SO i had a working qmx rev 2 - purchased assembled - i was swapping my antenna coax from another radio and poof the QMX no longer powers up . I also had a kit so i swapped out the PS boards . Still no luck . i tested the power supply boards individualy and they work fine . After furthur trouble shooting i discovered that the Lin_Reg_EN signal is stuck at .7 volts with 10V in . I tried tacking a pull down resistor ( 10k ) to ground and it lowered it by maybe 20 millivolts . Sounds like i have a bigger problem . Any suggestions

On Sun, Mar 30, 2025 at 08:37 PM, <ng433@...> wrote:

i discovered that the Lin_Reg_EN signal is stuck at .7 volts with 10V in

When left encoder is pressed?

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How did you test the SMPS boards, using the method from the QRP Labs Wiki?

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When the controller isn't running, LIN_REG_EN is pulled to 50% of "+12V". With 10 V in it should be around 5 V produced by R101 and R115. If the voltage is near 0 V LIN_REG_EN is disabled and no VDD is produced to start the controller.

0.7 V could come from a broken Q101, a broken Q110 or a fault at the mainboard. I recommend to remove all the subboards and check resistance for the mainbord VDD - GND and VCC - GND. Both should be some kiloohm. If only a few ohm something is wrong connected to the low resistance voltage rail.

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73 Ludwig

Also inspect carefully the interconnect of the LIN_REG_EN signal fromthe smps card to the main board, ensuring nothing there is pulling it down.

On Thu, Apr 3, 2025 at 01:25 AM, <ng433@...> wrote:

Any ideas where to start looking for the short?

Check all the elements connected to the VDD rail and all the places where the VDD rail goes. With some luck you will find a tiny solder whisker or a tiny part cut from an element during construction and shorting VDD now.

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and

may point to a broken controller.

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73 Ludwig