I have a P-95 that stopped working as well. After contacting Yamaha, they told me that the P-95 is discontinued and not supported. I decided to diagnose it myself. First, I am an electrical engineer so, I was able to dig into the problem much deeper than the average Joe. So, the P-95 is powered by a 12v DC adapter. My adapter was producing 14.5v DC which is actually fine. I found a YouTube Vid to show me how to crack the keyboard open. It is well worth you time to find this vid as there are 57 screws you have to remove. Also, the small panel on the back MUST be opened first and the 2 ribbon cables must be disconnected before your separate the halves. I was able to find a service manual on line. The P-95 and the P-85 have the same guts. You must actually download the P-85 service manual as it has block diagrams and schematics.
The power section is located on the AM board and is really a simple circuit. 12 volts run thru a fixed fuse, filtered and then is "switched" by a power transistor circuit ( TR-401, TR402). How this works is as follows, when the power button is depressed, the circuit is triggered to pass 12 volts through to two (2) 5 volt regulators. One regulator is draws alot of current and is mounted on a heat sink (IC401). This powers the computer portion of the keyboard electronics. Once the main computer chip sees power, it wakes up and then sends a second trigger signal to the 12 volt power circuit telling it to "stay on" until the power button is hit again.
In my case, I was able to simulate the power switch signal and saw that the 12 volt switched circuit was actually working. So, then I focused on the 5 volt regulators. I suspected that the high power regulator was possibly bad. Sure enough, the low power regulator worked fine but there was nothing coming out of the 5 volt high power regulator lead. I ordered a new power regulator (Part Number NJM7805) and received it about a week later. I removed the old part from the board and replaced it with the new power chip. I was able to test the board by applying power and then simulating the power button switch and the power supply circuit began working like a champ.
Power electronics can overheat and will eventually pop. It happens. I was fortunate to find the schematic and figure out what component had failed.
Here is the link to the manual I used:
Also, this is the AM board. The bad IC was the single transistor mounted on the left hand side of the heat sink in the picture. It was not difficult to de-solder and replace.
It's a shame that Yamaha is not supporting this keyboard any longer. It would be easy enough to keep a supply of AM boards on hand for replacement. Hope this helps others keep their keyboards up and running.