jQuery UI Livesearch works with English, but don't work with Cyrillic characters

Hello, I have a problem with jQuery UI Livesearch - it works perfect with English but do not work with Cyrillic - empty array and I have both in my database. I am using CI 3.1.5 and Firebird 1.5. When I using MySql (with like query) table is with charset utf-8 and Collation-utf8_general_ci I have the same problem - English works, Cyrillic don't. Here is my code:

My Model:

PHP Code:

public function get_autocomplete($search_data)
{ 


$data = mb_convert_encoding($search_data, "Windows-1251", "UTF-8");

// var_dump($data);

$query = $this->db1->query("select cl.name
from clients cl
where (cl.name containing  '$search_data')"); 

//var_dump($query);

return $query->result();

} 


My controller:

PHP Code:

<?php
 
class search extends CI_Controller{

public function index()
{
$search_data = $this->input->get("input_data");

$results =  $this->search_model->get_autocomplete($search_data);

if(!empty( $results))
{

$jsonData = array(); //Create array  
            foreach( $results as $row )  
            {  
                $jsonData[] = array(   
                                     'value' => $row->NAME
                                     );   
            }  
           $this->output->set_header("Access-Control-Allow-Origin: *, Content-type: application/json, charset=windows-1251");

echo json_encode($jsonData);
}
}
} 


My Ajax:

PHP Code:

$(".addClient").each(function() { 
   $(this).autocomplete({
  autoFocus: true,
   minLength: 2,   
   source: function (request, response) {
       $.ajax({
           url:  "<?php echo  site_url('search');?>",
           scriptCharset: "windows-1251",
           data : { 'input_data' : request.term},
           contentType: "application/json; charset=windows-1251",
           dataType: "json",
           success: function (data){
            var result = [];
            data.forEach(function (value, index) {
       result.push({label:value.value, data:value });
   });
 
   response(result);
}
      });
   }, 
});
  }); 


Edit

I tried with php without CI and it works. So I must change some CI settings.

My database settings:

PHP Code:

$active_group = 'default';
$query_builder = TRUE;

$db['default'] = array(
    'dsn'   => '',
    'hostname' => 'localhost',
    'username' => 'root',
    'password' => '*****',
    'database' => 'bulvestprint_mysql',
    'dbdriver' => 'mysqli',
    'dbprefix' => '',
    'pconnect' => FALSE,
    'db_debug' => TRUE,
    'cache_on' => FALSE,
    'cachedir' => '',
    'char_set' => 'utf8',
    'dbcollat' => 'utf8_general_ci',
    'swap_pre' => '',
    'encrypt' => FALSE,
    'compress' => FALSE,
    'stricton' => FALSE,
    'failover' => array(),
    'save_queries' => TRUE
);

 $db['firebird'] = array(
        'dsn'      =>  '',
        'hostname' => 'LOCALHOST',
        'username' => '*****',
        'password' => '*******',
        'database' => 'D:\Firebird_1_5\BULVEST_SPAS.GDB',
        'dbdriver' => 'ibase',
        'dbprefix' => '',
        'pconnect' => FALSE,
        'db_debug' => TRUE,
        'cache_on' => FALSE,
        'cachedir' => '',
        'char_set' => 'WIN1251', //same result with NONE
        'dbcollat' => 'PXW_CYRL', //same result with ''
        'swap_pre' => '',
        'encrypt' => FALSE,
        'compress' => FALSE,
        'stricton' => FALSE,
        'failover' => array(),
        'save_queries' => TRUE
    ); 


I tried mb_convert_encoding, iconv, change charset and nothing. The field in the database is win1251. 
What I am doing wrong?