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Hello, I just completed a QMX build and had a power-up issue.

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I used a lab PSU at 7V initially; QMX draws ~150 mA right away - not proper.

But at the same it did power-up and display lights up.

So I tried USB connection and was able to flash the firmware successfully.

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- it self-turns on;

- MCU probably thinks it is underpowered: as soon as it boots and displays normal view, it shows "Shut down!" and cycles;

- display backlight blinks randomly - again, power stability issue?

- draws too much power: I dared to try to turn it on with 9V and got almost 1A consumption.

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I double-checked all components, especially on power boards and found no fried parts or soldering issues.

I also checked the 8-pin power board connector; it is the only board getting the incoming voltage, is this correct?

I don't see constant grounding on PWR_ON pin (unless left knob is pressed).

There's no measurable impedance between power and ground on the power connector if the power board is removed.

I also checked for notorious Q103/Q104 short before building - not a known V4 problem, but just in case.

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What else I can check? I assume the problem is within 8-pin power board; could it be somewhere else?

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Thank you in advance.

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On Mon, Apr 7, 2025 at 08:31 PM, <andrey@...> wrote:

I used a lab PSU at 7V initially; QMX draws ~150 mA right away - not proper.

150-200mA is fine for QMX without firmware, in firmware update mode presenting USB drive.? Assuming the current draw happened after you pressed the power-on button.

But at the same it did power-up and display lights up.

If display light comes on without firmware installed, there is a problem.? It? uses 5V power supply, which is only started by firmware.? Are you sure this happened with no firmware installed?

So I tried USB connection and was able to flash the firmware successfully.

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- it self-turns on;

- MCU probably thinks it is underpowered: as soon as it boots and displays normal view, it shows "Shut down!" and cycles;

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If it 'self? turns on', and then does 'shutdown', without you pressing the power-on button, it likely means the power-on signal is stuck in the 'on' position.?? That would cause it to self-turn on, and the MCU, after booting, would see the power-on/off button asserted, and do its shutdown sequence.

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So the first thing to do is verify the power-on button from the encoder, and all of the connections of the PWR_ON signal to the main board (see the schematics, it goes through pin 1 of the 8-pin).? It is likely that the signal is shorted to ground (or to another low signal, like the adjacent PWM_5V pin) somewhere along that path.?

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This could also cause the display backlight to flash, since the MCU will start up the 5V, but then quickly shut it down again because it thinks you are pressing the power on/off button to shut it down.

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First solve that issue, and keep using the 7V supply, and then we can investigate the high current draw if it isn't resolved.

Stan KC7XE

Thank you Stan.

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I checked everything again but was not able to find anything wrong connectivity wise.

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I ended up powering up first power board (PCB#1) disconnected:

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"V_IN": 7.0V <-- fed from lab PSU; I get 400+ mA immediately!

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"+12V": 6.8V

"VCC": 6.0V

"PWM_5V": 5.6V

"ADC_5V": 3.0V

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Is this invalid way to test it?

This is a valid test for the 5V smps, if I understand correctly that you removed it and are testing it separately.

However, the voltage and current you are using could potentially cause further damage.? Can you limit your power supply current to 100mA?? or 250mA?

Since you are getting 400mA immediately, there is a short or malfunction somewhere in the 5V smps.

Please see this wiki entry for a complete power supply test procedure:

Be sure to do all the tests, and test all the pins.

But we already know two issues:?

1) You just reported PWM_5V being at 5.6V.? This is bad; it should be at 0V.? It has a pull-down resistor to gnd.? It is likely shorting to something else on the PCB#1 card, which is pulling it up high.? This will cause the 12V to constantly drive through Q108 and the D108 5.6V zener diode will conduct all of the excess current, trying to keep the voltage rail at no more than 5.6V, but it is getting up to 6V, driving the zener hard.? Since you are seeing 6V on VCC and 3V at ADC_5V, it means you haven't yet blown D108 - but if you keep feeding it 400mA, it may also blow.? So this problem with PWM_5V is likely what is drawing most of the current.? So one task is to find what is causing it to go high.

