Storytime: A3000 keyboard repair [degraded rubber spacers]

Arc/RPCs, peripherals, RISCOS operating system & ARM kit eg GP2x, BeagleBoard
shock__
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Joined: Sat Jul 29, 2017 9:21 am

Storytime: A3000 keyboard repair [degraded rubber spacers]

Postby shock__ » Fri Aug 18, 2017 9:17 am

As mentioned in another thread the BBC A3000 I was servicing for a friend had issues with its' keyboard.
Keys would either work unreliable, have "ghosting" (i.e. pressing G would work once, but once the keymatic would set it in would register as H) or work totally eratic (right half of the keyboard being either delete or enter, F12 opening the Task Manager instead of the commandline in RISC OS3).

First I expected either the keyboard connector to be bad which still had some battery gunk inside of it. Replacing the connectors only made the issue worse (before it would occasionally work to a degree where one could enter smaller BASIC programs). Next were the 74LS145 which also still had some gunk on it - those also checked out fine (but were socketed for easier replacement in the future in case they'd fail). Foil connectors were slightly trimmed to connect with a 'fresh' portion of the strenghtened part (they had become shiny exposing plastic and 2 contacts also had slight battery damage) - no improvement either. Broken traces couldn't be found either on the exposed part of the foil layers, nor on the PCB.

I was about to give up at that point.

As a last desperation move I disassembled the whole keyboard (which luckily is just the top mech + foil + backplate clipped together [contrary to i.e. IBM Model M keyboards which have melted plastic rivets which have to be cut off and then be replaced with bolts and nuts]) and connected just the foils ... and look there - the keys would register as they're supposed to.
Then it rang to me ... as I had previously heard of the issue I was facing for Commodore SX64 keyboards. The cheap rubber spacing applied to one of the foils of the keyboard had degraded to a degree where the contacts/traces would be touching/close to touching/bouncing. Next step was seperating the 2 layers which had slightly welded together but could be seperated without much of a struggle. As additional spacers I used strips of electrical tape (be sure to put them on loosely without any strain as the foil would retract otherwise) in a pattern so there'd be electrical tape on each side of the contact spots. A more elegant way would have been to add another sheet of plastic which just exposes the contact spots which would go between both sheets (as it is done on quite a few other keyboards [i.e. ZX Spectrum or Atari XL]) or custom made stick on rings.
In the end it would look something like this: http://i.imgur.com/KaAIt8G.jpg
Reassembling the keyboard it worked a lot better. Some keys still wouldn't work or ghost so some finetuning to my "electrical tape dreamcatcher" had to be done and 2 contacts were addtionally refreshed using a soft pencil (graphite powder/conductive pens might also work) - I think I had to do that 3-4 times.

But now the keyboard is working almost like new :) It also gives some slightly more pleasant tactile feedback.
Thanks for reading.

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