elixir, 149 bytes

Result

AC

Duration

1816ms

Code [DL]

defmodule M do
def f(x)do
c=IO.read(1)
x=case c do
:eof->1/0
"T"->0
"K"->x-50
_->rem(x+1,51)
end
IO.write(if x<1,do: "@",else: c)
f(x)
end
end
M.f(0)

stdin

                          T                       
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                                                  
                 K                                

stdout

                          @                       
                          @                       
                          @                       
                          @                       
                          @                       
                          @                       
                          @                       
                          @                       
                          @                       
                          @                       
                          @                       
                          @                       
                          @                       
                          @                       
                          @                       
                          @                       
                          @                       
                          @                       
                 @@@@@@@@@@                       

stderr

warning: this expression will fail with ArithmeticError
  /volume/CODE:5

** (ArithmeticError) bad argument in arithmetic expression
    /volume/CODE:5: M.f/1
    (elixir) lib/code.ex:767: Code.require_file/2

strace