got a suggestion about restoring a membrane keypad?

[slburris]

Hi,

I'm working on a project to restore a WaveTek 275 Arbitrary Function Generator.
It works, except the front panel doesn't respond very well to keypresses.  If you
keep pressing, eventually a button will respond.  Some buttons are worse than others.

Being lazy, I tried the cheezy way to clean this -- I sprayed some alcohol under the membrane,
hoping to flush out any dirt.  I didn't think it would work, and it didn't :-(

So I've taken the front panel apart and peeled back about half of the membrane.
Looks like the "keys" are copper pads which touch a wire matrix when pressed.  From
poking around with my DMM, it looks like the contact problem is happening between
the pads and the wire which runs underneath.

So my thought is to burnish the copper pads, probably with a pencil eraser, and
lightly run over the wires with an x-acto knife to clean off any oxidation.  Since this
keypad is probably unobtainium at this point, I was wondering if someone had experience
doing a restoration like this and had any better suggestions?

Scott

[saturation]

I would avoid a knife, if I accidentally cut something in unobtanium, it would be hard to get a substitute unless I buy another damaged device for parts since that keyboard is custom made.

I've used erasers too with great success.  But to avoid torsion forces, buy a cheap electric eraser, it could do all you want with the least work, and if it doesn't work it won't break the bank.

A Staedtler Electric Eraser, is sold in the US between $5-10 and the eraser refills is a box of 20-30 heads for $3.  The Sakura Electric Eraser one is Japanese original and costs $40!  They are 2x AAA powered.





Video:



http://www.artistsupplysource.com/product.php?productid=25326

The reason its cheap is just its an electric motor that spins an eraser head. 

[Mechatrommer]

Looks like the "keys" are copper pads which touch a wire matrix when pressed.  From
poking around with my DMM, it looks like the contact problem is happening between
the pads and the wire which runs underneath.

which part are you trying to erase? the top part of copper pad? or the underneath part where the pad and wire mesh contacting? the contact is underneath... u must have access to those area i think. erasing the top part wont do any good. thats the way i see it from the picture u showed.

[slburris]

I'm erasing the copper pad underneath.  I'm trying to just peel
back half of the switches to get access to the wires and pads.
That might not be clear from the picture.

Once the first half is done, I'll carefully peel back the second half
and repeat.

Scott

[NiHaoMike]

If it couldn't be fixed, solder in small pushbuttons. I once fixed a cordless phone with a bad power button by soldering a pushbutton from a broken VCR across the contacts, then carefully cutting the rubber button so it fits.

[Rhythmtech]

If it couldn't be fixed, solder in small pushbuttons. I once fixed a cordless phone with a bad power button by soldering a pushbutton from a broken VCR across the contacts, then carefully cutting the rubber button so it fits.

NICE!  way to keep a phone out of the trash heap.

[slburris]

Well, this repair is going badly.

Shining up the pads and wires with an eraser has cured some of the problems, but
it was necessary to peel back the backing tape that the copper pads are attached to.
Unfortunately, the tape isn't sticky enough to adhere well when put back.

The switches are vertical strips of copper pads.  When pressed, the pad touches
a horizontal wire, completing a row connection.  The strips touch a raised wire
(basically a wire with a little solder blob on it) for the column connection.
With the now poor adhesion of the backing tape, the column connection isn't
making good contact.

I've tried taping things down, using rubber bands, etc. and nothing seems to work
100%.  The "right" solution would be to move the copper strips to another large piece
of sticky tape and reinstalling, but I don't think I could remove the copper strips
without damaging them, nor could I align them well by hand.

I've thought about gluing the existing tape, but that would make this permanently
unrepairable from that point on.  I'm contemplating punching very small holes
in the backing tape and carefully soldering the copper strips to their column touch point.

If that fails, I guess I can make up a PCB with an array of microswitches and some
curcuitry to mimic the current membrane keyboard and logic.  That's more complex
than I was hoping for, but I'm running out of ideas.

Scott

[saturation]

You need a glue similar to what held the keypad together.  Glue comes in different adhesive strengths, the weakest is something like on post it notes.  I've used rubber glue which is easy to remove for hours; when it finally sets, its often a weak bond on plastic polymers, it will be easy to tease glued items apart, scrape off the glue, clean and redo.

Well, this repair is going badly.

