Well, this is frustrating. The original PCB had a G120 so I removed this from the first board and replaced it with an SCM20260N and it works fine. I can debug etc.
Second board not so good. It gets detected by the device manager and appears in TinyCLR Config as SC20260_SC20260 but when I try to connect to it, the status shows Connection failed.
I was able to update it by putting it into bootloader mode but after this, it still shows connection failed when I try to connect to it.
Sometimes it connects and sometimes it fails. When I try to set Extended Heap and External Memory, the same thing fails, and only the extended heap setting gets set.
Both devices are drawing the same current of 110mA at 13.3V input (I have an OKI-78SR3.3 providing the 3.3V onboard)