Onchange Tag Of A Listbox To Use The Values To Manipulate An Iframe - "can't Load Page" - Src Is Broken

I put the following line in the onchange tag of a listbox to use the values to manipulate an iframe:
document.getElementById('viewer').src=('Location_Files/')+(this.value)+('.htm') There it works perfectly but I wanted to do more so I decided to build a function around it:

function display()
{
document.getElementById('viewer').src=('Location_Files/')+(this.value)+('.htm')
}

When I put the function in the onchange tag instead, the iframe tries to work but I get a "can't load page" message like the src is broken. Now when I first made that line it took me hours to figure out that I can't use a "" because if I do it fails. So I went with "/" and that works but I wonder if that rules change when I specify it in the java script instead of in the onchange tag itself. Either way it still breaks if I try to use "".