Incorrect encoding during Ajax request - how to fix it?

Question

AJAX

I make an Ajax request - I get data in the wrong encoding.
added to javascript did not help:
contentType: "application/x-www-form-urlencoded;charset=windows-1251",
added to the php file to which the request goes - the coding has become better, but still not the same:
header(' content-type: text/html; charset=windows-1251');
How to fix?