NAD 3020i

I have a strange thing that happened with my NAD 3020i.
Powerled on front panel did not light up anymore and i had dc on speaker outputs and pre-amp out.
Sound was distorted and with hum.
I found out that Q 510 : 2SA1015 died in the power supply part .
When i replaced that , the green led worked again but still dc on pre-amp output.
When i used another preamp and connected it to the poweramp it sounds great and no dc on speaker outputs.
I looked further and found a thin pcb line burned .
It was the earth line between amplifier and pre-amplifier.
When i fixed this and tried again , both speaker fuses of 4 Ampere went out .
If i leaf the fuses out , the preamp works perfect ,no hum,no dc.
So, if i want the preamp to work ,i have to leaf the fuses out,if i want to work with the amplifier, i have to cut the earth between amp and pre-amp again.
Anyone an idee what could be wrong?
Thanks for help.
One possible is the short circuit in the transformer supply side. The preamp section shorted to the poweramp section either in the outside cable or inside the transformer.

Disconnect the earth broken line between amplifier and pre-amplifier, remove all 4 fuses, and test if these is short circuit between the preamp section and poweramp section.