UPDATE.
The Second replacement inverter also had issues. I was sent, and replaced, the POWER board. Not too difficult, actually not as bad as I thought.
Fault continued, (different one) and they sent me the OTHER main board, the INVERTER board. I replaced that too.
It works!!
So both main boards have now been replaced. Looks more complicated than it is. Biggest concern was the white goop they like to seal the plugs in with. Comes off pretty easy.
Only a few small ancillary boards left of the original device.
All tests so far have been good.
The takeaway is this. It's been a nightmare, taking over 4 months to resolve. HOWEVER, I will say that Sungold responded within 24 hrs of every email I sent, and sent new boards within a few days. While support is odd, email only, they have at least stuck with it and were prompt. If you have issues with a Sungold inverter, don't waste time asking for a return and replace. They like to send you parts (cheaper) and its not that hard to repair. Take this experience as a lesson in the system they use. I did end up with a working inverter.