Digital Piano - keys gone mad (keyboard matrix troubleshooting).

[Yamin]

Hi there,
I've got a Yamaha tyros 4 in for repair and the keys seems to have a mind of their own, some keys make sound, some comes out on full volume, some keys remains to sustain long after the key is released and some don't even make sound. Its not consistent meaning the same note does not behave the same all the time.
The keyboard had some issues in the past but it was typical velocity/dead keys and cleaning the rubber contacts solved the problem. However this time it seems this is not the case, I did clean the whole keybed nonetheless and to no surprise it didn't solve the problem. Did a factory reset and it didn't solve the problem either.
So I took out the oscilloscope and probed the scanning pins. Here is what I found:
1. All the pins marked BK (BK00-BK13) shows a nice square wave, with no noise. The 'on' time is longer than the 'off' time.
2. Pins marked with MK does not show a steady square wave but a weird 'spiky' wave - like a weird 'saw-toothish' wave. MK24 was the worst, also on one MK pin(can't remember which one) it showed a square wave superimposed over the wired sawtooth wave. 

I couldn't probe around the IC as I would have to disassemble the whole keybed. Was hoping I could get your guys advice on what to look for next and what could be the probable cause for this fault.

Appreciate the help.

[KevinA]

Does the self test routines (Page 65 onward) show anything?

[Audiorepair]

Before jumping to overcomplicated conclusions, I would take some time to assess the situation.

So, you say you have had this keyboard in for repair more than once before for dead/faulty keys, and it was dirty/worn key contacts strips that you cleaned and the problem went away.

But this time, even cleaning didn't solve the problem.


Could it not just be that the key contacts are just so worn now that no amount of cleaning is going to bring them back to life again temporarily, until the keyboard is brought back to you again?

[Yamin]

Does the self test routines (Page 65 onward) show anything?

I wasn't able to get into the test program this time as the way which involves getting in to the routine involves having to press certain notes on the keyboard while powering up. The notes pressed are not registering I guess.

Before jumping to overcomplicated conclusions, I would take some time to assess the situation.

So, you say you have had this keyboard in for repair more than once before for dead/faulty keys, and it was dirty/worn key contacts strips that you cleaned and the problem went away.

But this time, even cleaning didn't solve the problem.


Could it not just be that the key contacts are just so worn now that no amount of cleaning is going to bring them back to life again temporarily, until the keyboard is brought back to you again?

Sorry my wording was a bit misleading in my first post - it was in for repair once before and that time one note was dead and another note had velocity issue and cleaning the pad and strips solved the issue. The keyboard is not heavily played as the owner is not a full time gigging musician and he only plays it during his free time. Visually the pads doesn't look at all bad. The problem occured all of a sudden as per to the owner and like I mentioned the same problem doesn't seem to stick to a particular note eg: if I press the 'D' there might be no sound if I press it again it might make a sound and if I press it again the note may sustain after releasing the key. It happens randomly on random notes.