tcpdf cyrillic letters displayed as question marks "?"

Question

I had downloaded tcpdf and here you are - I have a problem.

simple html code

<p>Это тест</p>

works all right

$pdf = new TCPDF(PDF_PAGE_ORIENTATION, PDF_UNIT, PDF_PAGE_FORMAT, true, 'UTF-8', false);

$pdf->SetCreator(PDF_CREATOR);
$pdf->SetAuthor('Nicola Asuni');
$pdf->SetTitle('TCPDF Example 002');
$pdf->SetSubject('TCPDF Tutorial');
$pdf->SetKeywords('TCPDF, PDF, example, test, guide');

$pdf->setPrintHeader(false);
$pdf->setPrintFooter(false);

$pdf->SetDefaultMonospacedFont('freeserif');

$pdf->SetMargins(PDF_MARGIN_LEFT, PDF_MARGIN_TOP, PDF_MARGIN_RIGHT);

$pdf->SetAutoPageBreak(TRUE, PDF_MARGIN_BOTTOM);

$pdf->setImageScale(PDF_IMAGE_SCALE_RATIO);

$pdf->setFontSubsetting(true);

$pdf->SetFont('freeserif', '', 12, '', true);

$pdf->AddPage();

//simple html being shown ok
$pdf->writeHTML('<p>Это тест</p>');

$pdf->Output('example_002.pdf', 'D');

but when I try to convert my html page cyrillic symbols replaced by question marks "?"

$pdf->writeHTML($html);

Please help - I am googling for three days!

here is a link to my page so you can look at the source code http://u1159833.cp.regruhosting.ru/co_constructor/page.php