Carbon contacts under rubber keypads are known to wear-out over time. This article shows one way to repair them.
Carbon contacts under rubber keypads are known to wear-out over time. This article shows one way to repair them. I recently faced this issue with my Tektronix TD784D oscilloscope where some of the most frequently used keys (such as the "Clear Screen" button shown in Figure 1) required multiple presses or even stopped responding. This is also a common issue with remote control keypads.
Figure 1 - Problematic Rubber Key
The first thing to do is to disassemble the keypad panel so you can access the carbon contacts (Figure 2). In some cases, if you are lucky, the issue may be only dust accumulation which can be solved by cleaning the contacts with a cotton swab and alcohol. In my case however, simply cleaning the contacts was insufficient as the carbon material wore out and disappeared over time.
Figure 2 - Carbon Contacts Exposed
Note that you can test if the circuit still works by shorting the PCB copper end of the keyboard as shown in Figure 3. If the key works, this shows that the issue is in fact with the carbon side and not the PCB.
Figure 3 - Testing with Screw Driver
After doing some research on the web, I found some people used conductive compounds (such as this one from Amazon) to address such issues. The reviews on these products were mixed though, and since I didn't need so much material to repair just one or two buttons, I decided on a different approach.
I've had in my lab conductive adhesive copper tape (as shown in Figure 4), which I have used for some Electronics projects. (You can also get it from Amazon). It occurred to me that I could cut small pieces of material and glue the copper to the rubber pads. This would create a very conductive surface that should work fine for replacing the carbon contacts.
Figure 4 - Adhesive Copper Tape
To extract small copper disks from the tape, I used a punch of the kind you would use to punch through paper before mounting on a ring binder. Figures 5 and 6 show the results. If you don't have a punch of this type, you might be able to use scissors instead.
Figure 5 - Punching the Tape
Figure 6 - Copper Disks
Now it came time to attach the disks to the keys. Unfortunately, the adhesive in this tape wasn't strong enough to firmly mount the disks on the contacts, so I decided to use an epoxy glue instead (see Figure 7).
Figure 7 - Epoxy Glue
Figures 8 and 9 show the disks glued to different buttons. In the case of Figure 9, the button required two disks glued side-by-side.
Figure 8 - Glued Disk
Figure 9 - Two Disks Glued to Keypad
After letting the epoxy glue dry overnight, I re-attached the front panel and tested the fix. So far the buttons have been working very well and are very responsive again (See Figure 10). While time will tell if this fix proves reliable, they have been operating for many weeks now without a problem. Hopefully, this tip will be useful to the reader when confronted with a similar issue.
Figure 10 - Fixed!
Comments, questions, suggestions? You can reach me at: contact (at sign) paulorenato (dot) com