I am running a NAD 3150 integrated amplifier bought new many years ago. My problem is this: the right channel regularly drops out, i.e. goes silent. I can restore the sound by turning the channel knob fully to the right then turning the volume up full for a split second. The sound is restored but invariably drops later. I am not into electronics so am unable to test anything inside the case myself. Is this problem due to something simple like dirty controls? if so, how do I take the knobs off to get at the contacts? How should I start? Any suggestions and advice much appreciated.