C--------------------------------------------------------------------------
        subroutine ftmkyu(ounit,keywrd,comm,status)

C       modify a null-valued keyword 
C
C       ounit   i  fortran output unit number
C       keywrd  c  keyword name    ( 8 characters, cols.  1- 8)
C       comm    c  keyword comment (47 characters, cols. 34-80)
C       OUTPUT PARAMETERS
C       status  i  output error status (0 = ok)
C
C       written by Wm Pence, HEASARC/GSFC, July 1997

        character*(*) keywrd,comm
        integer ounit,status
        character value*80,cmnt*80
        
        if (status .gt. 0)return

C       find the old keyword
        call ftgkey(ounit,keywrd,value,cmnt,status)

C       check for special symbol indicating that comment should not be changed
        if (comm .ne. '&')then
              cmnt=comm
        end if

        value = ' '

C       modify the keyword record
        call ftmkey(ounit,keywrd,value,cmnt,status)
        end