Write bytes from textbox to file from basestream with C#

Basically I need to convert text in a textbox from UTF-8 to base16 (I think that is what hex is in) and write it to a file.

This but back words:

//Setup byte reader.
            FileStream fs = new FileStream(EditOpen.FileName, FileMode.Open);
            BinaryReader br = new BinaryReader(fs);
            long length = fs.Length;
            //Read bytes to textBox1.
            br.BaseStream.Position = 0x00001844; //Min loading address.
            byte[] PT = br.ReadBytes(0x00000428); //Amount of bytes to load (1064 to be exact).

            //Print string PT to textBox1 after converting to UTF-8 and replace 0's with DOT's.
            textBox1.Text = System.Text.Encoding.UTF8.GetString(PT).Replace("\0", ".");
            fs.Close();

The easiest way is to use a StreamWriter created with the correct encoding

  using(StreamWriter sw = 
            new System.IO.StreamWriter(fs, 
                System.Text.UTF8Encoding))
  {
     sw.Write(textBox1.Text);
  }