foreach (Control c in this.Repeater1.Controls)
{
HtmlInputCheckBox check = (HtmlInputCheckBox)c.FindControl("cbDelete1");
if( check != null )
{
check.Checked = true;
}
}

for (int i=0;i<this.Repeater1.Items.Count;i++)
{
HtmlInputCheckBox check = (HtmlInputCheckBox)this.Repeater1.Items[i].FindControl("cbDelete1");
if( check != null )
{
check.Checked = true;
}
}

foreach( RepeaterItem item in this.Repeater1.Items )
{
HtmlInputCheckBox check = (HtmlInputCheckBox)item.FindControl("cbDelete1");
if( check != null )
{
check.Checked = true;
}
}