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My recently built QMX+ has just stopped working.

I was running the diagnostics from the terminal and disconnected power from the barrel connector whilst still connected to the terminal via USB.

The QMX will no longer power up.? The screen does not illuminate.? It does not appear as a COM port or a USB memory device.? A click can be heard in the earphones when the 'pwr' button is pressed or released.? There is a slight dip on the 12v power when measured at the barrel connector but the QMX+ fails to power up. There is no evidence of damaged component.

Please can you suggest where I start fault finding?

Interesting.? I have disconnected power multiple times with the USB attached to the computer without any issues.

Did you have TX on in diagnostics when you pulled the power?? That could have caused some sort of issue.

The USB connection in the QMX only uses the ground pins, the central data pins, and pulls down the control pins to ground via resistors.? None of the power pins or other data pins are connected.? The data pins do connect directly to the processor, but I don't see how this connection could have anything to do with pulling power.? If the QMX was transmitting when you pulled the plug, due to the higher current being supplied it could cause a momentary large spike/bounce on the ground/power planes due to the sudden loss of input power, which may have damaged something.? (It's always better to power-down the QMX via the power button before pulling the plug).

Thanks Stan,
I was on the TX diagnostic page.? I don't think that I was transmitting at the time. The use of the terminal at the time of failure may be a coincidence.

I have measured the voltage on the jumpers JP105. I have 12.04v on the Vin pin. When I press the 'pwr' button I very briefly see: V+:12v and Vdd 1.28v but this might be a due to the response time of my meter. I do not see any voltage on Vcc.

?

How would I fault find the voltage regulator and buck converters on a QMX+ rev2?

How would I confirm a dead CPU?

Dead CPU is unlikely; lots of other failures much more likely.? Your power supply is not powering on the CPU, so it can't finish powering up the switching power supplies.

In other posts a common failure from a power fault was D109 - check it first.? And a power supply test process is detailed in the wiki: /g/QRPLabs/wiki/37128

Much of this same process is applicable to the on-board power supplies in rev2 - the design of the power supplies didn't change, only their placement.

Note that the general power-on process is this:

1 - pressing the power-on button turns on Q103/Q105, which puts the voltage from the input jack on the +12V rail

2 - this +12V goes to the input of IC101 which creates the initial 3.3V VDD voltage via D103, which is also connected at the top of D109.

3 - this VDD voltage powers on the CPU, which then sets up the switching power supplies, turning on the VCC via Q108 and switched VDD via Q111.? It turns OFF the IC101 3.3V (i.e. changing from the linear 3.3V supply to the switched one).? This all happens in about 1/4 second if all is working well.

So when you press the button and only 12V comes on, usually it means the initial linear 3.3V supply (IC101) isn't getting voltage to VDD to power up the processor.? If you can measure 3.3V at the output of IC101, but not at VDD, it often means that D103 is blown and shorting the VDD pin to ground.? Could also be other components or soldering issues.

On Fri, Oct 4, 2024 at 06:17 PM, Stan Dye wrote:

Note that the general power-on process is this:

1 - pressing the power-on button turns on Q103/Q105, which puts the voltage from the input jack on the +12V rail

2 - this +12V goes to the input of IC101 which creates the initial 3.3V VDD voltage via D103, which is also connected at the top of D109.

3 - this VDD voltage powers on the CPU

?

There are a few different failures that could cause the measurements you are seeing, including D109.

IC101 should output 3.3-3.6V or so, and you only see 2.17, so either the IC101 circuit is not putting out the correct voltage, or something downstream (including D109) is dragging down the VDD bus.

To further check the IC101 circuit, I would first measure the Vi pin, which should have 11V or so, if not, Q102 could be bad.? Then (if you have appropriate skills/tools) I would lift the Vo pin to separate it from the circuit, and measure its voltage.? If it is less than about 3.3V, IC101 is bad.? We already know D103 is good because you measure an appropriate 0.6V drop across it to VDD.

If all that is good, either D109 or something downstream of it is dragging down VDD.? I guess you should also test Q111: if it is shorted Source-Drain, it could possibly drag VDD down via R114.? When powered off, it should look like a diode in one direction and open in the other, easy to check.? Then the next logical check is D109.? May be easiest to just replace it, but you could also lift one end and measure it to see if it looks shorted or semi-shorted.