Quotation:

  dataType String
                     Default: Intelligent Guess (xml, json, script, or html)
                                    The type of data that you're expecting back from the server. If none is specified, jQuery will intelligently try to get the results, based on the MIME type of the response (an XML MIME type will yield XML, in 1.4 JSON will yield a JavaScript object, in 1.4 script will execute the script, and anything else will be returned as a string).

 

 

$.ajaxSetup({
        url: "http://......ashx",
        type:
"POST",
        cache:
true
       
///dataType: "json"    /// Remove
    });
    $.ajax({
        data: { param: value },
        success:


function(data) {
            
/// For this Intelligent Guess data is JSON

             /// But using Json to obtain the value is questionable        

             $.each(data, function(i, item) {
                    
var aaa = item.A;
             });
        }
    });

Trace : Http Header Info

  Accept

    */*

If Add  DataType:JSON

  Accept  application/json, text/javascript, */*

Who can answer?