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Just don't split the ajax setting into two parts. So your Tooltip code should be like:
<a onmouseover="var r = Math.floor(Math.random()*100); tooltip.ajax(this, '/Home/UpdateVotes/'+r,{success:callback2, responseType:'json', position:0});" class="tooltip">Tooltip Loading Demo</a>
Also make sure your callback2 function exists on the page.