To answer my own question and bring closure to this thread, I never found schematics for this inverter so I created my own, attached... for what they are worth. They should be used instead of the schematics I posted earlier in this thread. I now have a good understanding of this inverter design. Even so, I was never able to get it working, even after $200 in new components, but I got real close. There were more problems discovered after I fixed the previously mentioned things. The main issue, being a 10 ohm short somewhere on the 3.3V rail. This inverter has two MCU's and by lifting one pin on one of them U41 (Vrefhi), I finally found the source of the short. With that pin lifted, the bootup process got much further. The app was reporting the correct DC voltages and the relays started to click. But then the error message "Inconsistent Sampling" came up which makes sense since one MCU was seeing Vrefhi but the other wasn't.
So I ordered a replacement MCU, taking a chance that a firmware installation would also program the MCU. Unfortunately, it didn't so with the new MCU and all the shorts fixed, the error was "internal communication error". That also makes sense since the MCU wasn't programmed. So I now have an inverter for want of the software or a way to program it. If anyone has an old MIN series inverter that doesn't work, I'd be interested in it.
