JK1PDR

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Which SMPS board was bad, for 5v or for 3.3v?

What revision of the QMX assembly, rev 1, 2, 3 or 4?

Early 5v SMPS boards have a 0.5w protection zener, later boards have a 5w zener.

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Anyways, great news that you have solved this!

Replacing the PCM1804 successfully shows that you have

some very good soldering skills.

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Jerry, KE7ER

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On Wed, Nov 27, 2024 at 03:37 AM, Mitsu TOMATSU wrote: