I have to use some old 77 routines in my F90 code.
In the main program, is it mandatory to declare the 77 subroutines as "external"?
cause otherwise it says "This name does not have a type, and must have an explicit type".
Your error message seem to imply that the subroutine you are calling has been used as a function. The problem would be in your calling statement. Likely you have accidentally used a subroutine as a function, or made a typo in the program.