I Have Mercer 8KW inverter. I have had an F90 fault in the past 9 months. The technician at Mustek South Africa said that the control board was replaced, each time i had the ubit repaired. The control board has this number on it: RMA#2212009 31-550718-03G-CPCBA CNTL/AXPERT MAX II/VOL 8KVA 230V W/O COMM CE. What would be the cause of the F90 fault? It stops working once it displays this fault on the LED screen.