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QMX Hi Band Noise Problem

 

Okay, let's try this again. I Just completed my Rev 3 Hi band QMX. Rx sensitivity is okay and Tx power out is very good on all bands. Probably all needs a bit of tweaking, but pretty good so far.

However, there is constant noise present on receive on all bands, CW or digital, with antenna or dummy load, battery or power supply. It varies with volume setting. My Lo band QMX (Rev 1) with same attachments has no noise.

Attached is a recording of it on 20m FT8 frequencies. This is representative of all bands, all modes, DL or antenna. I am wondering where to start for troubleshooting. Any suggestions?


Observing waterfall in WSJT-X shows noise as well. 3104.jpg picture shows 20m, 18m, 15m, 12m, & 10m from bottom to top. This is with rig attached to dummy load and cycling through from 20m thru 10m. The other picture (3106.jpg) is on the antenna again cycling through from 20m through 10m. Each band has two cycles of FT8 data.?

Since the noise varies with volume, I doubt the output amp (5532) is the problem.?Similar reasoning for the DAC (4334).?Although I don't think the CPU is the culprit, I don't think it can be completely ruled out.?This would point to the noise likely coming before the D/A and audio amplifier sections.?So that leaves RF path, IC402/IC403 (3253), IC405/IC406 (4562) and IC407 (PCM1804) or a bad discrete component somewhere. I swapped both SMPS from my good working Lo band QMX, but no change in noise, so I doubt power bus issues.?

Next, I need to figure out how to troubleshoot this issue. I need some suggestions as to how to go about it.?
?

Dan
de NM3A at mail dot com



--
73, Dan? NM3A
 

Dan,

Take a look at this thread and see if it applies to your situation:
/g/QRPLabs/message/119153

JZ KJ4A
 

John,
Makes sense, but how do I make sure? Is the only way to replace it and see?
--
73, Dan? NM3A
 

Dan,

If you have access to a scope and a signal source, you should be able to replicate the beat note test I suggested to Martin. It was helpful to him in pinpointing the issue to the stereo ADC.

JZ?

On Mon, Mar 18, 2024, 11:32?AM Daniel Walter via <nm3a=[email protected]> wrote:
John,
Makes sense, but how do I make sure? Is the only way to replace it and see?
--
73, Dan? NM3A

 

Thanks. I do have a scope and I can use my XG3 for a signal source. I'll see what I can do.?
--
73, Dan? NM3A
 

OK Dan. GL! JZ


On Mon, Mar 18, 2024, 12:31?PM Daniel Walter via <nm3a=[email protected]> wrote:
Thanks. I do have a scope and I can use my XG3 for a signal source. I'll see what I can do.?
--
73, Dan? NM3A

 

Can't get a clean signal - old scope problem, I think. Anyway, inputs and outputs of IC405 and IC406 look very similar. I'm going to go with PCM1804 being bad. LM4562 is out of stock at DigiKey and Mouser anyway.?
--
73, Dan? NM3A
 

Dan,

I advise that you try again to get some confirmation that you have isolated the problem before replacing? the stereo ADC, as every repair or mod intervention poses a risk of damaging the radio.

JZ KJ4A?



On Mon, Mar 18, 2024, 2:03?PM Daniel Walter via <nm3a=[email protected]> wrote:
Can't get a clean signal - old scope problem, I think. Anyway, inputs and outputs of IC405 and IC406 look very similar. I'm going to go with PCM1804 being bad. LM4562 is out of stock at DigiKey and Mouser anyway.?
--
73, Dan? NM3A

 

Thanks for the heads up. I will keep working on it.?
--
73, Dan? NM3A
 

JZ,
Well, I am getting nowhere. All I see is noise, which is there whether I measure something or not, so I wonder if my old Tektronics 2246 has issues, although the 1kHz test measures fine. Not sure where to go next, short of just replacing chips or sending it off for repair ?


Interestingly, the rx sensitivity still seems very good, while the noise just keeps getting louder and more constant.?
--
73, Dan? NM3A
 

Dan,

It's possible that your scope and/or probe(s) are dogging you.?

If you are skilled and capable of SMD repair, maybe the best thing is to simply charge ahead with a stereo ADC change-out.

If not, Jeff Moore is very capable and has a Sterling track record.

JZ

On Tue, Mar 19, 2024, 12:40?PM Daniel Walter via <nm3a=[email protected]> wrote:

JZ,
Well, I am getting nowhere. All I see is noise, which is there whether I measure something or not, so I wonder if my old Tektronics 2246 has issues, although the 1kHz test measures fine. Not sure where to go next, short of just replacing chips or sending it off for repair ?


Interestingly, the rx sensitivity still seems very good, while the noise just keeps getting louder and more constant.?
--
73, Dan? NM3A
 

Yes, I've heard nothing but good news about Jeff. While I am not an expert, I have done SMD replacement successfully and believe I can do this one. The chips are on order, so I will do that next.?
--
73, Dan? NM3A
 

OK Dan and GL!...JZ


On Tue, Mar 19, 2024, 1:33?PM Daniel Walter via <nm3a=[email protected]> wrote:
Yes, I've heard nothing but good news about Jeff. While I am not an expert, I have done SMD replacement successfully and believe I can do this one. The chips are on order, so I will do that next.?
--
73, Dan? NM3A

 

One more piece to the puzzle. I managed to lift the Vcc leads to ICs405&406. The noise was unchanged. After resoldering those leads, the signal came through as before, so no harm (I don't think, anyway!) Just another point towards IC407.?
--
73, Dan? NM3A
 

That's a strong indication, Dan...JZ


On Tue, Mar 19, 2024, 4:18?PM Daniel Walter via <nm3a=[email protected]> wrote:
One more piece to the puzzle. I managed to lift the Vcc leads to ICs405&406. The noise was unchanged. After resoldering those leads, the signal came through as before, so no harm (I don't think, anyway!) Just another point towards IC407.?
--
73, Dan? NM3A

 

JZ,

Well, I got the PCM1804 today and installed it. That cured all the noise problems. So apparently I got a bad chip from the factory. Second one at least, as Martin also had this issue. At least I can't quite see how I would have ruined this chip without taking out others along the way.?


Thanks for your help in troubleshooting. Now just need to tweak the BPFs and LPFs to optimize things.?
--
73, Dan? NM3A
 

Picture?


--
73, Dan? NM3A
 

OK! Nice job on the repair, Dan!

I have seen other ADC failures occur, just a small few. Of all the components that connect to the 5V distribution, the ADC is the most vulnerable to overvoltage excursions. The analog front end gets damaged, leaving the digital portions (which run on 3.3V) still sort of functional.

In the worst cases I've come across, the ADC chip actually becomes very warm.

JZ



On Thu, Mar 21, 2024, 3:58?PM Daniel Walter via <nm3a=[email protected]> wrote:
Picture?


--
73, Dan? NM3A

 

Maybe IC407 needs its own Zener protection.?
--
73, Dan? NM3A
 

Dunno, Dan.

There are a lot of QMX out there now and this specific failure remains at quite a low level. Yes, that's no comfort to you as you had one of those failures ;-(

It is also a little unnerving to see three such failures in a week. Incoming
parts defects may also be part of the story.?

JZ


On Thu, Mar 21, 2024, 5:33?PM Daniel Walter via <nm3a=[email protected]> wrote:
Maybe IC407 needs its own Zener protection.?
--
73, Dan? NM3A

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