I am reposting this here from the Apple II forums, as I suspect that more Mac users are likely to hae an answer:
I recently had to do manual repairs to a bad 4-pin DIN connection on an ADB keyboard (658-4081), where the connector was so badly damaged that the pads were ripped due to stress--this is how I received it. The keyboard now works, but some of the keys produce doubled symbols.
Unlike other Apple II keyboards, the serial data all flows down one pin, and I am not sure what on the keyboard PCB could be damaged in such a way to cause this to occur. The only thing that I did, was to add some bodges to manually bridge the left-side ADB DIN port to where its traces normally lead.
Any suggestions? I am guessing that because the ADB port was not making a solid connection, part of the serial bus on the KB was damaged by intermittent contact, as if it was being hot plugged and unplugged. Thankfully, the //gs itself has no issues with my other ADB keyboard, but that is a later model, from an SE/30 or LC series machine, with the membrane keycaps, rather than the Alps switches, and I would prefer to use the Alps model
Before I open this up again to evaluate it, it;d be nice if anyone might point out any specific things to inspect on it.
Prior to this repair, the right-side port worked, and it did not have this issue, so it'd certainly a new symptom.
P.S. If any of you buy these, open them and check the pad connections on both ADB ports first, ere using them. If you can make the port wiggle with finger pressure, repair it first. It may be worthwhile to but a dab of resin down aling the side of the ADB DIN connector so that it cannot flex I have never seen the actual connector go bad, only the pads, because it wasn't well-anchored in this case design, so, I have never had a reason to remove and replace the connector itself. Thus, fixing it down in a more solid way, would be practical.
The ADB is not hot-swap safe, whatever people might want to believe.