The invertor board appears to be faulty. I have tested all traces in the LCD cable between the invertor board and the logic board and they are OK.

I don't see how the invertor board can be repaired. It's not the socket to the brightness actuator, the invertor also works so it must be the controller on the PCB.

It's pitty as chances are next to zero of finding an invertor board for sale on Ebay. It even isn't the same as in the Duo 270c xx(

If anyone has a spare one for sale, feel free to PM me.