I have just started working on a defunct Pioneer SX 850. When it arrived there was no audio and no relay “click”. The insides, besides being dusty looked OK.
So I started with the power supply and removed and replaced all the electrolytic capacitors with a mix of Nichicon low impedance and audio types (all 105c). There are two transistors which should also always be replaced as they are often prone to failure. Having done this and with unit connected to a variable power supply with a fats trip and current meter, I turned it on, and still now relay click. So I tested all the voltages across the Power supply, ans everything is now as it should be.
So the next board to work on is the protection circuit, again there are two transistors, that should be upgraded, and all the electrolytics replaced. There are two small electrolytics, Sanyo, that I replace with WIMA film caps. Again turned it on, and hey presto the pleasant sound of the relay kicking in, so I then ran some tones through the aux input and all is good. Next is the phono equalizer, where all the transistors are replaced with low noise types and the small value electrolytics by WIMA film capacitors and the other electrolytics with Nichicon Audio types.
so fa so good- now on to the main amp boards- photos to follow