Hello,

Basically, some of the keys on my electric piano stopped working just recently, and so I took it apart to try and find the problem.

In general, I understand how the system works. I.e., you press a key, which depresses a section of a rubber strip, which makes contact with a PCB (Printed Circuit Board), creating a certain tone. I admit, it's probably little more complex than this, but as I said, I think I understand generally how the system works.

What I don't understand is why there are two knobs on the rubber strip, which get depressed. One knob sticks out farther than the other, and it looks like there are two separate contacts on the PCB. Why two knobs? I'm racking my brain trying to figure out why you would need two, when you are only playing a single note. Also, it looks like the shorter knob only touches its contact if the key is really pressed hard. Does this makes sense anyone?

I have been testing every rubber knob and its contact, by touching the two and seeing what sound is produced. Fortunately, it sounds like every contact is producing some sort of sound. The problem is when I touch some of the rubber knobs to their contacts, more than one sound is produced. I.e., a simple chord is produced, when only one note's contact is touched. It doesn't seem like this should be happening, and perhaps means that the keyboard PCB needs to be replaced?

I would really appreciate any advice, or information from anyone who has an idea of what is going on. Thank you for your time,

John

PS, if this is the wrong forum, could you please let me know where a better place to post is? Thanks!

Is there any way you can take a picture of the PCB with the 'knobs', and post it here?

And a few questions:

0. What piano are you using, and how old is it?
1. Does each key has two knobs?
2. Do you get a different sound when you press both knobs at the time from when you press only 1 knob at a time?
3. Do you get a different sound depending on whether you press the short or the high knob?

I can answer the one about the knobs. Yes it's normal for one to only press when you hit the key really hard. It's because the taller one senses the lower velocities (ppp) and the lower knob senses the higher velocities (fff).

Hello Eternal and LesCharles73,

Pleased to make your acquaintance!

Since my last post, I have learned a little bit more about my problem, and can now stop calling these knobs knobs, and start calling them carbon contacts, which is what they are.

Thanks in part to you all and to further research, I believe I fully understand how the system works now. The longer carbon tip touches first, and the shorter carbon tip touches second (obviously); but what I didn't understand is that depending on the time interval between the first touch in the second, the tone will be played louder or softer... the logic being if you hit the key very hard the second carbon contactable touch very fast in the keyboard's computer will know to play that note loudly.

I don't think this makes too much difference (please enlighten me if I'm wrong), but I'll state it anyway. The keyboard is a Casio Celviano, ap-21 from 1999.

I have done some more testing, and have come to the conclusion that mechanically everything is working fine, but there is some sort of computer problem.

I have the service manual for my keyboard, and under troubleshooting when certain keys do not function, it suggests replacing the main PCB. I'm thinking that this is probably what I need to do (please let me know if I'm missing something extremely obvious) because of the following:

At first there were only six keys that were not working, then they worked again, and then they stopped working again. Next a number of keys were playing chords instead of single notes, and finally tonight more keys are not working, and other keys are producing chords instead of single notes.

Therefore, it seems to me that something is going bad in the software/hardware as opposed to keys not lining up with contacts (mechanical).

Again thank you very much for your time,

John

Sounds like either a software or hardware problem to me too. Sometimes the ribbon cables connecting the contact strip to the main PCB can become loose, but that should not cause one key playing chords intermittently like how you describe. Still, I would check to make sure they are pressed in fully and that they're lined up properly.

As for the carbon contacts, I'm not sure why they need two, but I do know that the longer one only transmits up to a certain MIDI value, and the shorter contact transmits the rest. There shouldn't be any time interval (or delay) between them being activated. The shorter contact should be activated as instantaneously as the longer one depending on the velocity at which the key is struck. Again, not sure exactly how it works scientifically, but it does and it is obviously not the source of your problem.

In my opinion I would just make sure all the contacts are clean and the ribbon cables are set correctly. Aside from that there is not much else you can do. Did you lift the contacts off the PCB strip and clean underneath? The rubber buttons/knobs/carbon contacts are removable on most keyboards. You should be able to just pull them off (and replace them easily) because they are held in by small rubber studs that go through corresponding holes in the board. Under these contacts is the actual carbon said carbon contact is contacting. They get dirty from time to time. I would GUESS a q-tip and rubbing alcohol would clean these by going over them gently, but I put "guess" in caps because I'm not sure of what chemical would be best or considered too abrasive. If nothing else, use purified distilled water.

If you've tried all this to no avail, I would put it back together and send it in for a diagnosis or repair before buying any parts yourself, since the problem could be either software or hardware and you have no way of testing.

Actually poptech has the right idea. These are simple ON/OFF contacts, not analog pressure sensors. So using two of them is a cheap way of building a velocity detector, just by looking at the time difference between the first one and the second one is pressed. In fact most modern DP's do the a similar thing, but with a single optical sensor instead.

Poptech - I wouldn't give up just yet. Unlike software, firmware rarely just goes partially bad. In a closed system like that, it either works, or it doesn't.

Unless you had some spills on your piano (in which case you should use alcohol to clean them), the failure is most likely mechanical.

My first guess would be that one of the rubber contacts is shorted, or intermittent. You can use an ohmmeter to check each one, and see if you find an odd one (measure resistance accross each contact - it typically should read OPEN/OL for all of them when not pressed, and 0 when pressed.

And like the other person stated - unplug all connections and plug them in again - it's a good idea to use contact cleaner (or alcohol if you don't have any) on each one, and air to blow it out.

Finally any removable integrated circuits should be unplugged, cleaned and reseated again.

Again, thank you both for your time.

I think I'm going to play around with it for a little while longer (i.e., test each contact with an ohmmeter) to see if I can learn anything more. Admittedly I am learning all of this from zero, and I certainly have my doubts if I can fix it or not, but there have been many projects I have taken on that I knew absolutely nothing about, and while I have suffered many a headache, usually it was worth it, as I learn something new in the process.

So, onto more testing. I'm sure I'll have more questions for experts like yourselves if I decide not to take it into the shop. At any rate, I'm not at the end of my rope yet.

Cheers,

John