got a suggestion about restoring a membrane keypad? - Page 1
Forum section: electronics / general

author: slburris
author_rank: Frequent Contributor
timestamp: July 02, 2010, 04:47:11 pm
content: 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 youkeep 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. Frompoking around with my DMM, it looks like the contact problem is happening betweenthe pads and the wire which runs underneath.So my thought is to burnish the copper pads, probably with a pencil eraser, andlightly run over the wires with an x-acto knife to clean off any oxidation. Since thiskeypad is probably unobtainium at this point, I was wondering if someone had experiencedoing a restoration like this and had any better suggestions?Scott

author: saturation
author_rank: Super Contributor
timestamp: July 02, 2010, 06:01:39 pm
content: 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=25326The reason its cheap is just its an electric motor that spins an eraser head.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 02, 2010, 06:43:33 pm
content: Quote from: slburris on July 02, 2010, 04:47:11 pmwhich 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.

author: slburris
author_rank: Frequent Contributor
timestamp: July 02, 2010, 08:33:03 pm
content: I'm erasing the copper pad underneath. I'm trying to just peelback 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 halfand repeat.Scott

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 02, 2010, 09:48:08 pm
content: 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.

author: Rhythmtech
author_rank: Regular Contributor
timestamp: July 02, 2010, 10:04:53 pm
content: Quote from: NiHaoMike on July 02, 2010, 09:48:08 pmNICE! way to keep a phone out of the trash heap.

author: slburris
author_rank: Frequent Contributor
timestamp: July 08, 2010, 03:49:50 pm
content: Well, this repair is going badly.Shining up the pads and wires with an eraser has cured some of the problems, butit 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 touchesa 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'tmaking good contact.I've tried taping things down, using rubber bands, etc. and nothing seems to work100%. The "right" solution would be to move the copper strips to another large pieceof sticky tape and reinstalling, but I don't think I could remove the copper stripswithout damaging them, nor could I align them well by hand.I've thought about gluing the existing tape, but that would make this permanentlyunrepairable from that point on. I'm contemplating punching very small holesin 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 somecurcuitry to mimic the current membrane keyboard and logic. That's more complexthan I was hoping for, but I'm running out of ideas.Scott

author: saturation
author_rank: Super Contributor
timestamp: July 08, 2010, 04:07:17 pm
content: 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.Quote from: slburris on July 08, 2010, 03:49:50 pm

author: slburris
author_rank: Frequent Contributor
timestamp: July 08, 2010, 04:12:42 pm
content: Quote from: saturation on July 08, 2010, 04:07:17 pmDo you mean something like rubber cement?http://en.wikipedia.org/wiki/Rubber_cementYou wouldn't happen to have some brand names in mind?Scott

author: saturation
author_rank: Super Contributor
timestamp: July 08, 2010, 06:05:09 pm
content: 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.Quote from: slburris on July 08, 2010, 04:12:42 pm

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 09, 2010, 05:18:11 am
content: 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.

author: DJPhil
author_rank: Frequent Contributor
timestamp: July 09, 2010, 12:44:11 pm
content: 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, oryou 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.

author: slburris
author_rank: Frequent Contributor
timestamp: July 12, 2010, 01:47:36 am
content: My attempts at gluing this have failed to work 100%, so Imoved on to plan B.With my smallest X-Acto knife, I punctured the tape holdingthe copper pads at the column contact points. Then withthe the soldering tip I use for SMD work, I carefully solderedthe copper strips to the contact points.And....It worked! I've probably made it more difficult to maintainthis membrane keyboard in the future, but every button nowworks 100% of the time.I'm guessing that the keyboard problem is the reason thisended up on Ebay in the first place. Now to run through thealignment procedure and start using it.Scott

author: DJPhil
author_rank: Frequent Contributor
timestamp: July 12, 2010, 04:38:40 am
content: Congratulations and well done!

author: saturation
author_rank: Super Contributor
timestamp: July 12, 2010, 11:21:47 am
content: Kudos to your repair job. Can you post pics?

author: slburris
author_rank: Frequent Contributor
timestamp: July 15, 2010, 03:32:08 am
content: So far I haven't been able to get a decent picture with aniPhone -- I may try a shot with my USB microscope and see ifthat works any better.Scott

author: Mitchell2099
author_rank: Newbie
timestamp: July 16, 2010, 05:55:24 am
content: - I already referred this to my friend.RegardsMitchell

author: slburris
author_rank: Frequent Contributor
timestamp: July 16, 2010, 06:26:49 pm
content: OK, here are some photos. The first photo is a shot of the overallkeypad. The copper pads are connected in strips that run verticallyand contact wires underneath which run horizontally. There are otherhorizontal contact wires which are supposed to connect thevertical strips via a small raised bump. But since these didn'tmake reliable contact anymore (see previous in this thread), Iattempted an unorthodox repair.The second photo is a closeup of the repair work. IN the upper leftquadrant of the picture, you can see where I carefully puncturedthe tape holding the copper strips together, and soldered thestrip to the underlying horizontal wire. This was done 9 times, onefor each column of pads.Scott