Shining up the pads and wires with an eraser has cured some of the problems, but
it was necessary to peel back the backing tape that the copper pads are attached to.
Unfortunately, the tape isn't sticky enough to adhere well when put back.

The switches are vertical strips of copper pads.  When pressed, the pad touches
a horizontal wire, completing a row connection.  The strips touch a raised wire
(basically a wire with a little solder blob on it) for the column connection.
With the now poor adhesion of the backing tape, the column connection isn't
making good contact.

I've tried taping things down, using rubber bands, etc. and nothing seems to work
100%.  The "right" solution would be to move the copper strips to another large piece
of sticky tape and reinstalling, but I don't think I could remove the copper strips
without damaging them, nor could I align them well by hand.

I've thought about gluing the existing tape, but that would make this permanently
unrepairable from that point on.  I'm contemplating punching very small holes
in the backing tape and carefully soldering the copper strips to their column touch point.

If that fails, I guess I can make up a PCB with an array of microswitches and some
curcuitry to mimic the current membrane keyboard and logic.  That's more complex
than I was hoping for, but I'm running out of ideas.

Scott

[slburris]

You need a glue similar to what held the keypad together.  Glue comes in different adhesive strengths, the weakest is something like on post it notes.  I've used rubber glue which is easy to remove for hours; when it sets on most type of plastics its a weak bond, it will be easy to tease glued items apart, scrape off the glue, clean and redo.

Do you mean something like rubber cement?
http://en.wikipedia.org/wiki/Rubber_cement

You wouldn't happen to have some brand names in mind?

Scott

[saturation]

Yes, rubber cement.  Whatever is easy for you to find near you.  Its fairly old technology, Elmers is easy to find at office supplies like Staples or Office Max, ~ $2-3.

You need a glue similar to what held the keypad together.  Glue comes in different adhesive strengths, the weakest is something like on post it notes.  I've used rubber glue which is easy to remove for hours; when it sets on most type of plastics its a weak bond, it will be easy to tease glued items apart, scrape off the glue, clean and redo.

Do you mean something like rubber cement?
http://en.wikipedia.org/wiki/Rubber_cement

You wouldn't happen to have some brand names in mind?

Scott

[Mechatrommer]

i'm not really sure what are you talking about since i dont have it infront of me, you know better. this is where DIY skill and think of yourself creatively should come in handy. but when you mentioned about soldering, if i'm not mistaken, that would be plastic all over, you can screw that up easily and make a nonreversible repair (or damage), think and decide carefully, try to sort out any other possible way... before you are attempting nonreversible process. just my advice.

[DJPhil]

If it were me I'd try to rebuild/redesign the panel in question, that way I could try as many different things as needed without permanent damage.

I can't quite tell what you'd be up against there from the pictures, but it might help to consider making a flexible pcb. There are board houses that do it, or you can try it diy style. I don't have any personal experience with flexible stuff (still trying to get the normal stuff to behave), but I'll definitely be trying it out eventually for just such occasions as this.

Hope that helps.

[slburris]

My attempts at gluing this have failed to work 100%, so I
moved on to plan B.

With my smallest X-Acto knife, I punctured the tape holding
the copper pads at the column contact points.  Then with
the the soldering tip I use for SMD work, I carefully soldered
the copper strips to the contact points.

And....

It worked!  I've probably made it more difficult to maintain
this membrane keyboard in the future, but every button now
works 100% of the time.

I'm guessing that the keyboard problem is the reason this
ended up on Ebay in the first place.  Now to run through the
alignment procedure and start using it.

Scott

[DJPhil]

Congratulations and well done!

[saturation]

Kudos to your repair job.  Can you post pics?

[slburris]

So far I haven't been able to get a decent picture with an
iPhone -- I may try a shot with my USB microscope and see if
that works any better.

Scott

[Mitchell2099]

- I already referred this to my friend.

Regards

Mitchell

[slburris]

OK, here are some photos.  The first photo is a shot of the overall
keypad.  The copper pads are connected in strips that run vertically
and contact wires underneath which run horizontally.  There are other
horizontal contact wires which are supposed to connect the
vertical strips via a small raised bump.  But since these didn't
make reliable contact anymore (see previous in this thread), I
attempted an unorthodox repair.

The second photo is a closeup of the repair work.  IN the upper left
quadrant of the picture, you can see where I carefully punctured
the tape holding the copper strips together, and soldered the
strip to the underlying horizontal wire.  This was done 9 times, one
for each column of pads.

Scott