Damaged capacitative keyboard
 

I have an E-mu electronic music keyboard that was damaged when someone
spilled a drink on it. Four of the keys stopped working after the mishap.

The keyboard works, I believe, by the same principle as a capacitative
computer keyboard. When you press on a key, a black circular part
plunges against a circuit board which has at the point of contact a
corresponding pair of interlocking metal plates (embedded in the board).
I believe the interlocking metal plates are acting as a capacitor, and
the black circle is some kind of insulator that gets between the
plates and changes the capacitance. There is a strip of 61 of these
capacitors on the circuit board, one for each key on the keyboard. They
are all connected to diodes to form some sort of matrix. When a key is
pressed, the keyboard senses the change in capacitance. In any case,
that's the best of my understanding.

So after the spill, I opened the keyboard up and cleaned off the residue
of the drink from the board and the black circles with isopropyl alcohol.
After that, 3 of the keys started functioning again. However, the
fourth is still malfunctioning. It works occasionally, but most of the
time, it doesn't work. I've tried cleaning it more, but to no avail. No
amount of cleaning seems to be able to improve its function.

I've found by fiddling around that if I change the angle at which the
black circle plunges onto the capacitor, the key works. Unfortunately,
it's not possible to change that angle when the keyboard is assembled.

My question is, what could be going wrong? What happened to that
capacitor when the drink spilled and what could I do to fix it?

Any ideas greatly appreciated.

Damaged capacitative keyboard
 

"Hamad bin Turki Salami" wrote in
message .ibm.net...
I have an E-mu electronic music keyboard that was damaged when someone
spilled a drink on it. Four of the keys stopped working after the mishap.

The keyboard works, I believe, by the same principle as a capacitative
computer keyboard. When you press on a key, a black circular part
plunges against a circuit board which has at the point of contact a
corresponding pair of interlocking metal plates (embedded in the board).
I believe the interlocking metal plates are acting as a capacitor, and
the black circle is some kind of insulator that gets between the
plates and changes the capacitance. There is a strip of 61 of these
capacitors on the circuit board, one for each key on the keyboard. They
are all connected to diodes to form some sort of matrix. When a key is
pressed, the keyboard senses the change in capacitance. In any case,
that's the best of my understanding.

So after the spill, I opened the keyboard up and cleaned off the residue
of the drink from the board and the black circles with isopropyl alcohol.
After that, 3 of the keys started functioning again. However, the
fourth is still malfunctioning. It works occasionally, but most of the
time, it doesn't work. I've tried cleaning it more, but to no avail. No
amount of cleaning seems to be able to improve its function.

I've found by fiddling around that if I change the angle at which the
black circle plunges onto the capacitor, the key works. Unfortunately,
it's not possible to change that angle when the keyboard is assembled.

My question is, what could be going wrong? What happened to that
capacitor when the drink spilled and what could I do to fix it?

Any ideas greatly appreciated.


For that type of soiling use water and detergent.

--
Regards ......... Rheilly Phoull

Damaged capacitative keyboard
 

On Sat, 19 Nov 2005 00:00:08 -0700, Hamad bin Turki Salami
wrote:

I have an E-mu electronic music keyboard that was damaged when someone
spilled a drink on it. Four of the keys stopped working after the mishap.

The keyboard works, I believe, by the same principle as a capacitative
computer keyboard. When you press on a key, a black circular part
plunges against a circuit board which has at the point of contact a
corresponding pair of interlocking metal plates (embedded in the board).
I believe the interlocking metal plates are acting as a capacitor, and
the black circle is some kind of insulator that gets between the
plates and changes the capacitance.
[snip...snip...]

I think that you'll find that it's even simpler than that. The pair of
metal plates are simply connected to a "row" and a "column" in the
keyboard matrix and the contact is made by a conductive patch on the
base of the key plunger.

Cleaning the spilled drink probably removed the conductive coating. Try
painting the contact patch area with a conductive paint. Radio Shack has
p/n 640-4339 that would probably work, as would the conductive paint
from a "rear window defogger repair kit."

--
Rich Webb Norfolk, VA

Damaged capacitative keyboard
 

Hello, Rich!
You wrote on Sat, 19 Nov 2005 16:10:02 GMT:

RW I think that you'll find that it's even simpler than that. The pair of
RW metal plates are simply connected to a "row" and a "column" in the
RW keyboard matrix and the contact is made by a conductive patch on the
RW base of the key plunger.

RW Cleaning the spilled drink probably removed the conductive coating. Try
RW painting the contact patch area with a conductive paint. Radio Shack
RW has p/n 640-4339 that would probably work, as would the conductive
RW paint from a "rear window defogger repair kit."

