Dielectric grease != heat sink grease. The latter is specially formulated to fill microscopic pits and voids with material that's a good conductor of heat. It should be available anywhere you can get transistors etc that go on heat sinks.

Pioneer receivers are notorious for their pistol-hot power supply regulators. Some people relocate the big transistors to the amplifier's main heat sink. While you have the PS board out, you might as well replace the electrolytic caps, because they've been in the oven for too long. Replace with models selected for high temperature and long life, so you don't have to do it again.

Just wanted to say that I found this thread after experiencing the exact same issue on my SX-780. I re-soldered the 3 regulators on the board and it now works perfect. Thanks!

I cannot even count the number of receivers that can be fixed by just two service operations. The first is a visual inspection, followed by removing and replacing the solder on any overheated joints. Typical are the aforementioned regulator and pass transistors, and a lot of the time, high wattage resistors. They just degrade the solder on the PCB over time, and sometimes even lift the foil.

The next biggie is dirty switches. Over and over again a unit comes in with "powers up but no sound" ... and turns out to be nothing more than the TAPE MON switch having oxidized enough to simply prevent any audio output. A shot of contact cleaner or D5 and she's good to go. Of course such units get every pot cleaned, shafts lubed, and the rest of the switches cleaned as well.

For the other 25% of the units..... some can be extremely painful to fix. Surprisingly few bad electrolytics however. Should they all be replaced ? Probably but... the intent here is to "just fix the unit". If someone wants one fully restored, I pass. One cannot charge enough to replace every electrolytic in an old Pioneer, or even a Marantz, if you have to charge for your time.

And as far as the protection relay, I have also seen a few of these that have intermittent contacts or don't make contact at all. Depends on how long the thing has been sitting unused.

One of the things that cause problems in old Pioneer receivers is defective transistors. Some types are known to get noisy, especially 2SA726 and 2SC1451.

I am slowly restoring an old SX-1010, which has been full of intermittent problems, affecting the protection circuit. Having rebuilt the tone control, power supply, and protection boards, I am going to rework the driver boards. One of the bias pots is very touchy, and needs to be replaced. Those, as well as the DC offset pots, can cause the problem.