What about the back lights?
If there's markings on the inverter Bon 'Back lights ON' , the marking may be on the power supply board that connects to the inverter, get a 3 volt battery and with the set switched on apply + to Bon terminal and the - on the battery to the chassis, this should turn the back light ON just to check and will by-pass any error shut downs, providing the power supply is OK, I assume you have checked the relevant electrolytic capacitors.
Sometimes the high voltage ceramic capacitors coupling the tubes, creates this problem but you have ruled that out by changing the inverter board. I'd be thinking the back lights myself, that's why I suggested forcing them to come ON so you can check.
Dave
