[codefox1]

Well, I am assumingh that the thing plays after the thump and isn't blowing a fuse. If it is a tube radio, it might be something silly as a transformer whose wax or varnish has dried out, but it could be some sort of protective device on a solid state set. Some of the latter used relays to shield the more delicate components from damage if there was a short to the output (i.e. speakers) etc. I'm afraid you will have to be more specific. Tell us more!