Yours was literally the only YT video I could find that did work on the same model of piano I have! I am dealing with a similar issue where just my Bb5 is acting up---when pressed with a considerable amount of force, the note will play again at max volume. Of course, it started doing this right as I began to use it to perform on weekends and occasionally for practice throughout the week. Getting it professionally repaired would be at least $69 and a whole week at the shop, which feels a bit silly for a single key, so I'm trying to figure out if I can do this myself. Although I'm not very well versed in such repairs, and I don't have all the same tools either...

How did you get the panel off?

Sure, my Bb5 has the bug mentioned below.
Slightly muted, press it hard and let go and get hammer on sound level.
I'm a retired software engineer who repaired small engines for about 10 years
so I look forward to taking this thing apart without nitrile gloves (greasy engines).
[ update: those keys ARE a greasy mess...I was wrong ]
-- Key removal tip helped.
-- Cleaning tips helped.
-- Paper clip to set rubber switch mat helped.
-- Removing all video of disassembly did NOT help.
LATER:  Yes I fixed the problem with that 1 key.
Something had been spilled on the keyboard and got into those contacts.
How could we all spill something on the same key ??? I do not know.
Now its time to put it all back together.
Oh 51 exterior screws removed to take shell apart and lift keyboard up
enough to get keys out.  3 on the inside.  You WILL need a magnetized
phillips head screw driver.
I did NOT remove ribbon cable.
Parts left over:  4 screws.  Not a clue where they go. Problem solved....
Thanks.
--  North Central Florida.
-- 10.6.2026

Damn. I'm having just about the same problem with my Casio PX-780M. The woofer starts thumping when i hold the D5 key solidly. It's several years old but been hardly used. Seems a shame to turf it but it sounds like it's not worth fixing.
Edit: only happens in the "Piano" instruments sounds, not in "Strings", "Organ", or other instrument sounds.

Did you maybe try and put a graphite on the rubber strips, (on the black dot part) tho like the lead of a pencil, just rub it on the rubber contacts? I had similar issue with my digital piano but it didn't make the pop buzz sound tho but maybe it would work. It's a trial and error, I failed a lot of times before actually making it work.

Also maybe the power adaptor is faulty, try it with a new one to see if it fixes it.

I'm planning to buy this also Casio digital piano cheaply it had the issue like this same exact like yours but on lower keys, though Bright Piano isn't affected while all the 4 piano tones are affected lol, wondering if it's even worth to buy (just a spare piano tho) but still it's money.

Man, you sure do a lot of talking! I can't believe I've been watching this for 16 minutes just to learn you're not solving the problem.
Does anybody here have a solution? I would hugely appreciate!

Sure would've been nice if you showed how you opened it up in the first place. Oh well.

Have you isolated this as being the contact by first playing that note via Midi In?

thanks for this video! it's my exact same model casio, and my problem is that the D-flat/C# key works only half the time that i press it, regardless of how much weight or force i use on it. Just finally got the keys to pop out and gonna go get some anhydrous alcohol to clean up the contacts

i have this privia px150 with 4 sunken keys, what to do? some solution? only casio digital piano have problems ?? or this happen to any brand??