It’s quite common for the standby supply transformer(T2)to fail,as it’s permanently connected to power.
Check that you have the two standby supplies +15and -15 at the emitters of TR3 and Tr4.
Check that there is about 23 volt dc at the relay coil.
If none of the above are present,check for about 60 volt AC between P1-1 and P1-2 on pcb 01.
If not present,then the transformer T2 has failed.This is a special part made “in house” by B&O,so you need to ensure that you order the correct one for your country.
You can work on the amp/psu assembly by connecting ac power to P4 on pcb 01,but you should find the looms are just long enough to allow connection from the input sockets,if you prop something under the assembly when it’s out of the casing.
It is possible for the relay RLI to stick open too.As you can see,only one set of contacts are employed.