asp.net with javascript

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I have textbox whose text is assign through checkbox item selected
using javascript.
I also have two dropdown whose value is interdependent each other which is assign through .cs server code in c#.
while selecting dropdown value textbox value lost.
how i retain textbox value ?????

i already used update panel and save checkbox selected value in hidden field and then assign it to textbox in javascript.
textbox is readonly

help me

Javascript function:

<script type="text/javascript">

     function Checkinternationallocation(checkBoxList) {
         var options = checkBoxList.getElementsByTagName('input');
         var arrayOfCheckBoxLabels = checkBoxList.getElementsByTagName("label");
         var s = "";

         for (i = 0; i < options.length; i++) {
             var opt = options[i];
             if (opt.checked) {
                 s = s + ", " + arrayOfCheckBoxLabels[i].innerText;
             }
         }
         if (s.length > 0) {
             s = s.substring(2, s.length); //sacar la primer 'coma'
         }
         var TxtBox = document.getElementById("<%=txt_combo_international_location.ClientID%>");
         TxtBox.value = s;
         document.getElementById('<%=hide_international_location.ClientID %>').value = s;
     }

 </script>

:html code for textbox:

<div class="rightPositioncontrol">
                  
                           <asp:TextBox ID="txt_combo_location" runat="server" Width="450" onChange="tab();"  ToolTip="Location(s) of Job"></asp:TextBox>
                           <cc1:TextBoxWatermarkExtender ID="TextBoxWatermarkExtender5" runat="server" TargetControlID="txt_combo_location"
                               WatermarkText="Select Location(s) of Jobs" WatermarkCssClass="watermarked" />
                           <cc1:PopupControlExtender ID="PopupControlExtender3" runat="server" TargetControlID="txt_combo_location"
                               PopupControlID="Panel_location" Position="Bottom">
                           </cc1:PopupControlExtender>
                           <input type="hidden" name="hide_location" id="hide_location" runat="server" />
                           <asp:Panel ID="Panel_location" runat="server" Width="450" Height="200px" ScrollBars="Vertical"
                               BackColor="AliceBlue" BorderColor="Gray" BorderWidth="1">
                               <asp:CheckBoxList ID="chk_location" CssClass="radio_button_txt" runat="server" Width="450"
                                   onclick="Checklocation(this)">
                               </asp:CheckBoxList>
                           </asp:Panel>
                       </div>

thanks in advance