Here's a step-by-step guide to opening and cleaning the contacts on the PX-5S and similar Casio keyboards.
At the end of the guide is a series of pictures showing how I installed LEDS in the keyboard, and also a potential fix for the infamous Bb-key velocity problem.
Before you start: A bed makes a good surface for disassembly, but I found it easier to disassemble my keyboard right on the Z-stand. This allows access to all sides of the keyboard (including, importantly, the bottom) without having to roll it over.
IMPORTANT: Remember that these are small screws and they all thread into plastic! Be sure to set the screws when installing, and use little torque! It's better to have to re-snug them later than to strip the threads by installing them too tightly.
Here's a short video on how to set threads: https://www.youtube.com/watch?v=X1QjwVYbrDs
One more thing to note: All the screws are the same except the three internal keybed screws (distinctive because of their head shape) and the screws securing the access panels on the bottom (distinctive because they're smaller than the rest of the screws).
Tools you'll need:
#2 Phillips screwdriver (magnetic-tipped is best)
#1 Phillips screwdriver
Small pick for releasing the ribbon connector
Small applicator (I used cotton swabs but it's easy to leave strands of cotton so be aware of that)
Follow these steps:
NOTE FOR STEPS 5-7: I usually start by removing the #1 Phillips screws and the access panels, then removing all the #2 Phillips screws. Again, I recommend doing this whole operation with the keyboard upright on it's stand if possible. If you perform Step 7 with the keyboard inverted as shown, you will need a magnetic-tipped screwdriver in order to extract the screws.
Addressing the Bb Velocity Issue:
If you're here, you're probably already familiar with this issue so I won't go into detail describing it. Cleaning the contacts works well, but the problem will undoubtedly pop up again and most of us don't have the time to pull the board apart every few months to re-clean them.
I have a theory that the reason the issue affects this particular key (although it can pop up with adjacent keys also) is that the rubber dome strip doesn't fit flat to the PCB at this point. The rubber dome strip bridges a gap in the PCBs, and the spacing isn't quite perfect so the strip has a slight "wave" to it. This allows moisture to get underneath and contaminate the contact points.
My potential solution was to cut the rubber dome strip, and remove a little bit of material from one of the cut faces to allow the strip to rest flat. However, it should be noted that this will allow the cut ends to "flop" due to the spacing of the mounting dowels on the rubber strip. To secure the cut ends, I used a drop of hot glue on the front and back.
Make sure the hot glue goes around the top of the rubber, and underneath the bottom of the PCBs to form a sort of "I" shape. This will harden into a sort of clip to hold the rubber to the PCBs. This technique is necessary because hot glue won't adhere to the rubber.
Time will tell if this solves the moisture issue.
The Bb issue is back! This time I am of the opinion that it's not condensation collecting as I suspected before, but that the factory-applied grease may be breaking down and the oily residue making it's way underneath the silicone dome strip via capillary action.
I replaced my chopped dome strip with a new one, and this time, to eliminate the "wave" in the strip, I decided to move the outer PCB a little bit to the left to widen the gap between it and the center PCB, thus taking up the slack in the dome strip and forcing it to lie flat.
First I disassembled the PX5S and removed all the keys over the left-side outer PCB. Then I removed the seven screws which secured the outer PCB.
The outer PCB is aligned in position by a small tab on the plastic chassis, which nests into a notch on the PCB to register it. In order to shift the PCB outward, I cut this tab off with nail clippers and smoothed the remains by shaving it with an X-Acto knife.
Here is the tab before I cut it off.
When I lifted the PCB off of the plastic chassis, I noticed an oily trail that had collected underneath the PCB. I suspect that this is a separation from the factory grease that was used to lube the key pivot points.
Just for fun, I installed a cotton ball in the space between the PCB and the plastic chassis, underneath the gap. The theory is that any further seepage will be collected by the cotton rather than allowed to creep up over the PCB and under the dome strip. Whether or not this will have any effect remains to be seen.
In order to shift the outer PCB to the desired location, I had to remove a bit of material from the inside of the screw holes.
With the PCB shifted over, the dome strip can now lie flat with minimal ripple. Hopefully this will prevent the oily residue from contaminating the contacts.