Sharp photo copier fault

[Max Harding vk3jin]

I have a sharp photo copier model Z-88 bought in for repair that has this
fault. it prints a total black page . all manual adjustments make no
difference ,I have checked the drum and rollers and all are clean , the
paper path is free and not jamming.

can anybody help with a cause , or site have been through Google with no
luck.
I am an electronics tech . my customer thinks i can fix anything but this
has me stumped.

thanks in advance.
Max.