Export problem

Hi,
we have oracle 8.1.6SS server and now when we try to export using exp utility it ends with an error ORA-00904 when exporting synonyms. I tried to find invalid synonyms, but I could not find any method how to get it. Using other ways I realized that we have about 180 invalid java classes on which are public synonyms.
This problem occurs even when we do try to export schema without any synonyms.

                            Thanks in advance Ales