Hi Ludwig,
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Thanks for your reply!
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What do you mean by "normal"? Before SMPS is working the display should be blank and dark.
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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How do you know the controller is working and starts the SMPS in this state?
Because the USB connection and terminal app is working.
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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?
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
