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Old 12th Nov 2023, 2:10 pm   #1
Join Date: May 2005
Location: Adelaide, South Australia
Posts: 531
Default Conductive Rubber Keypad Repair

I have a DSO that is approx 6 yrs old, it replaced a Tek 453A which I really liked for it's ease of use, quick warmup time and bright sharp trace, but was far from fond of the amount of room it took up on the work bench - I do miss it though.
A few weeks back I used the scope to observe an amp waveform and noticed that some of the functions wouldn't work when the appropriate button was pressed. The scope came apart reasonably quickly and after checking for bad solder joints I removed the switch PCB, all of the push switches are the conductive rubber membrane type, the conductive rubber pads on the 'faulty' switches were quite a bit smaller than the rest of pads, to accommodate an LED that indicated the function in use, the pads looked ok but on checking the ON/OFF resistance and comparing with the others I found there was no change in the switch resistance of the faulty switches when pressed, the good switches went from 19K/off to 15K/on whereas the faulty switches stayed at 19K. With nothing to lose I painted a small amount of wire glue onto the affected pads, dried it with a little help from heat gun and it all worked, however I'm not confident this is going to last, I'd like at least 3 years or so of use - does anyone know of a proven repair that will last some years. The wire glue is I believe water based PVA glue mixed with carbon/graphite powder, and I've seen repair kits on the 'bay that consist of a selection of various sized conductive pads that are fixed/glued onto the original membrane.
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Old 12th Nov 2023, 2:31 pm   #2
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,082
Default Re: Conductive Rubber Keypad Repair

I've repaired a few keypads which have gone this way and it works but if you go the route of coating the original buttons with something conductive, ideally black flexible conductive stuff rather than hard silver metallic stuff, then you may have to resign yourself to having to do this repair again every few years.

If you use the black rubbery stuff, one thing I recommend is to roughen the surface of the original conductive 'pill' with fine grade sandpaper before applying the new coating. If you just paint it onto the original smooth-worn surface then it will only be a matter of weeks before the treated keys stop working and may even found to be sticking 'on', the reason being that the little black discs of painted-on carbon haven't adhered to the pills very well and are now detached from them and may even be stuck to the staggered contacts on the keypad PCB instead of the rubber pill.

I've considered using those stick-on replacement conductive discs but in those cases it's all going to be about how good the adhesive is. If you go that route, let us know how it goes for you.

If the rubber pills of your keypad press onto interleaved copper or gold (rather than carbon) contact fingers then what you can consider doing is carefully soldering very small low profile surface-mount switches to the interleaved contact fingers, so that now the only thing the rubber key has to do is to press down on the surface mount switch. With this method, it all depends on the spacing of the interleaved contact fingers and whether the solder pins of the SM switch can be made to land conveniently on one contact finger from each side, and also whether there is enough physical space between the un-pressed rubber key and the keypad PCB to physically accommodate the switch.
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