I checked the database table and found it was in latin1_swedish_ci so i've changed it to utf8_general_ci then I changed the collation from latin1_swedish_ci to utf8_general_ci for all the fields. 
Still it's giving me the same error.
Illegal mix of collations (utf8_general_ci,IMPLICIT) and (utf8_unicode_ci,IMPLICIT) for operation '='
Thanks,