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Another QMX Receive Issue Thread

 


QMX Low Band, bought pre-assembled.? Have not even opened the case to peek inside.?

I have made contacts with it, as recent as about a week or so ago, both CW and FT8.? Have had .014 on it.? This weekend I loaded .017 on it (after the fix) and all of a sudden it went deaf.? I think when it did was coincidental.? AGC on, off, variety of configs I've seen in threads here and just tried myself, no change.? I can hear the faintest signals sometimes, but swapping the cable between another radio where I can confirm W1AW is on 7/14.047.5 at +10 over, it should be blowing out my eardrums and I can't even hear it, only static.

So I loaded up diagnostics.? It looks like everything's right until I get to the audio/rf filter sweeps, and guessing the image sweep should be a little more pronounced as well.? These look VERY abnormal, while the LPF sweep looks fine as do all the voltage levels in diagnostics.

Put .014 back on it, factory reset, no change.? I haven't dropped it, as far as I know nothing irregular has happened.? I'm guessing I'm going to have to open this thing up, a task I dread doing hence why I bought pre-assembled, and not having assembled it I don't know much about its inner workings.? Any guidance based on what I've given?? I have a multimeter and a brave heart.? Also a good soldering iron and a steady hand but I reeeally don't want to have to use it if I don't have to.

Thanks in advance
Jason KO4NDP

LPFs look good


Abnormal?


PTT/TX, SMPS looks ok I think?




40m audio filter sweep (80 looks similar)... not quite right...


RF filter sweeps, not quite right....
 

Also, Rev 2 board (so the paperwork says), 12v, order?73417 fwiw, received pre-built assembled in case.

Disassembled and took photos of boards
 

The radio going deaf sounds a lot like this issue:

I don't see that mod on the board photos you posted.
 

I was JUST reading that and about to post that this issue sounds a lot like the symptom describes.? My unit shipped Dec 13, after the whole fiasco in Nov (Oct?) and I assumed it had the fixes.? Well, crap.? Any tests recommended to verify this is indeed the issue?? But it sounds like I may be replacing a fried "IC402 with a new 74CBT3253" as well as installing the?1N4148?

Symptoms are: lack of audio; running the RF Sweeps etc in the terminal tools shows nonsense low audio values. It is important to note that there are multiple possible reasons for no audio, most issues are attributable to improperly installed toroids, specifically failure to properly remove enamel and ensure a good electrical connection. However if you become certain that you have a loss of audio then this indicates that the parasitic oscillation (on a transmit) has destroyed IC402 and it will need to be replaced with a new 74CBT3253.

This modification is a preventative measure, it prevents the occurrence of destructive parasitic oscillation. If your IC402 is already damaged, the diode won't fix it; you need to replace IC402 AND then install the diode.?
 

Very similar problem as mine. suggest to check TxRx switch MOS.
 

Thanks Howard.? I read your thread and it did sound similar.? Mind telling me what you tested on the Q508 123 mosfet?? So many threads with FET checking and voltage confirming and many have different voltages and, man, it's hard to keep up with all of them for us non-engineers.

Also going to check John Z's recommendations:
Checking the bias voltage to the mux usually provides a good test of 3253 health. It can be measured at a number of easily accessible points, such as any terminal of T401, or on C406. It should be approximately 2.5V relative to ground. Any reading that deviates significantly from that, up or down, is a strong indication of mux failure.
Thanks again
 

Jason,

You can test that transistor easily with an ohmmeter.

With power off, measure resistance from gate to source, from gate to drain, and from drain to source. The readings should be 100K? ohms or more. Significantly lower values suggest a failed transistor.?

Good luck....JZ

On Wed, Feb 21, 2024, 12:50?PM Jason - KO4NDP <jason.turnage@...> wrote:
Thanks Howard.? I read your thread and it did sound similar.? Mind telling me what you tested on the Q508 123 mosfet?? So many threads with FET checking and voltage confirming and many have different voltages and, man, it's hard to keep up with all of them for us non-engineers.

Also going to check John Z's recommendations:
Checking the bias voltage to the mux usually provides a good test of 3253 health. It can be measured at a number of easily accessible points, such as any terminal of T401, or on C406. It should be approximately 2.5V relative to ground. Any reading that deviates significantly from that, up or down, is a strong indication of mux failure.
Thanks again

 

Thanks John

Cap 406 to DC ground is 4.65V.
Same with the T401 just to be sure.
So, out of proportion as you suggested.

Q508 BSS123 FET - G-D 4M ohm, D-S 490K, G-S 7M+.

Anything else I should consider checking? (and how :)? Thanks, and thanks in advance!
Jason KO4NDP
 

Jason,

I think you nailed it. The band switch mux is dead and the T/R switch transistor is OK.

