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QMX Build Challange

 

I received a QMX and built it.? ?I've built a number of QRP kits all successfully, but not so for this little guy.? I attempted to powered up using a regulated power supply at 7v and 250 mA but without success.? In my efforts to get things going, I resoldered every joint and inspected for bridging.? ?In the end, I clearly identified a component (Q109) on the 3v power supply that had a hole melted through the package.? ?I've just order the spare power supply boards from QRP Labs, but don't know where to look for the cause of the current failure before installing and destroying another 3v power supply.?
?
Any ideas for me.
?
Regards,
Wilson Little (NT5AT)?
 

The 3.3v SMPS provides power for the STM32 controller. While you are waiting for new SMPS boards, I would check resistance between VDD and ground per to rule out a controller short, per the wiki. /g/QRPLabs/wiki/37422

gl de Russ, va3rr
 

Hello Wilson,
?
I suggest you first remove both power supply boards and verify that there are no shorts on the Vcc and Vdd lines.? The pin assignments from the bottom view of the QMX board are in a PDF.?
?
I assume this is a QMX and not a QMX+.? They are different in board layout.? There are also differences in board revisions.? Supplying the volunteers with both helps us give you better suggestions.
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There is also a test process in the files section of the group for the power supply boards:
?
I hope the above helps.
73
Evan
AC9TU
 

One question: which version of kit did you have?? v1, v2, v3, v4?
There are several different possibilities for a burned Q109.
All of them come from a short on Vdd or one of its control lines (LIN_REG_EN, PWM_3V3).
These shorts could come from solder bridges, or a tiny flake of solder that migrated somewhere, or from mechanical issues: the tolerances are very close when assembling, and metal surfaces can easily touch together.? So in addition to inspecting soldering, also look very closely at the assembled unit (without bottom cover), and see if anything from the various assemblies is touching.? Typical issues are the LCD mounting tab that needed bent over, the 10 pins connecting the display to the mount board, the encoder bodies, and the back side of the power connector - maybe others.? Also the soldering of the connectors to the SMPS: the connector 'pins' are a bit longer than the pads (if not trimmed), and can touch or create solder bridges to other SMPS components on the top side.
?
After inspection, first thing to do is measure resistance to ground on each of Vin, +12V, VDD, and VCC, on the main board, without the SMPS installed.? And then the resistance between each of these voltage rails.? The resistance to ground should be at least a few k-ohms.? There should also be at least a few k-ohms isolation between each of them.? Also measure the resistance on the control pins to both 3.3V and 5V SMPS (PWR_ON, PWR_HOLD, PWM_5V, LIN_REG_EN, PWM_3V3).? They should have no continuity to any other pin on the connector, and have a high resistance to ground and Vdd and +12V.
?
Hopefully those inspections and measurements will show something that could have caused the over-current that burned Q109.?
Good luck, Stan
 

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Thank you all for your input and support.? I have a rev 4 QMX (not QMX+).??

?I did a lot of work this morning resoldering every joint before identifying the failed 3.3v power supply component.? I didn't see a bridge, but that doesn't mean one wasn't there.??

I did check all the resistance measurements recommended to id a short, and all had excellent resistance.? ?9.3k ohms was the value of Vdd to ground, every other measurement was higher (ranged from 25K to 1.5M).? At this point I don't believe a bridge still exists.??

I feel more comfortable, attempting a startup with another 3.3 v power supply bd.

Again, THANK YOU.

Regards,
Wilson (NT5AT)

From: QRPLabs@groups.io <QRPLabs@groups.io> on behalf of Stan Dye via groups.io <standye@...>
Sent: Tuesday, April 8, 2025 12:06 PM
To: QRPLabs@groups.io <QRPLabs@groups.io>
Subject: Re: [QRPLabs] QMX Build Challange
?
One question: which version of kit did you have?? v1, v2, v3, v4?
There are several different possibilities for a burned Q109.
All of them come from a short on Vdd or one of its control lines (LIN_REG_EN, PWM_3V3).
These shorts could come from solder bridges, or a tiny flake of solder that migrated somewhere, or from mechanical issues: the tolerances are very close when assembling, and metal surfaces can easily touch together.? So in addition to inspecting soldering, also look very closely at the assembled unit (without bottom cover), and see if anything from the various assemblies is touching.? Typical issues are the LCD mounting tab that needed bent over, the 10 pins connecting the display to the mount board, the encoder bodies, and the back side of the power connector - maybe others.? Also the soldering of the connectors to the SMPS: the connector 'pins' are a bit longer than the pads (if not trimmed), and can touch or create solder bridges to other SMPS components on the top side.
?
After inspection, first thing to do is measure resistance to ground on each of Vin, +12V, VDD, and VCC, on the main board, without the SMPS installed.? And then the resistance between each of these voltage rails.? The resistance to ground should be at least a few k-ohms.? There should also be at least a few k-ohms isolation between each of them.? Also measure the resistance on the control pins to both 3.3V and 5V SMPS (PWR_ON, PWR_HOLD, PWM_5V, LIN_REG_EN, PWM_3V3).? They should have no continuity to any other pin on the connector, and have a high resistance to ground and Vdd and +12V.
?
Hopefully those inspections and measurements will show something that could have caused the over-current that burned Q109.?
Good luck, Stan

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