As the previous poster says, it's a simple contact arrangement . These
keyboards commonly use a row of bottle shaped rubber contacts which have a
conductive coating on the contact face. It's more common to find that loss
of the conductive coating is the cause of contact failure and much more
rare is loss of the etched matrix. The only proper repair is to obtain a new
set of rubber contacts from the makers or agent. When you de-greased the
matrix did you clean the rubber contact face? If so, it's likely that that's
the problem.

With best regards, 3T39. E-mail:

Damaged capacitative keyboard
 

On Sat, 19 Nov 2005 16:10:02 +0000, Rich Webb wrote:

On Sat, 19 Nov 2005 00:00:08 -0700, Hamad bin Turki Salami
wrote:

I have an E-mu electronic music keyboard that was damaged when someone
spilled a drink on it. Four of the keys stopped working after the mishap.

The keyboard works, I believe, by the same principle as a capacitative
computer keyboard. When you press on a key, a black circular part
plunges against a circuit board which has at the point of contact a
corresponding pair of interlocking metal plates (embedded in the board).
I believe the interlocking metal plates are acting as a capacitor, and
the black circle is some kind of insulator that gets between the
plates and changes the capacitance.
[snip...snip...]

I think that you'll find that it's even simpler than that. The pair of
metal plates are simply connected to a "row" and a "column" in the
keyboard matrix and the contact is made by a conductive patch on the
base of the key plunger.

Cleaning the spilled drink probably removed the conductive coating. Try
painting the contact patch area with a conductive paint. Radio Shack has
p/n 640-4339 that would probably work, as would the conductive paint
from a "rear window defogger repair kit."

I do not see how this can be the case. I am unable to make a note sound
by running a wire between the two plates. I am therefore skeptical that
the keyboard works by a simple contact principle. Also, the surfaces of
the metal plates (all of them, not just the damaged ones) are black and
don't conduct well.

Also, it does not seem that the problem is with the black circle that
plunges down. If I shift the rubber strip that the black circles are
attached to, I can realign the black circles with different keys.
When I do that, the problem doesn't shift. The same keys malfunction
and the same keys work, so the problem can't be with the black circles.

Isn't the principle behind common computer keyboards capacitative switches
like I'm describing? My impression is that this kind of switch is much
more reliable than a simple contact kind of switch and would certainly be
favored in a high quality device that is subjected to relentless use,
like a musical keyboard.

Damaged capacitative keyboard
 

"Hamad bin Turki Salami" wrote in
message t.ibm.net...
On Sat, 19 Nov 2005 16:10:02 +0000, Rich Webb wrote:

On Sat, 19 Nov 2005 00:00:08 -0700, Hamad bin Turki Salami
wrote:

I have an E-mu electronic music keyboard that was damaged when someone
spilled a drink on it. Four of the keys stopped working after the
mishap.

The keyboard works, I believe, by the same principle as a capacitative
computer keyboard. When you press on a key, a black circular part
plunges against a circuit board which has at the point of contact a
corresponding pair of interlocking metal plates (embedded in the board).
I believe the interlocking metal plates are acting as a capacitor, and
the black circle is some kind of insulator that gets between the
plates and changes the capacitance.
[snip...snip...]

I think that you'll find that it's even simpler than that. The pair of
metal plates are simply connected to a "row" and a "column" in the
keyboard matrix and the contact is made by a conductive patch on the
base of the key plunger.

Cleaning the spilled drink probably removed the conductive coating. Try
painting the contact patch area with a conductive paint. Radio Shack has
p/n 640-4339 that would probably work, as would the conductive paint
from a "rear window defogger repair kit."

I do not see how this can be the case. I am unable to make a note sound
by running a wire between the two plates. I am therefore skeptical that
the keyboard works by a simple contact principle. Also, the surfaces of
the metal plates (all of them, not just the damaged ones) are black and
don't conduct well.

Also, it does not seem that the problem is with the black circle that
plunges down. If I shift the rubber strip that the black circles are
attached to, I can realign the black circles with different keys.
When I do that, the problem doesn't shift. The same keys malfunction
and the same keys work, so the problem can't be with the black circles.

Isn't the principle behind common computer keyboards capacitative switches
like I'm describing? My impression is that this kind of switch is much
more reliable than a simple contact kind of switch and would certainly be
favored in a high quality device that is subjected to relentless use,
like a musical keyboard.



Check out: http://computer.howstuffworks.com/keyboard3.htm for the various
types of pc keyboards.

As you'll see, the rubber dome types employ carbon contacts. Cheap to
produce, which is why you can get them dirt cheap -- even FREE w/rebate --
nowadays.

The so-called "touch-sensitive" musical keyboards are different. Some have
pressure sensors under each key (aftertouch control), some only sense the
key-down velocity, others offer all the above and more. AFAIK simple carbon
contact pads alone (i.e., pc keyboard type) cannot offer velocity (or
pressure) sensing.