There are 2 ways I can think of:
- On the Convert - Look&Feel page enter the correct encoding in the box labelled 'Input character encoding' as you do the conversion
- Open the TXT file in a decent text editor (e.g. Notepad++ if you're a Windows user) and force the TXT to be converted to 'UTF-8' or 'UTF-8 without BOM' encoding (please don't ask me what the difference is
). Then reimport the TXT into Calibre for a standard conversion. It goes without saying that you should visually check that the quote characters look correct in the TXT file before reimporting to Calibre.