2) You previously reported auto power-on/shutdown, so either a) or b):

a) the PWR_ON signal is shorted low, either on the PCB#1, or on the main board.? So check it in this PCB#1 test - it should be pulled up to almost Vin.? If it is even a couple of volts lower than that, it will auto-power-on, and you need to find the reason.? If it is properly at Vin, check on the main board the resistance between this control pin and all the other pins in the connector - they all should have a very high resistance.?? If any have a low resistance or continuity, that is a problem that must be fixed.? If all of this is OK, then check b):

or b) the PWR_HOLD signal is shorted high, either on PCB#1 or on the main board.? This does the same thing as PWR_ON, but is the opposite polarity.? It has a pull-down resistor, and should stay at 0V on the PCB#1 tests.? If it is higher than that, find out why.? If not, check the signal at the connector on the main board - it should have high resistance to all other pins on the connector.

One possibility is that your PWM_5V signal may be shorted to your PWR_ON signal - they are adjacent pins in the connector, and that would cause both of the symptoms you are seeing.? But the PWM_5V may be a little to high for this explanation, so maybe it is something else.

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I hope this is helpful.?? Good luck

Stan KC7XE

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Your measurements reveal your problem.

Note that PWR_HOLD is going up to 5+V in each of the measurements.

It should be at 0V (until the processor brings it high to hold the power on).?

When it is high, it turns on the power.? So it somehow is shorted to something, or a component has failed such that it goes high.

It should only be connected to R105, which pulls it down to ground, and the gate of Q104.??

I don't see anything obvious in the photos of your board.??

If Q104 has shorted internally, the +V on its drain could pull up on PWR_HOLD.? So I may be tempted to remove it and see if the tests start working.? But before that, check the spot I circled in your photo here, and make sure that the drain of Q104 is not shorted to the drain of Q103 - if shorted, that would indeed burn out Q104, and likely cause the problem you see.

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And note that yes, as you increased voltage, zener D108 begins to conduct.? It is a protection diode, to keep the voltage from soaring above 5V if there is a fault.? It is a 5.6V zener, so it will go up to 5.6V plus a little more depending on how much current you push through it.

I hope this helps.? If you remove Q104, or at least pick up its drain terminal, and the supply tests start working, you can likely just replace it and have a working board.

Stan KC7XE.

Hi Andrey,

At least there is some progress.? But this is very strange.? I have not seen anyone report multiple failures on the control line transistors before.?? As for your PCB#2 test, to know further what is wrong, we would need to measure voltages on LIN_REG_EN, PWM_3V3, IC101 input, and Q102 gate while you have it powered with 5.6V.

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I have no idea what the root cause of this could be.? A short of Vin or +12V to the control lines could burn the transistors - on QMX note that such shorts can happen because of close assembly tolerances - make sure nothing is touching together when fully assembled.? You can carefully check this with a light and a magnifier with the bottom cover off.? Pins and tabs from the display card can touch other items; examine the bodies of the encoders, the back of the power in jack, etc.

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Yes, some main board faults can indeed burn out the power supplies - but usually that is from over-current, and the failures are typically different.? I think you reported that you measured ohms between all of the smps connector pins to the main board (each with respect to all others), and found none with low-ohms.? So at least the main board shouldn't have a power short to any of the pins.? If you put in new/fixed power supplies, and power up with your voltage at 6-7V and a 250mA current limit, you won't burn anything out, but will enable further testing.?

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I guess it is possible that you got a set of SMPS boards that was populated with defective BSS123 FETs, but I've not heard anything like that reported before, either.? Note that you can order for US$10 a new set of SMPS boards from qrp-labs if that is better for you than replacing the components.

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Stan KC7XE

I had a VERY similar issue with a QMX+. Powered up by itself, but mine did not power down and cycle back and forth.

The fix on my QMX+ was to replace Q104, the BSS123 that PWR_HOLD turns on.? No short to ground on either PWR_ON or PWR_HOLD.

I didn't record all the values like you did since it worked fine once it powered itself up, but I do recall that the Q104 BSS123 seemed to meter out ok in circuit. Maybe it was leaking just enough Source to Gate under power to turn on Q103 and Q105?

Easy enough to change if have any around or can get them locally, or you can order a new set of SMPSs from Hans.

Hope that helps!

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Randy, N4OPI