Hello Ben,
I had the same issue and after Hans's explanation on what is going on in the software I switched to a different platform (went from Windows to Raspberry Pi) and the TX signal appeared.? As stated in Hans's message, he has updated the firmware to let us know what is going on in the software.
A quick summary of my transmit issue was the USB link with the Windows10 computer was dropping out and a tone was not being sent to the QDX so it never went into a signal transmit, it was just ready to transmit.
Note, that computer still does not link properly with the QDX.? I have other computers running Windows10 that do.? The issue is the USB link dropping.
73
Evan
AC9TU
PS; be careful probing around the PA MOSFETs.? A brief twitch and both MOSFETs on one side of the push-pull amp let out the smoke.? It also took out IC5, the 74ACT08 PA Driver.? Asl me how I know.? Let's say I have practice now with repairing the full transmitter power chain.? The QDX is a very small device for this 69-year-old body to handle.? Extra care is required on my part.
