First of all you should download the service manual:

https://elektrotanya.com/marantz_pm7000-pm8000_power_amplifier_sm.pdf/download.html

By the way: please use this download button

Andi K. wrote:

> Problem: The main (primary sided) fuse (3.15A/250V) gets destroyed, when
> connecting preamp w/ power stage. As long as the power stage is open,
> the fuse is OK - having no sound, of course

One of the many questions: Does the Powerstage work with another signal 
e.g. a signal generator when the premap is disconnected?
If yes, did you observe the output of the preamp with an oscilloscpe 
yet?

Andi K. wrote:
> Hi everyone!
> doing electronics since nearly 40 years now - but believe it or not: i
> joined the forum today...! This looks like one of those cases where reviewing 
the schematic and verifying power distribution would be a good starting point. 
Many embedded issues ultimately come down to voltage stability or initialization 
timing. Components such as 
YKC22-0874V(https://www.jotrin.com/product/parts/YKC22_0874V) are often used in 
compact electronic assemblies where dependable operation is important.
> Background is an issue w/ the Marantz PM8000 amp, being capable of
> switching between AB- and Class-A mode.
> Problem: The main (primary sided) fuse (3.15A/250V) gets destroyed, when
> connecting preamp w/ power stage. As long as the power stage is open,
> the fuse is OK - having no sound, of course. FYI: These two functional
> blocks are connected via two 100R resistors on each channel.
> Currently i do not have any ideas anymore... So i would be glad for any
> hint!
> Thanks
> Andi (lcdguru)

If the fuse only blows when the preamp and power amp are connected, I'd 
check for DC offset or oscillation coming from the preamp. A scope 
measurement at the preamp output could reveal the culprit.