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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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