Check Box

Here is my function

<script language="javascript">
document.form.B1.disabled = true;

function changebutton(){
if (document.form.C1.value = "ON"){
document.form.B1.disabled = false;
return true;
}
else {
document.form.B1.disabled = true;
return true;
}
}
</script>

So far it's doing what I want it to do, when the document loads it disables
the button, and when the check value is clicked it would enable the button.
Now here is the problem, when I uncheck the checkbox the button is not
disabled.