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Sorry my bad. I thought 5V had a linear regulator as well. What I observed was that after long-pressing the power button, the LCD would turn on normally and show some text (this is probably when the controller is still running on the 3.3V linear regulator). After sth like "initializing audio", all the characters would become solid squares, but the backlight was still on (this is probably when the controller has shut down the 3.3V linear regulator).

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Because the USB connection and terminal app is working.

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Sure. It's probably not important it was USB-C. I was trying to power my QMX+ with a fixed 12V PD trigger connected to an Anker 36W USB-C power brick. I was able to turn it on partially - the LCD was up for maybe 1 sec, then it suddenly shut down. I tried 2 or 3 times then I realized this particular power brick might have some issues. (BTW I've been using the 67W one with 9V and 12V PD trigger all the time without any issues). I just double checked, the 36W one does generate the correct voltage, so I think QMX+ never saw 20V from it. But I guess when QMX+ was shutting down the 3.3V linear regulator, it probably triggered some protection on the 36W power brick which would cause voltage spikes somewhere and that fried IC503 somehow.

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73 de N6HAN

Hi Stan, thank you so much for your analysis!

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I used a fixed 12V PD trigger, so it's probably not the 20V issue. I'll take a closer look later.

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You're absolutely right! I realized this as well. It looks like when the controller is still on the linear regulator, the 5V SMPS and LCD control works fine (at least partially).

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I'm removing IC503 now. Without power, the resistance between Vdd/GND is 2.6k so I guess the short is either on the Vcc rail, or only happens when the 3.3V SMPS is running.

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Thanks again!

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73 de N6HAN Zhenxing Han

On Sat, Mar 29, 2025 at 06:12 PM, @Ludwig_DH8WN wrote:

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When the controller starts to use SMPS, the LCD goes all blocks so I guess the 5V rail is damaged.

Strange. In some failure situation going to Diagnostics may reduce input current. Do you know your supply voltage every time?

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Sorry, failure. It should be:

but if I try to access the hardware diagnosis function, QMX+ crashes and the USB connection is lost.

Strange. In some failure situation going to Diagnostics may reduce input current. Do you know your supply voltage every time?

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73 Ludwig

On Sat, Mar 29, 2025 at 04:19 PM, Zhenxing Han N6HAN wrote:

The steady power consumption after SMPS bootstrap is about 22W so something is probably shorted.

This would mean 1.8 A @12 V! To much and dangerous for the QMX.

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What do you mean by "normal"? Before SMPS is working the display should be blank and dark.

How do you know the controller is working and starts the SMPS in this state?

Strange. In some failure situation going to Diagnostics may reduce input current. Do you know your supply voltage every time?

When seeing a damage something very bad was happening.

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Could you tell a bit more about? Is it important it was USB-C? How did you feed your QMX by the USB-C PD power supply and how did you feed it later?

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73 Ludwig

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On Sat, Mar 29, 2025 at 08:19 AM, Zhenxing Han N6HAN wrote:

Long story short, I managed to damage my fresh built QMX+ by trusting a USB-C PD power supply too much...

If you use an external trigger module with a PD power bank, it is easy to accidentally let it switch to the 20V mode and break your QMX.? I know this from my sad experience.? But now I only use a fixed PD trigger cable, and it always works fine.

The symptoms are that I'm still able to power it up and the display is normal before SMPS kicks in. When the controller starts to use SMPS, the LCD goes all blocks so I guess the 5V rail is damaged.

I'm not sure your analysis is correct.? How are you determining when the SMPS kicks in?? The 5V supply is only an SMPS supply, and it is what powers the display LCD and backlight.? So if the LCD ever comes on, it is because the 5V SMPS is already going.? But it could still be damaged in some way

Visual inspection shows 74ACT08 is damaged as well.

So this could be from an over-voltage on your 5V rail.? Did the fault happen while transmitting, or receiving?? If transmitting, something that blows your finals could also blow this device.

The terminal application is still working (so 3.3V is fine?), but if I try to access the hardware diagnosis function, QMX+ crashes and the USB connection is lost.

Hmmm... this should hold a clue.? Ludwig, have you seen this?

The steady power consumption after SMPS bootstrap is about 22W so something is probably shorted.

Yes, that is way too high for any mode of QMX+ operation.? So something definitely is shorted.? Your receive mode power consumption should be about 1W.? Do you have a way to limit the current into QMX?? For further debugging, until you find the fault and get a successful power-on at normal current draw, you should limit the current to 250mA to avoid damaging additional circuits.? Also it is best to reduce the voltage to about 7V.? Since things are kinda-sorta working a bit, your 3V and 5V protection diodes seem to have saved you from extensive damage, and are not likely damaged themselves.

I have an unbuilt QMX+ board that I can take components from. Really appreciate anyone could help me narrow down the components to check/repair. I was trying to use QMX+ SSB to work some WPX SSB contest stations... Really wish I could still make it... ?Many thanks!

It is really hard to say what all is damaged.? It looks like you didn't hit it with 20V like I did to mine,? since yours isn't totally broken.? I blew Q108, D109, IC403 and the TVS input protection diode, D110.? But it seems your D108, D109 and D100 are probably fine, since all voltages still power on.? Maybe you are lucky and only IC503 is blown - and it could be shorting the 5V supply down after the receiver/transmitter are configured.?

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So my best shot-in-the-dark guess at an action plan is to remove IC503, and see if the short is gone, preferably by measuring the resistance between Vcc and gnd, which should be a few k-ohms.? Then powering up with a current-limited supply to see if it comes up, and things start working again in receive mode.? Then replace IC503 with a new one, and power-up with a current-limited supply, check the diagnostics, except LPF sweep and SWR sweep (which will return poor results until you increase the current limit).? Then if all looks good, increase the current limit and try an SWR sweep, LPF sweep and diagnostic mode Tx. ? You may find that your finals are also smoked and need replaced.

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Good luck.

Stan KC7XE

Long story short, I managed to damage my fresh built QMX+ by trusting a USB-C PD power supply too much... The symptoms are that I'm still able to power it up and the display is normal before SMPS kicks in. When the controller starts to use SMPS, the LCD goes all blocks so I guess the 5V rail is damaged. Visual inspection shows 74ACT08 is damaged as well. The terminal application is still working (so 3.3V is fine?), but if I try to access the hardware diagnosis function, QMX+ crashes and the USB connection is lost. The steady power consumption after SMPS bootstrap is about 22W so something is probably shorted.

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I have an unbuilt QMX+ board that I can take components from. Really appreciate anyone could help me narrow down the components to check/repair. I was trying to use QMX+ SSB to work some WPX SSB contest stations... Really wish I could still make it... ?Many thanks!

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73 de N6HAN Zhenxing Han