Hi
OK so I still have at least one more fault.
I was able to get through all of the self-tests OK. No errors.
I was able to calibrate the oscillators and output level control.
Further testing showed AM and FM modulation is not working. It was, but not anymore.
The FM modulation sort of works, but there is a step change at a certain modulation frequency.
Enabling AM modulation does not produce side bands, but it does change the output level.
The LCD display is showing what I would expect to see. That indicates that the firmware is OK. I haven't created a fault with the nvRAM.
The manual has a diagnostics section for modulation, so I will work through that.
So, do I expect to need or use FM/AM modulation? NO.
Do I need to fix it for any other reason? NO.
Am I going to fix it?? YES of course.
It would just annoy me to know that it is broken when I know I can fix it.
The block diagram, with annotated modulation signal flow, is attached.