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Testing for a dead QMX board

 

I'm absolutely gutted about this.? Having bought a Rev 1 board of the QMX when they were released, I finally finished it tonight.? I've built the QCX and QDX, and found this one the hardest so was very happy? to see it finally come alive. All was working fine - rx & audio - but the right hand (tuning) rotary would only go down in frequency, not up.?
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I traced it back to a short between the middle pin and the far right pin, re soldered, and fixed. Putting it all back together I must have dislodged some undetected (I checked it all with a DMM before first power up) whisker on the power board as it was pulling ~250mA before I'd turned it on.? The MCU was hot to the touch before I yanked the power.? I found the short and fixed it, but now the QMX won't turn on.? There's correctly no power draw in the off state anymore and pressing the left encoder draws 3mA, but then nothing.
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In the "off" state Q103 on the power board is showing 8.93v.? Q105 comes to life with 8.93v when the rotary on button is held.? I'm getting nothing on IC101 on the other power board.
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Is there a way to check if the STM32 is bricked???
 

"I'm getting nothing on IC101" means what? No 8.93v going in?? No 3.3v going out?
My guess is the latter.
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Use your dvm to check resistance from the 3.3v rail to ground.? Should be several Kohms.?
I'm guessing it's under 100 ohms, due to a short inside the processor, caused by an over-voltage event on the 3.3v rail.
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Assuming a low resistance to ground on the 3.3v rail, I suppose you could try lifting
the 3.3v pins on the processor, see if that removes the low resistance on the 3.3v rail.
Or you could take a current limited bench supply set to 3.3v and 300ma max, use that to drive the 3.3v rail and see
if the processor still gets hot.
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Swapping out the processor is a tough job.?
First off, you need a processor pre-programmed from Hans to accept his encrypted firmware.
Jeff Moore has a stock of those, and a lot of experience in swapping out processors.
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Jerry, KE7ER
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On Tue, Feb 18, 2025 at 06:27 PM, Nathan W wrote:

In the "off" state Q103 on the power board is showing 8.93v.? Q105 comes to life with 8.93v when the rotary on button is held.? I'm getting nothing on IC101 on the other power board.
 

Hello Nathan,
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Sorry to read about your experience.? I can sympathize with the same thing that happened to me.? In my case, the STM32 was fried.? I started removing the easiest-to-replace ICs connected to the 3.3-volt line, testing every time I removed one.? It came down to only the STM32 left when I called it a lost cause.? The shipping and replacement costs get very close to the price of another kit without the case.
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I bought and built another QMX kit, being careful when testing it out of the case support.? The mechanical tolerances are very tight, and it is easy to cause a short when testing.? I put tape over the end of the power barrel connector, as that seems to be where the short occurred.
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73
Evan
AC9TU
 

Thanks, Jerry.
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Between gnd and vdd on the main board, with the SMPS removed, I'm seeing .5 ohm.?
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Unfortunately, swapping out the MCU, even if I had a pre-installed bootloader, isn't something I have access to the equipment for.
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It'll have to be repurposed as a weight for throwing antenna lines over tree branches. :)
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Hi Evan,
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Funnily enough, I did actually put kapton tape over that barrel "just in case" but it was a solder bridge on one of the SMPS that bit me.? I really don't know where it came from.? I checked visually, checked with the meter, and as I mentioned it did power up long enough to install the firmware and listen to some ft8 signals on the various bands.? I didn't even touch the power boards when I was debugging the rotary switch problem.? It'll forever be a mystery as to how 9v and vcc soldered themselves together while I wasn't looking, but alas, that's the deal when you're building kits that are faaaar beyond ones ability if not for the excellent instructions to follow.
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I think I'll have to build something dead bug style out of Sprat to pick my confidence back up.? Would it be offensive to put a FOXX2 in the QMX case? hihi?
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Nathan, with 0.5 ohm resistance on the 3V line, you may have a different, simpler problem.? Usually a processor failure results in something more like 20-30 ohms, if I am not mistaken.? So I would look for another of those mystical solder whiskers.? BTW, I clean my boards very carefully - but still find little solder whiskers - they hide very well then migrate to where ever they want to go.
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But if you are ready to give up on it, consider sending it to Jeff W1NC - he is skilled at both finding those shorts and replacing the processor.? See his info at ?/g/QRPLabs/message/137552
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Stan
 

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Hi
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Unfortunately 0.5 Ohms is a common values for a dead MPU, I know I have had 2 of them die with that result, all in all replaced 3 of them, the last one showed 75 Ohms VDD TOto GND.
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Nathan where are you located. ?
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73
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Ian G4GIR
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----- Original Message -----
From: Stan Dye via groups.io <standye@...>
Reply-To: <[email protected]>
Sent: 19/02/2025 19:26:34
Subject: Re: [QRPLabs] Testing for a dead QMX board
Nathan, with 0.5 ohm resistance on the 3V line, you may have a different, simpler problem.? Usually a processor failure results in something more like 20-30 ohms, if I am not mistaken.? So I would look for another of those mystical solder whiskers.? BTW, I clean my boards very carefully - but still find little solder whiskers - they hide very well then migrate to where ever they want to go.
?
But if you are ready to give up on it, consider sending it to Jeff W1NC - he is skilled at both finding those shorts and replacing the processor.? See his info at ?/g/QRPLabs/message/137552
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Stan
 

On whatever revision the next board is a few pre soldered PCB jumpers (that can be easily broken / re made) along the supply rails would be useful service aid.?

Unsoldering the processor, ripping a few tracks and finding it was fine (but now bent and abused) is going to be pretty depressing.? ?
In the mean time remember it is often possible to carefully lift any single IC leg and that slicing PC tracks for fault isolation is a much under rated approach?
 

Hi Ian.
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I'm up in, a currently very wet, Cumbria.?
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Bruce, I think the PCB test points is a great idea.? I don't have enough hands to hold the prescribed jewelers loupe whilst operating a soldering iron and scalpel.?
 

On Thu, Feb 20, 2025 at 10:07 AM, Nathan W wrote:
I don't have enough hands to hold the prescribed jewelers loupe whilst operating a soldering iron and scalpel.?
A headworn magnifier may be what you need.? besides freeing up a hand, it provides binocular vision. ?
I prefer these to the kind which pivot on the headband, they block both air and light.
73, Don N2VGU
 

2nd Jeff Moore can fix this!
 

I’m more than happy to fix this. ?I get rigs from all over the world to bring back to life. ?Tariffs and expensive postage is a pain. ?But at least it’s an option.

Also, I’ll be at Hamvention in Dayton most likely if that helps.
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--
73
Jeff Moore
W1NC
 

Jeff, you are a real asset to the QRP Labs community. Thanks for all you do.

Mike Krieger


On Sat, Feb 22, 2025 at 10:25?PM Jeffrey W Moore via <jeffreymoore=[email protected]> wrote:
I’m more than happy to fix this.? I get rigs from all over the world to bring back to life.? Tariffs and expensive postage is a pain.? But at least it’s an option.

Also, I’ll be at Hamvention in Dayton most likely if that helps.
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--
73
Jeff Moore
W1NC

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