Roland D-50 key contact cleaning
The Roland D-50 is an amazing synthesizer, but its keyboard circuitry also has some small issues that may show up after all those years.
To detect the key velocity, there is a pair of electrical contacts below each key. Due to their arrangement, the time between closing the two contacts varies depending on how hard you hit the key. The contacts are protected by rubber caps, but small dust particles (or as my case when I got my D-50 five years ago, a piece of tin foil 
) may get below the caps and prevent the circuits from closing. Depending on which contact the dust ends up on, the key no longer sounds at all (I have never experienced this), or it always sounds at full velocity (like in my case). It is also possible that this only happens randomly and not every time the key is pressed.
Luckily, it is possible to fix this issue by cleaning the circuit boards. There are various tutorials on the internet which helped me fixing the problem. However, some of these tutorials contain unnecessary steps or otherwise contradict each other, so I decided to share my experience with the procedure here. Still, you may read the other tutorial and watch the video to get a more complete idea of what you need to do. Read the whole tutorial first before doing anything!
Step 1: Opening the device
To open the D-50, it has to be put on its top. Mount it on some books or similar boxes to avoid resting the synth on the joystick and pitch bender. Remove the five screws on the back of the device and the screws on the bottom of the synth. You do not need to remove the four feet.
Step 2: Removing circuit boards
Once the synth is open, it's time to get the main board and key scan board out of the way. The main board is the big circuit board that sits in the middle of the synth. I decided to not unmount the board entirely, since I am not a masochist and do not want to disconnect all the connectors on the mainboard and ruin my fingers. I also wanted to cut a minimal amount of cable ties. I ended up cutting the two cable ties that hold the shielded cable at the bottom, as that will also help removing the key scan circuit board entirely. After that, I started disconnecting connectors on the mainboard until I could swing it away to the back of the synth.
Technically, you probably only need to unmount the key scan circuit board if you want to fix any keys below it, but I still recommend unmounting the board and keep it safe. To do this, you need to disconnect some more connectors below the board.
Once this is done, you can remove the six screws the hold the keyboard assembly in place. Have a look at the other tutorial if you are unsure where the screws are located. Also disconnect the aftertouch ribbon cable at the right end of the keyboard assembly from the board. Now you should be able to move the keyboard assembly out of the synthesizer. Be careful when doing so. 
Step 3: Fixing the keyboard
To remove the keys, you first need to remove the springs that hold them in place. Note that black and white keys use springs of different length, so keep them separate! The springs for white keys have fewer coils than the springs for black keys. You will need to remove a number of keys next to the faulty key - how many pretty much depends on the length of the rubber strip below the keys, which varies across the keyboard. If you are unable to find the ends of the rubber strip, just remove more keys.
After the springs are gone, you may have to loosen the plastic film strip that holds the keys in place on the back of the keyboard assembly. You can pry it open carefully with a screwdriver. You can then remove the keys by pushing them below the film strip, but note that black keys cannot be pushed out before surrounding white keys are gone.
Now that the keys are gone, gently put the paper strips below them to the side (e.g. by fixating them with a screwdriver). Below them, you can find the rubber strips which need to be removed gently. Make note of how exactly the rubber strips were inserted (both vertical orientation and the horizontal "offset" is important!). Once those are gone as well, you can clean the contacts on the circuit board using rubbing alcohol (preferrably 90% or more) which you put on a q-tip. Be sure to clean the surroundings and the rubber caps from any debris as well - you do not want to repeat this procedure again too soon, do you?
Step 4: Putting everything back in place
Let the board dry a bit and put the rubber strips back in place. If they do not seem to fit at first, you possibly rotated them accidentally. Turn them by 180° and try again. Remember to put the black keys back first, then the surrounding white keys. Now you can put the springs back on the keys, and make sure the film strips on the back sit firmly again. If it no longer glues to the board afterwards, some tape or glue may help with keeping them in place.
Now you can mount the keyboard assembly again, reinstall the screws and connect the aftertouch ribbon. Reconnect the key scan board and install the screws. Do the same with the main board, and put some new cable ties on the shielded cable. Close the unit, put the screws back in place and you're done! 