Thanks Axel for confirming the theory.
I just went from two non-working Beo5s to two working Beo5s 🙂
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I tested with parts from two remotes, new flat flex cables (seen here) and three battery packs: two older Sanyo packs (blue cells with yellow wrapping, bought years ago, suitable NTC, one dead pack, one working) and the new one (green wrapper).
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I replaced the flat flex on one remote as the old one had a sharp bent and the connectors were somewhat fiddly too after several insertions and removals.
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New flat flex next to the old one. I bought a 8cm one (link for the current
link at the time of writing) which does fit inside the case (needs a bit of fitting) and is easier to handle during disassembly. Shorter cable would be ideal but I couldn't find one. Cable specs: 30 pins, 0.5mm pin spacing, Type A (contacts on the same side).
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This is the new battery pack into which I replaced the "more correct" NTC from the dead older pack. While I presume this is not a good practice (the NTC should be matched to the specific cells), I expect the range to be about there for the remote to detect a hot cell, and at least with the USB charging the current is so small that unless the cells go short, I guess it'll be fine.
For reference, this is how the display module (actually the CPU board) looks inside. The clip-up connector for the flat flex is so much easier to work with than the push-to-insert one in the keypad board.
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The display case is held with clips on the edges, it can be opened and closed with not too much marks on the casing.
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Both remotes appear to work now, and least they both charge (first with USB as I could plug in my USB power meter to keep checking that the charge kept going and then with the original charger (400mA), keys work and with camera I can see the IR leds beaming.
Neither remote is in too good condition, both have worn key legends and scratches on the aluminum parts etc and one has a bit loose display mounting. Both remotes have my somewhat hacky configuration from years ago (that january 2017 as seen in the display is probably correct), when I only started to grasp the configuration tool before the first remote died (I presume it was a bad battery, then the later remote I bought probably had a bad ribbon cable and/or bad battery too, and I couldn't figure this all out then).
Now I need to get the configuration utility installed and start hacking with it again to see if I can further polish the configs!