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?