Encoding::UndefinedConversionError

menu.to_s.encode('UTF-8', invalid: :replace, undef: :replace, replace: '?')

This worked perfectly, I had to replace some extra characters but there are no more errors.

What do you expect for "\xC2"? Probably a Â

With ASCII-8BIT you have binary data, and ruby cant decide, what should be.

You must first set the encoding with force_encoding.

You may try the following code:

Encoding.list.each{|enc|
  begin
    print "%-10s\t" % [enc]
    print "\t\xC2".force_encoding(enc)
    print "\t\xC2".force_encoding(enc).encode('utf-8')
  rescue => err
    print "\t#{err}"
  end
  print "\n"
}

The result are the possible values in different encodings for your "\xC2".

The result may depend on your Output format, but I think you can make a good guess, which encoding you have.

When you defined the encoding you need (probably cp1251) you can

menu.force_encoding('cp1252').to_json

See also Kashyaps comment.

If you don't care about losing the strange characters, you can blow them away:

str.force_encoding("ASCII-8BIT").encode('UTF-8', undef: :replace, replace: '')