I believe that the correct procedure is to squawk 7700 for about a minute and then set your transponder to 7600. This gets the controllers attention and then lets us know that you have a radio problem. Oh please do not squawk 7700 constantly if you have lost your radios.
Radio failures are handled differently from an emergency (7700) so if your only problem is that your radio quit then 7600 is the correct code to squawk.
Mike
NATCA BNA