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I need help troubleshooting an issue with my QMX+ 3.3v power supply board (PCB #2).

After assembling my QMX+, applying power, and pressing the encoder switch, I saw 170 mA current draw but no LCD backlight and no USB port recognition.

I removed the two buck converter boards and tested them independently according to Chris' G5CTX suggestions. ?The 5V board passes all tests.

The first test of PCB#2 resulted in a low 3.0-volt output from the linear regulator circuit and an unexpected 46 mA current draw. ?Testing the buck converter circuit with PWM_3v3 worked as expected with Vout tracking Vin and no current draw.

Tracing voltages through the regulator circuit shows 5.5V input at "+12V", 5.3v at the junction of Q102 drain and regulator input, 0.6V at regulator ground, 3.8v at regulator output, and 3.0v at Vdd. ?The 0.8v drop across output diode D103 seemed a bit high, but otherwise nothing jumped out at me. ?LIN_REG_IN and Q101/Q102 enable and disable the linear regulator as expected.

I was ready to blame the hardware until I remembered I have the same board from a yet-unbuilt QMX. ?That second board gives me exactly the same result. ?I couldn't have two bad boards, could I?

Any suggestions are very much appreciated.

Bruce, W8BH

On Thu, May 30, 2024 at 10:04 AM, Bruce Hall wrote:

The first test of PCB#2 resulted in a low 3.0-volt output from the linear regulator circuit and an unexpected 46 mA current draw. ?

This is with PCB#2 not connected to anything?? If it is plugged in then there's a lot on the main board that will take power.

Looking at the circuit Q111 might conduct through its diode but there doesn't seem to be an obvious path to ground.? R110 and R112 might take some current, much less than a milliAmp.

"After assembling my QMX+, applying power, and pressing the encoder switch, I saw 170 mA current draw but no LCD backlight and no USB port recognition."

This is normal. You then need to load the firmware operating system from your computer. After loading, it should automatically boot with the model selection screen and the backlight on.?

What's not normal is not seeing the QMX show up on your computer. Try a different cable or a different computer.?
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73, Dan? NM3A

Hi, Chris.? PCB#2 was tested according to your posted suggestions- not plugged in.? I thought the current draw might be from a bad regulator or diode, but both of my PCB#2 boards give the same result.? ?I am not sure what I should check next.

I think I found the path:? Vdd through the Q111 body diode, as you suggested, to 47-ohm R114, to Q110 and then to ground.? ?The unconnected LIN_REG_EN input is pulled high via R101, therefore Q110 is turned on.? ?I measure 2.4 volts across R114, which would account for ~50 mA current.

Perhaps this current draw is normal after all?? It would only occur during startup with the linear regulator.? ? My Vdd is still rather low (3.0 V).

Bruce, W8BH