Replace the mux chip and install the recommended diode modification at the T/R switch and you should be back in business.

If you feel insecure about doing that, Jeff Moore's repair service is a great alternative.

VY 73 JZ KJ4A?

On Wed, Feb 21, 2024, 6:42?PM Jason - KO4NDP <jason.turnage@...> wrote:
Thanks John

Cap 406 to DC ground is 4.65V.
Same with the T401 just to be sure.
So, out of proportion as you suggested.

Q508 BSS123 FET - G-D 4M ohm, D-S 490K, G-S 7M+.

Anything else I should consider checking? (and how :)? Thanks, and thanks in advance!
Jason KO4NDP

 

Will take a stab at it, chip ordered (and a bunch of spares - glad the shortage is over), got plenty of the diodes on the shelf, we'll see what happens in a few days.? I really appreciate it John.??
 

OK Jason!

A word of caution on installing the diode modification: An SMD diode (sod-323 body) is preferred over a glass body axial diode. The glass diode can be difficult to install without lifting pads on the PCB. Any attempt to form the diode leads while attached to a pad will likely lift and tear that pad. Ugh!

Wishing you success, JZ


On Wed, Feb 21, 2024 at 9:29?PM Jason - KO4NDP <jason.turnage@...> wrote:
Will take a stab at it, chip ordered (and a bunch of spares - glad the shortage is over), got plenty of the diodes on the shelf, we'll see what happens in a few days.? I really appreciate it John.??

 

John, got some ICs in and did the fix tonight.? If you wouldn't mind glancing over this and giving me some experienced feedback before I power it on again it'd be much appreciated.

Masked it off with some duct tape so hot air wouldn't affect much else.? Came off rather clean.? After this photo I added some flux and wicked a bit of the raised solder off.


Verified old to new and started tacking it on.? Looks decent from the top.




Until I get it around to about this angle and I see pin 1 especially is a little blobby but ok, nothing _looks_ touching.


But I go ahead with the diode which turned out good (imo).? But then I get another angle at the 1-8 pins and see it looks much different in that light.? Especially 1, 4, 8, and maybe 2.? Guessing it's worth going back and putting some more flux on and reflow.? Thoughts?


Thanks so much for your advice.
 

Lo and behold, of course the back cover didn't fit thanks to the diode, I knew that but didn't remember when tacking it on.? So I took that off and put another on closer down at board level.? Since I was in there with the iron hot I reflowed the IC.? Cleaned it up, put it back together, and powered it up.? Sounds coming through again, nice and clean.? Yay, happy night.? Thanks for the advice John!
 

OK! That's great news, Jason!?
Congrats on the repair and VY 73 OM.
JZ KJ4A?

On Fri, Mar 1, 2024, 9:09?PM Jason - KO4NDP <jason.turnage@...> wrote:
Lo and behold, of course the back cover didn't fit thanks to the diode, I knew that but didn't remember when tacking it on.? So I took that off and put another on closer down at board level.? Since I was in there with the iron hot I reflowed the IC.? Cleaned it up, put it back together, and powered it up.? Sounds coming through again, nice and clean.? Yay, happy night.? Thanks for the advice John!

Martin
 

I had the same problem and refitted the IC and installed the diode.
Despite the diode, the QM went deaf again after some FT8 transmissions. ??
 

Martin,

Check L503 to see If it has been damaged ( open circuit).

JZ KJ4A?


On Tue, Mar 5, 2024, 1:17?AM Martin <martin@...> wrote:
I had the same problem and refitted the IC and installed the diode.
Despite the diode, the QM went deaf again after some FT8 transmissions. ??
 

Hello - After upgrading to .017 my QMX quit receiving.? I've looked at the RF filter sweeps and they look exactly like the ones you posted.? I assume my unit has the same problem - darn!!? It's interesting that this seems to happen after upgrading/installing .017.? I had been getting/receiving calls on 1_00_014 but decided to upgrade to 1_00_017 due to the WSPR problems with earlier versions.? Should have left well-enough alone!? And my QMX is a pre-built unit and I'm not looking forward to sending it off to get it fixed. - Gary
 

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Try a firmware reset and see if it helps.

73,
Cliff, AE5ZA



On Apr 18, 2024, at 15:09, deckerg42 via <deckerg42@...> wrote:

Hello - After upgrading to .017 my QMX quit receiving.? I've looked at the RF filter sweeps and they look exactly like the ones you posted.? I assume my unit has the same problem - darn!!? It's interesting that this seems to happen after upgrading/installing .017.? I had been getting/receiving calls on 1_00_014 but decided to upgrade to 1_00_017 due to the WSPR problems with earlier versions.? Should have left well-enough alone!? And my QMX is a pre-built unit and I'm not looking forward to sending it off to get it fixed. - Gary

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