tournedos:
You didn't plug it into the PHONO socket by chance?
This should not be a problem on a standard Beomaster 4500, as it does not have a RIAA on the phono socket (Called PH/TP2) -so in this case the phono socket should be as all the other sockets. However - IF the previous owner has had a RIAA module mounted, then you can not connect a Tape-recorder to this socket. Try the Tape1 or Aux sockets.