Hi @swellenuwe
Check if there's 2.6V AC on the main board connector JFN701 pins 2 & 4, JFN702 pins 2 & 3 and front board JFF101 pins 2 & 3.
If both voltages get to JFF101 OK then if could be a faulty fluorescent tube in the display (Marantz p/n HQ300580R, FL DISPLAY model VFD29-1403FS VSLVFD291403FS)
If there's no fluoro tube AC voltage appearing on main board JFN701 pins check the connection on the main transformer.
Here's the [document|42244|service manual] that will help.
The main pcb schematic is on p.15 (0f 61)
The front pcb is on p.13
The wiring block diagram is on p.7 although I think that there's a mistake as it shows FN710 going to the standby board and not the main transformer. JW104 on the standby board goes to JFN102 on the main pcb JFN102 does not go to the main transformer as shown