My English is not perfect, the same with my piano-playing.
I´ve got an old Korg Concert 4000 with electronic damage (SMD-condensator around the D/A-Converter had sporadic shorts because of moisture-damage, so it makes a crackling noise).
I repaired this fault (the first time i ever opened a DP!) and also cleaned and greased the whole keyboard (13 hours of work!).
Now I am very happy and try to learn to play piano.
But there are two strange behaviours:
When i press the keys C, D, E, F, G simultaneously and release only the C-key, it makes a loud sound!
Another strange thing:
When i press and hold C-sharp and then press C, the tone for C
is not normal!? These two things happened in all octaves over the keyboard!
So it is no fault in one key. I have NO idea, what´s wrong. Processor? Firmware-bug?
Does anybody have an idea?
I uploaded a short video with sound:
http://homepage.hamburg.de/Katharina/piano.avi
I know, this is a very old piano and can´t be compared with
the DPs of today, but it was for free and it makes no sense for me to buy a new one.
With friendly regards from northern Germany!
Andreas

I don't think something that severe could be a software bug... everyone else would surely notice it.

Key decoding is faulty and velocity measurement is faulty.
This must be a shortage or interruption in the keyboard scanner matrix.
Or/and broken or polluted contacts in the keyboard.

But the "bug" is in every octave?!


When only ONE key is pressed, everything is o.k., the same when
two ore more other keys are pressed. I can press C and E or C and G together or one of them first, then the other: it´s fine!
Therefore i don´t think it´s velocity measurement.


If it is an interruption, the key would not work.


I cleaned all contacts on the keyboard, checked all solderings and connectors. The bug is in every octave, so it´s unlikely to be a damaged key or key-contact.

So i must search, check, try this & that and hopefully i will find the bug someday.

Yes the keys are scanned in a matrix.
If there are for example 12 columns for 12 tones and 8 rows for 8 octaves and if there is a short or breakage then this can happen.
(I dont know how the matrix is organized)

I would check for broken soldering points first. Mostly these PCBs are single sided and the soldering can break.
Use a strong magnifying glass. Sometimes the breakage is very fine and only visible with strong light, magnification and experience.
For each contact there should be a diode. Check these carefully and any through-hole components like resistors ICs and connectors. If you see something like a pale ring roundabout the wire in the soldering point, then it might be broken.

If the piano has a transpose feature then try this. If you transpose, but the problem stays connected with the same keys, then you can be sure that it is a keyboard problem.
Also check if there is something like a system reset.
Check here for the service manual:
http://www.wdgreenhill.com/manu/korg.htm
Its not in the list, but ask this guy via email. He has other stuff or might know a compatible model.

BTW, I am in Schwabenland near Stuttgart ;-)

Best,

Peter

BTW here is an offering for the service manual:
http://www.guitar-parts.com/products/1385/Korg-Service-Manual-C4000-C5500-C30-C40-C50.htm

But check yourself if you find it cheaper or for free......