First, thanks to all who responded !
Second, I still have no idea what's wrong.
(besides of a broken CPU)

So, as I see it (could be wrong) but the cpu boards of ka675, ka680,
ka690 are the same, 
but only different speed grades (aka l4002-aa, l4002-ab, l4002-ac)
They look pretty much simililar, there is only one different oscillator.

about memory: What I have here are two l4004-cp boards, which are
64MByte each.
My guess is, that this is the ms690 memory.

And, the cpu stays in "F" on the LED, it looks it even doesn't try to
test the memory.
(if I remove the memory out of the 4000/400 system, I get at least the
message, that there is no memory)

And, if it helps anybody, the backplane in my system is a: 
5019353-01 or 5419354

(this number is easy to find, near the fourth memory connector)

Still looking for ideas ;-)

cheers