così?
codice:
<input type="radio" value="2" name="PH" onClick="javascript:document.getElementById('cp2').value='3600';"> 
<input type="radio" value="quellochevuoitu" name="PH" onClick="javascript:document.getElementById('cp2').value='--';"> 
<input type="text" size="6" id="cp2" name="cp2" value="--" readonly>