And those ADB keyboards are somewhat harder to replace than their PC cousins arent they? I recently aquired my first ADB keyboard and it took some effort to find one online at a descent price shipped. Did it happen suddenly ?

Anyway, never opened one of them but from the description you give, looks like one of those "thin film" contact scheme. Those from my experience can fail just like that. Since you have 4 keys that fail in a row, my guess would be its some sort of broken trace in that "ribbon" . not likely to be mechanical or interface problem i would say.

If you have a multimeter i would do continuity checks at various places and try to discover where something is broken. Then from this point think of a way to fix it. Visual inspection of the area around the keys might give you hints also.
good'ol troubleshooting!

Good luck!

Btw : 36 screws? are you serious? Thats almost as much as are used to hold the door plate in my vintage 1950 Philco refregirator... and this thing is totally overkill

MaxWar wrote:

Did it happen suddenly ?l

I think so, the mac doesn't get much use these days but one day these keys stopped working and never have since.

MaxWar wrote:

If you have a multimeter i would do continuity checks at various places and try to discover where something is broken. Then from this point think of a way to fix it. Visual inspection of the area around the keys might give you hints also.
good'ol troubleshooting!

Ok thanks, that's one for a rainy day then.

MaxWar wrote:

Btw : 36 screws? are you serious? Thats almost as much as are used to hold the door plate in my vintage 1950 Philco refregirator... and this thing is totally overkill

Yep, scattered over the plate pretty evenly. Sort of why I was wondering if the problem might be anything else, don't want to take them out if I don't absolutely have to 🤣.

Edit: since come across a lot of these keyboards on ebay and other forums, all with some keys not working.

Woohoo, fixed it! Thanks for the inspiration Maxwar.

Naturally I took out all the screws and found the rubber cups and two layers of cellulose film with what looked like photocopied circuitry on them.

But I also found a small pcb on the back of the keyboard unit with some contacts on it that press onto the ribbon-cable-ends of the keyboard contact-films. Those contacts were dull on the PCB and so were the corresponding patches on the ribbon cable. Rub with IPA, put it all back together [...and take apart again to put in the rubber cup I found after reinserting 36 screws... that didn't need removing in the first place... ] and it all works fine! Yay, Mac lives again 🤣.

Great job! feels good doesnt it ? I love to fix stuff.