I wrote this up a while back for my Model M forum, but seeing as that forum has zero traffic, and this one does... I'm going to go ahead and post it here.
This one isn't so much a modification as a repair... however, these directions will also work to convert a Lexmark board (possibly only ones built in 1993) to use an SDL cable.
Anyway, my 1993 1391401 now works.
A power surge took out the controller in this board - it powered up (caps and scroll came on, then num lock, then they all went out, as normal), but didn't actually respond.
I have a Lexmark board that had a good controller, but a damaged cable (two broken pins), so it didn't work.
So, I decided to do something about this.
This document was written after modding this revision of the controller, which was used in 1993:
Note that it only has two connectors for the ribbon cables, and also, note the J2 location on the PCB. J2 is filled by a four-pin connector (and J1 empty) on a Lexmark board. (The controller I modded had J2 occupied)
This controller may be the only one that works, I haven't tried it with other versions.
No pics, other than the one above, unfortunately, but...
First, open the keyboards. Use a 5.5mm deep socket. It's not 7/32", like most sites say. 7/32" will work, but it'll slip off a lot, and generally be annoying. Remove the cable on the SDL-equipped (that is, real) board. Four bolts, that are in REALLY tight (they dig pretty deeply into the plastic when originally assembled), accessible from the bottom rear of the keyboard.
On the SDL-equipped board, the actual keyboard assembly, with the controller, will lift right out. Lift it via the rear of the assembly, though.
On the fixed cable board, it's sorta tricky. Lift the keyboard assembly up a little, then disconnect the keyboard cable from the controller. The keyboard cable will still be attached to the grounding point on the steel plate of the keyboard assembly, but we'll remedy that. Go ahead and feed the cable through the plastic base of the keyboard, though.
Grasp the ribbon cables and carefully pull them straight up. Yes, you get flat ends. Deal.
On both keyboards, use an 8mm (IIRC) socket and a flathead screwdriver (3/16" is what I used) to undo the ground bolt. Be careful - it may be very easy to snap. Don't ask how I know. (At least I was careful on the other one...)
Take the two controllers, and desolder the SDL port from the first controller. (FWIW, any controller can be the donor controller for the SDL port.) There will be eight solder joints to undo - two for grounding, and six for data. Do not attempt to undo the "solder joint" that has nothing connected (next to the six for data) - I'm not sure what it's for, but trying to undo things unrelated to what you're working on is never a good idea.
Then, solder the SDL port onto the new controller. You don't need to remove the J2 connector, just add the J1 connector. Luckily, there's nice solder blobs ready to go for you, but you may need to add more solder.
Once everything's cooled off, and you've treated your burns... (I TOLD YOU TO BE CAREFUL WITH THE SOLDERING IRON!
) take your new controller, and carefully push the ribbon cables into the sockets. Hook up the SINGLE ground cable (no double ground, as the SDL port uses the PCB for grounding) and test it before bolting everything back together. It should work. If not, check your solder joints, cable, and keyboard port.
Now that you're done, remove the cable, and slide the front of the keyboard assembly into the front of the case. Push the controller down into place (tricky), and make sure it's aligned. Set the assembly down onto the alignment post, then bolt the case back together.
Plug in your cable, and you're done!
