Maxent P420142X1 - not showing image when

Maxent P420142X1 - not showing image when connected through AV or Component input. Sound comes in for a second when input is selected or sound fixed/variable is changed in the OSD-menu. Screen stays always blue, except when cable box (OPtmum) or DVD(philips) player are connected when the screen is green. Audio input works well on RGB and DVI but only after changing sound in the OSD-menu (fixed-variable, doesn't matter as long as it is switched from one to another, sounds comes in but if monitor is turned off or input is changed, sounds goes off and one has to go back to select on menu) Annoying!
Any hint?