Asp.net Modal Edit Partial View Not Keeping Dropdownlist Value - ajax

Question

When the modal pops open the values that were originally selected for the drop down list have been changed to the first item on the list. How do I keep the values that were originally selected?
Controller:
public ActionResult AddEditExam(int exam_id)
{
exam exam = new exam();
ViewBag.credential_id = new SelectList(db.credentials, "credential_id", "credential_code", exam.credential_id);
ViewBag.credential_status_id = new SelectList(db.credential_status, "credentials_status_id", "credentials_status_description", exam.credential_status_id);
if (exam_id > 0)
{
exam ex = db.exams.SingleOrDefault(x => x.exam_id == exam_id);
exam.exam_id = ex.exam_id;
exam.credential_id = ex.credential_id;
exam.credential_status = ex.credential_status;
exam.exam_desc = ex.exam_desc;
exam.exam_last_update = ex.exam_last_update;
exam.exam_passing_score = ex.exam_passing_score;
exam.exam_time = ex.exam_time;
}
return PartialView("AddEdit", exam);
}
Partial View: Slimmed down to drop down list
#Html.LabelFor(model => model.credential_id, "credential_id", htmlAttributes: new { #class = "control-label col-md-2" })
#Html.DropDownList("credential_id", null, htmlAttributes: new { #class = "form-control" })
#Html.ValidationMessageFor(model => model.credential_id, "", new { #class = "text-danger" })
#Html.LabelFor(model => model.credential_status_id, "credential_status_id", htmlAttributes: new { #class = "control-label col-md-2" })
#Html.DropDownList("credential_status_id", null, htmlAttributes: new { #class = "form-control" })
#Html.ValidationMessageFor(model => model.credential_status_id, "", new { #class = "text-danger" })
<script>
$(document).ready(function () {
$("#btnSubmit1").click(function () {
$("#loaderDiv").show();
var myModelBody1 = $("#myForm").serialize();
if (!$("#myForm").valid()) {
return false;
}
$.ajax({
type: "POST",
url: "#Url.Action("index", "exams")",
data: myModelBody1,
success: function () {
$("#loaderDiv").hide();
$('#myModalAddEdit').modal('hide');
window.location.href = "#Url.Action("index", "exams")";
}
});
});
});
Original View: Modal
<div class="modal fade" id="myModalAddEdit">
<div class="modal-dialog">
<div class="modal-content">
<div class="modal-header">
×
<h3 class="modal-title"><strong style="color:#ff9c21">EDIT </strong> Exam</h3>
</div>
<div class="modal-body" id="myModelBody1">
</div>
<div class="modal-footer">
Update
Close
</div>
</div>
</div>
Original View: Modal Script
var AddEditExam = function (exam_id) {
var exam_id = exam_id;
var url = '#Url.Action("AddEditExam", "exams")?exam_id=' + exam_id;
//var url = "http://localhost/NAHP/exams/AddEditExam?exam_id="+exam_id;
$("#myModelBody1").load(url,
function() {
$("#myModalAddEdit").modal('show');
});
}