Hello,
I could solve my network problem (on 4000/90).
I dismounted everything until I got the naked motherborad in my hands.
Then I managed to open the ethernet switch button which choose between AUI
and BNC. ACtually the problem was in the switch itself, it did not work.
I opened it but once opened it was not possible to close again and anyway
the switch mechanism did not work at all.
So I looked at how it could work and I have hardwired on the motherboard
the switch pinouts so that they could allow AUI to being set.
I tryed to figure out it As I was not sure of what I Was doing and I took
a decision on how to hardwire the switch button.
After a little work of hardwiring on the motherboard (in the rear part
near the BNC there are the 18 pin end points which were then attached to
the button switch) I could be luckyly succesfull!!!! I could hardwire the
switch to point for the AUI port to being activated and now it works!!!

bye all

Rick