My Privia PX-850 developed a loud key (i.e. full volume regardless of the key actuation force). I suspected an issue in the sensor of the key, and opened up the piano to try and repair the issue. I could not find nice information online on disassembly of this piano, so I'd like to share some pictures with you here, perhaps it will be of use to someone.
I started by removing the backpanel. That was easy, but be aware of the cable connecting the lid sensor. There is a connector that allows disconnection.
Next I removed the keyboard cover. A screw holds the metal bar in place on the spring mechanism. Loosen the screw and slide out the bar. Next, the whole cover can be slid out of the rails.
Next, I removed the whole keyboard assembly out of the cabinet. In retrospect than may not be necessary to remove the pianokeys and access the sensors, but this is how I did it. I loosened the center electronics cabinet and loosened all connectors. The high density flat cable can be unlocked by lifting up the plastic top ridge in the pcb connector. The right speaker casing also needed to be removed as it covers some screws.
I removed the front panel with the controls. It is attached to the keyboard assembly through several posts, unscrew these from the front panel. Also, At 1/3 of the length next to the grille two screws keep together two segments. After detaching, both halves easily come out. I removed the tape and ziptie connecing the cables to the white keyboard assembly.
The keyboard came loose with 3 screws on top and a whole bunch on the bottom. After this, the whole assemble could be maneuvered out of the back with relative easy. Take care to support it well over the length.
Next, I loosened the retaining bracket behind the affected keys. Also, the black posts for the front panel had to be removed in that segment. After removal, I could take out first the white keys, then the black. The keys hinge between white plastic tabs, after prying and spreading them a bit the key can pop up, and move back and out. I removed keys until a whole sensor segment was exposed..
I loosened the rubber strip from the membrane switch array gently. It is stuck in with soft rubber pins in holes on the pcb.
Each key had 3 contracts (tri sensor!) at different heights, I suspect the connect sequentially and stroke force is deduced from the timing between them.
Finally, what I came to do. I gently cleaned both the carbonized rubber contacts and the zigzag pcb segments with isopropyl alcohol and a sort of cotton tip applicator.
Then reassembly. The rubber segment was hard to get seated in the holes, until I found that a paper clip wire can poke in the holes on the top and nicely push in the 'legs' below in their holes. Next the keys went back in, secured by the brackets, everything back in the cabinet, and try to remember where all screws came from.
In the end, I powered up the instrument, and yay! the piano key is now touch sensitive again. Mission accomplished.
Over here I made a movie of the action being actuated.