hifi engine

hitachi ha-5700

Hi guys! I've been lurking in this forum for quite some time, but it's the first time I'm posting. I have a Hitachi HA-5700 which I bought new, back in 1979. I've been using it since then and it has never been opened for maintenance or repair. A month ago I noticed that when I turn a switch or a knob a popping or a crackling (static?) noise is heard and sometimes the volume level drops. When I stop moving them the noises disappear and the sound is great as ever, but nevertheless it's annoying. Is there any remedy to this? Any input will be highly appreciated.