I am a know-nothing when it comes to stereo equipment. A friend gave me an indefinite loan of his Fisher 400 vacuum tube receiver. We love it not just for the sound, but because the tuner actually picks up stations most crappy modern equipment won't. But about two months after we got it, the tuner stopped picking up ANY signals at all. Anyone have a clue what might have broken? Is the tuning function controlled by one of those tubes?