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I have several devices (calculators, remote controllers, …) with rubber keypads where some keys are dysfunctioning. As I understand it, this is because of the carbon pill since things work fine if I use something in metal to make an electric contact…

Is there any way to repair this at home? In the past, I tried to clean the carbon pill with alcohol, with no significant success. So is there a known-good way to clean, repair or replace the carbon pill?

EDIT: Here is a photo of the rubber keypad: rubber keypad

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  • \$\begingroup\$ Is it failing to close or always closed from leakage? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 28 '19 at 15:52
  • \$\begingroup\$ @SunnyskyguyEE75 It’s failing to close. \$\endgroup\$ – user2233709 Apr 28 '19 at 16:00
  • \$\begingroup\$ Then show areas of missing carbon \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 28 '19 at 16:01
  • \$\begingroup\$ @SunnyskyguyEE75 Do you mean a photo of the carbon pill would help? I don’t see any carbon obviously missing, but it is not effective to make a, electric contact. \$\endgroup\$ – user2233709 Apr 28 '19 at 16:06
  • \$\begingroup\$ The carbon may have a flexible binder that is an insulator, so washing off the exposed carbon does not help much. You would have to apply more carbon crystals with a similar binder or replace the button. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 28 '19 at 16:07
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I use household aluminium foil. With round punches or using a knife or scissors i make small alu-patches to cover the 'rubber' keypads.punched or cut alu-pads To stick the alu-pathes on the conductive silicone pads use silicone glue (ordinary silicone sealant to bond glass will do. Other types of glue even super glue (cyanoacrylate) won't work on a silicone surface on the long run). Apply a smal (!) amount of the silicone glue on the pad with a small screwdriver and attach the alu-foil to the pad using tweezers. Finally press down the foil on the keypads and firm the edges over the pad (cover the pad with a sheet of paper and then press gently with a finger. Check if the foil is centered on and firm over the pad so that there are no sharp edges that can scratch the printed circuit on the board when the key is pressed. [a well centered and firm mounted alu-foil[2] [nine pads repaired[3]

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You can purchase a kit that renews the flexible conductive surface of the keypad buttons. The ones that I have used in the past worked very well.

Be aware that the shelf-life of the repair product is quite short once the bottles have been opened. In other words, gather all of the devices to be repaired together and have them opened up and ready to go. Then and only then should you open the repair material bottles and mix up the compound.

Working life of the mixed repair compound is very short. Mix enough to do one full keypad. Then mix up another batch to do the next keypad. And so on.

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  • \$\begingroup\$ I once tried to repair such a keypad with a product designed to repair cars windscreen defogger circuits. It worked but the repair did not last long… Do you think your kit would be better? Any reference? \$\endgroup\$ – user2233709 Apr 28 '19 at 17:23
  • \$\begingroup\$ I've used the product from MG Chemicals many times with no problems and no complaints. FWIW: I entered the following string into a Google search box "keypad rubber button repair kit" and the very first item in the list that came up was the MG Chemicals product. \$\endgroup\$ – Dwayne Reid Apr 30 '19 at 0:26

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