Sticky Keys for Yamaha Digital Piano YDP-121

Suppliers here

https://www.google.com/search?q=yamaha+model+YDP+161+piano+board

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use wd 40 and a soft clothe apply on the sticky areas give it time and clean after .

A YDP uses the same key type as a basic Yamaha Clavinova. As you have stripped the keyboard to reveal the keys take a look at the end where the key hinges. There will be an arrow pointing to one side. You will need to remove att the screws holding the key base (black plastic ) to the wooden base. Slide the whole keyboard back a couple of inches, push down where the arrow points between the keys, the offending key will pop up. Slide forward and jiggle out. Replace key. reassemble. If the tag on the back of the old key has snapped off remove the piece of debris that might be there before refitting.

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replacement keys can be found online

http://www.beckettsmusic.co.uk/shop/scripts/prodView.asp?idproduct=17431

Is the board missing? If not, just repair the board. Here is link to manual close to your unit:
http://elektrotanya.com/yamaha_ydp-131_131c_sm.pdf/download.html
Scroll down to near bottom and wait for "download your manual" in lower right corner. Click to download. This is the 131 model but should be VERY similar to your unit.

On SOME Yamaha pianos, there is an electrolytic capacitor 1mfd/50v that becomes leaky and causes this. On those with the particular main board the capcitor is at location C94. On other pianos there are other caps that bypass the wire from the pedals that are ceramic. Likely one of these has shorted. There is also a possibility that the pedal contact has shorted. Test for this by unplugging the pedal cable. If it still sustains, then problem is internal. Unless you are adept at electronics this is NOT a DIY repair.

The elektrotanya website link proposed in the other solution here did not work for me. So with a similar sticky keys problem on my YDP-113 yamaha digital piano, i just started taking it apart.

I found that the problem was that the sticking keys actually had small cracks at the hinge end. See attached photo for a picture of the hinge end of the key. The cracks are very small and hard to see, but if this is your problem, you will see it in a good light. Having the key apart will also allow you to clean the sides well in case there has been a bubble gum attack or similar problem.

After learning that replacement keys are extremely expensive (like $15 per key...) I decided to move the bad keys to the highest and lowest notes on the keyboard (88 key keyboard so there is some seldom used territory). The center several octaves are now free of sticky keys and this is good enough for now.

The procedure on my YDP-113 was:
1. Remove black screws from back panel of unit to allow top to slide forward and then lift off. Set aside.
2. Roll the roll top all the way closed and then simply lift up to remove. Set aside.
3. Remove the six screws which attach the roll top gear rail (3 screws on each side panel). Set aside the gear rails.
4. Remove 2 screws in the key top bracket, and 1 screw from each end of the key top. "Key top" is my name for the small strip which lies along the top of the keys. If you have an official or better name, please share.
5. Remove the 10 or so large screws and two small screws which hold the keyboard panel down in the unit.The keyboard should now be able to slide a few millimeters toward the back of the unit.
6. Remove 1 screw to take out the power switch panel. The front of it hooks in with a tab, so slide it back and then it will lift out.
7. Careful now. I felt this was the one operation where I had an opportunity to ruin the unit if I would have fumbled: Lift the keyboard up slightly and rest its front 'legs' on the front edge of the case. Careful of small wires to not rip them. Careful of the circuit boards that are underneath the keyboard frame. The objective here is to allow the keys to come out toward you so that they can be removed and replaced or swapped. You can kind of see in the cracked key end photo, how the keyboard is up and partially out of the unit.
8. You now can do the actual swapping. To remove a key, pry up very gently in back of the key with a small screwdriver. It will pop up. Then gently work the key around to free it from the return spring without bending that spring. Replace with a new key or swap will a good working key in a little used position. Note that B and E are interchangeable, also C and F. Some other keys also look like they could interchange though I didn't personally try it.
9. Reassemble in reverse order of disassembly.

Yamaha keyboards can be prone to sticking keys due to the rear part of the key cracking around the return spring - if this is the case the only solution is replacing the affected key modules. However this problem generally only occurs with long term use by very physical players.
Otherwise the problem may be caused by a build up of dust / fluff between adjacent keys - try raking out the spaces with a thin piece of fairly rigid plastic (maybe a credit card might work?)

Check if it might be dragging on the edge of the case. The restoring force for some of these comes from a silicone rubber dome that is oart of the contact. SOMETIMES the dome fails.

I wrote up and photographed for my similar YDP, see solution 2 under this post:
http://www.fixya.com/support/t8881187-1_key_not_spring