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Set RadComboBox value from javascript
function OnClientSelectedIndexChanged(sender, eventArgs) {
    var item = eventArgs.get_item();
        var ddl = document.getElementById('ctl00_plh1_Test1_Dropdown2_RadComboBox1_DropDown');
        ddl.selectedIndex = 0;

}

<telerik:RadComboBox ID="Dropdown1" runat="server"
                             NoWrap="true" Width="250" OnClientSelectedIndexChanged="OnClientSelectedIndexChanged">
                            <CollapseAnimation Duration="200" Type="OutQuint" />
                        </telerik:RadComboBox>


<uc2:RadComboBox ID="Dropdown2" runat="server" DdlAutoWidth="true"></uc2:RadComboBox>

var mycombobox = $find("<%= MyUserControl.FindControl("RadComboBox1").ClientID %>");

var mycombobox = $find("<%= RadComboBox1").ClientID %>");

mycombobox.clearSelection();

<rad:RadScriptBlock runat="server" ID="RadCodeBlock">
   <script type="text/javascript">

   </script>
</rad:RadScriptBlock>

var radComboBox = <%=YourComboBox.ClientID %>;

radComboBox.SetValue("someValue");

function onComboBoxSelectedIndexChanged(s, e) {
    var ctrl = '<%= Dropdown2.FindControl("RadComboBox1").ClientID %>';
    if (ctrl) {
        ctrl.set_selectedIndex(0);
        ctrl.set_text('');
    }
}