apple ][ (europlus) no characters. beep ok, but only white square no promt.no message

hello
i try to repair an apple ][ europlus.
power supply is ok.
almost all TTL chips new.

if i start up, it beeps once.
but no message shows. no characters at all. only a white square where the cursor should be
i can hit return or any key and the square moves as it should.
also i can type in (blind) a simple programm (forx example 10 print,20 goto 10 ) and run it then the square disapears.
and if i press control c. it returns.

so basic interpreter is working. just no character.

any ideas.
where are the characters generatet or stored?
someone has the rom dumps. i dont have a second apple ][ to test...

any ideas?

thanks
f.