My Psion 5mx keyboard works well when the backlight is off, but produces spurious characters when the screen backlight is switched on. This is inconistent - holding down the space key might produce several lines of spaces, with occasional newline, n, 6 or ' characters. Other keys might produce different results. I haven't seen any extra characters appearing when keys are not pressed.

I have swapped out the motherboard and the keyboard separately, with the same results, and running on mains or battery power doesn't affect anything. I don't have a spare working screen to swap.

Because the screen ribbon cable and the keyboard ribbon cable are in contact when they pass through the case to the motherboard, I am assuming that some interference is occuring between the two when the high voltage backlight is on.

Is there something I could put between the two ribbon cables that might reduce this, or anything else I might do to improve the situation?

  • 1
    I would try a piece of aluminium foil sandwiched in kapton tape between the two ribbon cables to see if that helps. The people on the electrical engineering StackExchange may be able to offer better advice. Commented Mar 29, 2023 at 6:59
  • If anyone else thinks this would be better on Electrical Engineering SE, please vote to close & move. Commented Mar 29, 2023 at 17:36
  • 5
    Ten bucks says it's a bad capacitor in the CCFL inverter causing RF interference.
    – Sneftel
    Commented Mar 30, 2023 at 10:23
  • 1
    @Sneftel If you add a little detail, and repair instructions, that could make a good answer.
    – wizzwizz4
    Commented Mar 31, 2023 at 23:35
  • @MarkWilliams Electrical Engineering's /help/on-topic page lists “consumer electronics such as media players, cell phones or smart phones, except when designing these products or modifying their electronics for other uses” as off-topic. Do you have a reason to believe this question is on-topic there?
    – wizzwizz4
    Commented Apr 11, 2023 at 16:53


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