Hi Hans,
Thanks again for the great QRP Labs products.
You're right about me getting my wires crossed somewhat. I was thinking about the following QMX issue when I made the comment:?. I was also thinking of the 1.5V SMPS transmit bias circuit in the QDX.
I had an unexpected failure of my QDX during initial testing of the diode modification I described. The failure occurred while powered by 5V from the USB cable and then connecting 12V, and the symptoms were similar to what Gunnar explained in his original post. I believe it was a Rev 4 QDX. It has been a few months since this happened, and it sounds like I need to revisit this scenario.
The initial intent of the diode solution was to allow 5V operation via the existing USB connection, while also allowing 12V operation when desired. This solution seems to work reliably as long as, when your want to run full power, you connect 12V first.
