IBM PS/2 L40sx and PS/2e M4-1 Space Saver keyboards

[ThinkDan]

Is this of interest here? I see information on standard and SSK Model M membranes but couldn't find anything specific to the M3 variant.

Following some discussion over keyboards - including the fate-tempting phrase "I wasn't expecting you to have a..." - a thinkpads.com forum member kindly sent me an M3 keyboard from a scrapped L40sx in order to try to fix a failing M4-1 keyboard for a 9533 PS/2e. My M4-1 had been put away some 15 years ago in a fully working state, and now was registering widespread key failures, which pointed to membrane faults:



If you're not familiar with these, the M3 was used on the PS/2 L40sx and CL57 luggables, and begat the M4 and finally the M6 as used in the ThinkPad 355/360/370 & 700/720/750/755 models. Not sure what happened to the M5. The M4 external keyboard came in two versions, the M4-1 having a TrackPoint II embedded. Of course, Admiral Shark has as much detail as you could want on these here - well worth a read.

M4-1 shown above the L40sx keyboard:



The L40SX keyboard fits directly into the M4 housing... like it literally slips in and screws down in the right places. Ribbon cables present in exactly the same place, with the same number of connectors/tracks. Of course, that's what the M4 was designed for. I overlaid each L40/M3 membrane over the equivalent M4 membrane, and they were functionally the same. (This strongly suggests to me that the M4 keyboard family are housings for the M3 keyboard assembly, i.e. not a distinct generation of Model M in and of itself.)



Inside the M4 casing is a simple circuit board to manage the PS/2 keyboard and TrackPoint interfaces, and the status indicators:



Turning either keyboard assembly over, the metal backplate slides to the left once the central retaining clip has been depressed, revealing the membranes:



The Model M membrane tracks are well documented, but the M3 / M4 differ in layout, and in number of tracks presented at the two ribbon connectors (9 and 19). Tracing each track from the connector to the key contacts, it became clear that the M4 upper membrane had three tracks failed within the ribbon part - inaccessible to repair. The L40sx upper membrane checked out fine, and was a direct swap, restoring full function:



Now the only issue is finding the PS/2e... I know it's in a box here somewhere...

[sharktastica]

Good to see you here!

I certainly think they're interesting (and I always defend that IBM buckling sleeve keyboards are underrated!) Thank you taking the time to post the membrane layouts - whilst I have M4s, I've been put off the idea of completely opening up the inner assembly in case I broke anything (and its not like its as easy to find new parts for these as full-size buckling spring Model Ms).

Regarding the outgoing M4-1 membrane assembly; depending on how bad the traces are, it may be possible to 'patch it' up with conductive ink. Obviously, it's not as good as a fix as a completely new and working membrane, but it may be worth holding on to the old one in case there's an opportunity to patch it and use it again.

[ThinkDan]

Hi Shark!

You could comfortably argue that the typing feel of the buckling sleeve keyboards was one of the foundations that built the ThinkPad brand, and certainly garnered the loyalty of early users. Contemporary reviews praised the keyboards as the best in the notebook business. It was rated as the notebook for serious writing.

As to repairing these membranes, it doesn't look good. Everything traces fine on each track from just inboard of the 180 degree flex where the ribbon becomes the membrane, then tracks fail between that far side of the flex and the ribbon connector. The ribbon itself is encapsulated on each side, so there's no simple access to identify and bridge any cracks present. I'm certainly not going to start shaving the plastic coating off...!