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Touch Pad Repair

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  • Touch Pad Repair

    Being a two-grouper commercial machine, my Grimac of course has two lots of touch pads, one for each group. The left side work reliably.

    But the right has always been hit and miss, mainly miss. Touch the pad and usually it doesn't respond.

    They are about $250 each, a lot of money for what is a very, very simple circuit board and five micro-switches soldered to it.

    After years of frustration, I decided to dismantle the right bank and see whether there was anything obvious which caused the problem. I've fixed electric window winder switches on my cars, and hoped these might have similar impediments: carbon traces around the contacts.

    Four little screws secure the pads to the machine. Off they came. Easy.

    Another 4 screws, from memory, secure the circuit board to the casing. Off they came easily enough.

    As I said, the boards are very simple etched boards which connect the springy micro-switches to a multi-pin socket, which then connects via a ribbon cable to the computer controller.

    Unfortunately, or fortunately, I could see nothing obviously wrong. No dry solder joints, no broken circuits.

    Further dismantling would entail taking apart each switch, which are not designed to be taken apart.

    I should have taken pictures, but I didn't, so I'll try and use a thousand words instead.

    Each switch has a small thin metal cover, less than a centimetre by a centimetre. At each corner, the covers have a tiny hole, about the diameter of a sewing needle. Little plastic prongs come through those holes, and, during manufacture, they are clenched with heat to stop the covers falling off.

    I was about to give up and put everything back together when I thought I'd give it a shot easing off a cover. Nothing to lose, after all.

    Very gently with a knife blade I prised it off at each cover. Of course, the plastic prongs, the thickness of sewing needles, broke, so there was no way they could be re-deployed.

    I found a small almost flat brass disc about six mm diameter inside the switch. When you touch the pad to run the pump, the disc flattens out, and the centre thus touches a contact and sends voltage to other contacts on the outside edge. Let go of the switch, and the springy disc bounces back from its flattened state and stops the contact with the centre electrode.

    The disc looked grubby, enough to insulate the contact areas. I gave it a quick clean and turned my attention to how I would reassemble everything, having broken those little prongs.

    Ah, what about my electric hot melt glue gun? Worth a try.

    So I Put the cover back on the micro-switch, tacked little globs of glue all around the edge, cementing the cover to the switch body.

    Put the circuit boards back in the base, and secured the base back into the machine.

    Well, to my surprise, that one touch pad switch I fixed, works beautifully.

    I am now encouraged to remove the touch pad once again and do a similar repair to the remaining 4 switches.
    Attached Files

  • #2
    Great outcome mate...

    You may not need to disassemble each switch though. Grab a pressure-pack of 3M Contact Cleaner, or a De-Oxit Kit and squirt through any tiny crevice you can, on the outside of the switches such that all contaminants can be carried away. Works a treat on most older switch assemblies, especially those that can't be replaced or are too expensive...



    • #3
      Great idea, Mal. I'll look see if there's any way of spraying such a solvent inside without dismantling.


      • #4
        Originally posted by robusto View Post
        Each switch has a small thin metal cover, less than a centimetre by a centimetre. At each corner, the covers have a tiny hole, about the diameter of a sewing needle. Little plastic prongs come through those holes, and, during manufacture, they are clenched with heat to stop the covers falling off.
        Sounds like they're "tactile" switches, they'll run you about a dollar a piece from Jaycar and the like;

        Click image for larger version

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        Probably easiest (and most reliable) to just replace them.


        • #5
          Certainly look like those and others your post led me to on the web. Dirt cheap. As I said, there's little more to them --well nothing more actually--than a plastic case, a printed circuit board with no solid state parts, the switches, and an LED at each switch. Perhaps a couple of dollars to mass produce...making the $250 for each group most unjustifiable in my opinion.

          Anyway, today I decided to remove them once again and bring more of the intermittent right group into reliable play.

          Unfortunately, Mal's suggestion of spraying the insides with cleaner was not possible as the tactile switches are totally concealed.

          I took pictures this time and hope to post them tomorrow.


          • #6
            Apologies for my delay in posting this.

            As you recall, the right touchpad on my two-group Grimac machine worked intermittently. Dismantling it revealed that the contact surfaces on the switches had tarnished and a cleanup with emerycloth or very smooth sandpaper should make them operable.

            The circuit board is held within a plastic case with 6 small screws. There really is nothing in these touchpads that warrants the extraordinarily high price of $250 each. Perhaps parts which have the word "coffee" share the same affliction with boating materials. Place the word "marine" in front of an item and suddenly the price quadruples.

            the circuit board with switches and front of enclosure

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            The rear of the circuit board with terminal for a ribbon cable to the controller.

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            It was a matter of very carefully prising open the lids of the switches to access the tarnished internals.
            I did that with a small screwdriver.

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            The contact disc(right) next to the extension which presses down on it when you touch the touchpad. Sanded off the tarnish with fine sandpaper.

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            Inside the switch, with tarnished electrical terminals which the springy disc connects when pressed. Used sandpaper and a screwdriver to brighten them up.

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            Then replace the thin steel cover, and since the plastic prongs--one at each corner and the thickness of a sewing needle--had to be sheared off to remove the cover, I used a hot melt glue gun to cement it down.

            A little messy with globs of glue everywhere, but hopefully it all holds together for many years yet.
            Last edited by Javaphile; 17 July 2014, 12:51 PM. Reason: Fix attachments at OP request


            • #7
              Yikes, the picture layout didn't quite come out the way I meant it to.
              Perhaps a kind moderator would care to remove the links and replace them with the appropriate pictures?
              Thanks in advance


              • #8
                You'll need to make another post with the last picture as each post can only have 5 attachments.

                Java "All done" phile
                Toys! I must have new toys!!!


                • #9
                  Thanks for the cleanup Javaphile. I'll re-post the last pic tomorrow. It's on my other computer.


                  • #10
                    And finally, the hot melt glue with its messy globs. Of course, more careful workmanship would leave a cleaner glue joint, but the switches are out of sight so probably unnecessary.

                    Attached Files


                    • #11
                      Well done mate...