I have an old Pioneer receiver that I got for free because the guy said it didn't work. He took it to two different places and they both told him that there was absolutely nothing wrong with it. So I plug it in, turn it on, and for about three seconds everything seems normal until the display starts flashing POWEROFF and then it shuts itself off. I just thought I'd ask someone on here if they have ever had a problem like this before I go tearing it apart.
Something is shorting the output side of the receiver. Assuming you are using different speakers and speaker wire than the person who gave it to you, you should have all output jacks free from connections, volume turned all the way down, then turn the receiver on. If receiver shuts down, you should contact Pioneer or try to identify the short if your are comfortable with doing that.
I have an old Pioneer VSX-411S. I had the same problem. Mine would work for maybe 30 mins to an hour and then just die for no reason. It turned out to be a crack in some circuitry around the front display where the function buttons are. The local Pioneer repair guy couldn't find the problem, so I took it to an independant and he found the problem in 15 mins. All it took was a small solder job to fix it.