keyboard fault: 1 key 2 letters

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keyboard fault: 1 key 2 letters

Postby Dege » May 16th, 2011, 2:49 am

I have a great keyboard, a BenQ x-touch 800, its' very confortable and I love it, but a year ago it started to act strangely:
when I press some key it output 2 or 3 characters.
Ex:
8 -> 81 or 18
i -> qi or iq and block num
The result? The keyboard is totally unusable and I've had to buy a 10€ replacement.

Now I've discovered this great forum and I want to try to fix the problem, but I don't know where to start.
So far I've opened the keyboard and took some pictures of the controller board, everything seems fine to my eyes.
Here are the pictures:

Image
Image
Image
Image
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Re: keyboard fault: 1 key 2 letters

Postby k-ww » May 16th, 2011, 6:33 am

If the IC [under the black blob] has gone wonky, you're out of luck.

You didn't show the keyboard part of the keyboard - if it's a folded sheet of plastic that the keys press on, there might be moisture in there [spilled drink?]

That could cause the IC to see two keys that have a common element in the XY matrix to cause the other to occur - if you type 1, do you also get 8, etc?

Also, if the replacement keyboard is the same type, I would swap the electronics board between them, and see if the fault tracks the electronics, or the keys.
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Re: keyboard fault: 1 key 2 letters

Postby st2000 » May 16th, 2011, 7:53 am

Keyboards are really cheap. You sure you can't be happy w/another?

You didn't say the type (QWERTY, DVORAK, USB, PS/2).

Probably doesn't matter. If anything works, it is probably not the electronics.

Probably just needs a cleaning.

Here's my argument: If it is like a laptop keyboard (very thin) you are probably looking at trying to fix bent parts. Very difficult to get back to the original look and feel. I would say impossible. If it is ordinary (like maybe 1.5cm or thicker), it will be tedious work to disassemble, clean and reassemble all the parts. Mainly because the keys will most likely fall out. But...

I'm guessing - 'cause there are no picture of the key switches...

There's probably a bottom plastic PCB, a plastic sheet with holes for each key, a top plastic PCB and a rubber
"dimpled" sheet on top. On top of that the plastic key float. A variation might be to only have the bottom PCB plastic sheet and the top rubber "dimpled" sheet with conductive spots on the under side.

I would look for:
1. Alignment problems.
2. Spilt liquids.
3. Dust & Dirt.

And mind the static electricity. That will spell the end to the electronics if you discharge into sensitive parts.

-good luck
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Re: keyboard fault: 1 key 2 letters

Postby paulgeering » May 27th, 2011, 9:07 am

Hi,

You can test if the problem is with the control board or the plastic circuits.

Disconnect the plastic circuits from the control board, like you did in this picture:
Image


Connect the control board to your computer and open a program (Notepad, TextEdit, etc.) so you can see if the keyboard can send letters.

Using a short piece of wire (or a paperclip) connect one of the contacts labeled R0-R17 to one of the contacts labled C0-C7.
Image

This is like pressing the buttons on the keyboard.

Repeat this until you find the combination that produces "q" or "1".

If you still get "qi", "iq", "18" or "81", then your control board is broken.
If, using this method, you can make it just type "q" or "1", then the problem is with the plastic circuits.

Paul
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Re: keyboard fault: 1 key 2 letters

Postby paulgeering » May 27th, 2011, 9:39 am

There are 144 different possible connections you can make.
Some of them will not type a character.

Hope this helps.
Paul
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Re: keyboard fault: 1 key 2 letters

Postby Dege » May 30th, 2011, 9:44 am

Thanks a lot, I've done the test that you suggest, and all went fine, no double letters.
Here's another set of pictures of the keyboard itself, what can be the cause of this problem?
http://imageshack.us/g/96/imag0012cgf.jpg/

edit: The keyboard have a PS/2 connector.
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Re: keyboard fault: 1 key 2 letters

Postby st2000 » May 30th, 2011, 10:41 am

@Dege
Note: Even if you paid $80+ for this keyboard several years ago you can probably replace it for less then $20 now.

With that said if you want to continue just for the fun of it - that's ok. Just know that some of us would toss it in the "take apart bin" and get another.

Going on...

While your picture are great. The only one that tell us anything is the last. And even then we don't see the bottom of the rubber mat. And we do not know if there are 1 or or more plastic sheets under it.

Also note, these types of keyboards operate best when new and are difficult to get working like new after something has gone wrong. It's not like fixing a car.

So, saving you the time to take more pictures let's just talk about the two most probable ways this keyboard is put together...

1. Rubber mat is just that, a rubber mat. And there are at least 2 plastic PCB sheets under it. With clean hands or gloved hands separate the plastic sheets. We are interested in cleaning the ones with printed circuits on them. The top plastic sheet should have a printed circuit on the under side and the bottom should have a printed circuit on the top side. How you clean these sheets of plastic is up to you and how dirty that are. I suspect they will appear clean. In that case I would be suspicious some oxidation is making it hard to get a good contact. I might try rubbing them with a clean dry cloth. I might even try some alcohol. After wards inspect the sheets. Any lint left behind will probably do more harm then what was there (if anything) before.

2. The rubber mat has conductive "dots" under each key which touched the plastic PCB under it. Again, with clean hands or gloved hands handle the plastic PCB sheet. Again you are looking for problems where the PCB traces are exposed under the conductive "dots". Again you want to remove anything that can get in the way of a good connection. If there are multiple plastic sheets you might want to separate them for better cleaning of the exposed PCB traces. As for the rubber mat, I never have had good luck cleaning these. Lint sticks to them like a magnet. I would inspect the dots for signs of dirt or residue from spilled liquids and "rinse" off what I could. Again inspect the dry parts for any lint left behind.

As you can see, more then $20 of work. And the results may be a keyboard less functional then the one you started with. But as always...

-good luck.
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Re: keyboard fault: 1 key 2 letters

Postby Underling » May 30th, 2011, 1:12 pm

here are some tips I've found helpful when rebuilding keyboards.

1. If you end up with problems the first time you put the keyboard back together just rip it apart again and realign the plastic sheets. It can take three or four tries to get everything aligned to the point where everything works.

2. Install every screw every time you test it.

3. If you wash anything with water then use rubbing alcohol to rinse it off. The alcohol washes out the water and then dries without a residue.

4. Let everything dry thoroughly before reassembly.

5. They are hard to kill so if something goes wrong you can probably fix it. The keyboard i'm using now I rebuilt like three years ago. I just took off the controller board and washed everything else under the tap. Then rinsed it with alcohol. It works fine now.
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Re: keyboard fault: 1 key 2 letters

Postby Daid » May 31st, 2011, 2:32 am

On the last photo you can see that the "sheets" inside the keyboard show signs of rust. More then I've ever seen in a keyboard. You should take it apart 1 step further. You might find more rust in it, making shorts.
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