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On Thu, Apr 3, 2025 at 10:26 AM, <k2cwcqrp@...> wrote:

a 12 VDC power adapter measuring 13.4 VDC output at open circuit

If this is the power supply you have been using to operate your QMX, that is what caused the problem.? 13.4V is too high a voltage for QMX, as explained (in boldface red) in the assembly and operations documents.? Hans recommends using 2 simple silicon diodes in series in the power cable to lower the voltage, or you can use a buck or buck/boost converter.? Using 13.4V will eventually cause problems, like possible damage to your finals and to your power supplies.? If the supply voltage goes down lower than 12V during TX, it is still a problem, since it will bounce back up to a too-high voltage.? And if it droops to lower than 12V during receive, then it doesn't have the current capacity to properly power your QMX, and will cause operational problems.?

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And if you weren't using that supply, good.? But you shouldn't use it for diagnosing the problem, either, unless you can turn it down to about 7V and set a 250mA current limit.

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But back to the current problem -

If all is operating correctly, your current while pressing the button should go up to about 200mA for about 1 sec, then settle down to about 100mA after the processor starts up the SMPS supplies.? None of that appears to be happening.? Which means the 3.3V linear supply isn't even turning on - and that just requires hardware switching, so something is definitely broken.?

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If you are up to doing some measurements, disassemble the QMX enough to remove the two power supply units, and test them independently using the procedure in the wiki. ?/g/QRPLabs/wiki/37128.? If one or both of these is bad (and you don't want to try fixing SMD stuff) you can order a new set from qrp-labs for $10.

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Also on the main board, measure the resistance of the 12V, Vcc and Vdd pins with respect to board ground and with respect to each other.? These should all be at least a few k-ohms, if any is instead just a few ohms, that means something on the main board is shorting and needs to be found and fixed before installing and using new/fixed power supplies.

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Or you can punt and send it to Jeff W1NC and he will fix it for you - but where's the fun in that?

Good luck - let us know how it goes.

Stan KC7XE

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