Just acquired a Marantz SR-640 stereo receiver but the tuner doesn't work. Neither AM nor FM. No signal strength lights light up and no FM Stereo light, no matter which antenna I use or what station I tune to. No static sound either. In fact, in 'Tuner' mode I get no sound at all, except a faint "hiss" when the volume is way up. It's as if the tuner section is 'disconnected'. All the other inputs (phono/line) work great. Any ideas where to start?