What is the best approach to trace the audio signal path if one of the channels starts crackling intermittently when the volume is adjusted? It would be incredibly helpful to find a detailed technical map of the internal boards to locate the exact position of the fuses and relay switches. My goal is to perform a thorough cleaning of the interior without accidentally disconnecting any sensitive wiring or damaging the original factory solder joints.

Does the top cover of the unit gets unusually hot after only twenty minutes of use? Have you ever had any experience measuring current in any devices? If you don't really understand this and don't know what fuses look like, then it's better not to interfere with a screwdriver inside, because there is a high probability that you will simply break it. You can view the service manual for Sansui AU-D607 on the ManualMachine website, where there is a diagram of how and what is located inside, but I hope that you will just clean it and that's it. If you have a slightly crackling sound when you change the volume, then it could simply be due to dust inside the buttons and you can remove it with a special spray for contacts, but very carefully.