SOLUTION - HOWTO with pictures
All,
As promised here is my result of fixing a couple of membranes.
I will try to summarize all the process, for those that are interested to have a single reference.
The structure of the post follows (in two different posts due to the limitation in the number of images to include in each post):
- 1.- Why were the membranes broken ?
2.- First attempt and error
3.- Second attempt – Bare Conductive Electric Paint
- 3a.- IBM model M 1396790
3b.- IBM model M 1391401
4.- Conclusions
The pictures are hidden to avoid a longer post
1.- Why where the membranes were broken ?
IBM model M 1391401: I procured this keyboard in ebay, USA. The seller told me that some keys were not working. He told me that the following keys were failing. I attach the text where the failure was described;
As for the non-working keys,
esc ` 1 tab q a z shift lctrl lalt g h + ] ' enter rshift rctl ralt
prtscrn scrllock pause home end uparrow leftarrow kypd+ kypdenter
kypd0 kypd.
At the time of procurement, I had no experience with IBM model M but I found repairing the keyboard an interesting challenge
The problem that I found (initially) is described in some pictures that were in the previous posts:
IBM model M 1396790: I procured this keyboard in ebay, Europe, Germany. The previous owner drop beer in the keyboard. I am not sure when did it happen. I do not know how long the membrane was exposed to the beer.
2.- First attempt and ERROR (do not try this !)
Initially, I tried to use copper tape to fix the membrane. The tape that I used had some glue on one side. This glue was not conductive. Thus I had to solder it to the conductive line in the membrane. This was extremely difficult. I eventually managed to do it.
I spent various hours, It initially worked but as soon as I put together the keyboard, the soldering broke in the part of the membrane that bended to the controller.
Following I attach some pictures. The result does not look very nice. But as I said, it was not easy.
3.- Second attempt – Bare Conductive Electric Paint
I got it following the recommendation of "cookie"
The ink takes some time to dry. It will become conductive only after it gets solid.
It is also important to notice the the resistance is significant as explained later in the post.
3a.- IBM model M 1396790
I started by replacing the copper with the ink. The resulting resistance was approximately 0.4k ohms.
In order to access the conductive line, I had to scratch the plastic protection on top. This plastic is only present in this section. If you have to do it, please do it gently. If you go to far you will destroy the conductive line. If you do note scratch enough you will not arrive to it. Always use a tester before "painting" on top. You may not have conductivity at both ends.
I checked the conductivity and I had to repair other sections on the membrane:
I attach also images from the different membrane layers. You will notice that I had to write down the keys.
3b.- IBM model M 1391401
I had some non-working keys; "g", "h", "b", "n", "left-ALT", "space", "0-num_pad", ".-num_pad" . I found the following errors in the membrane that I corrected:
I attach pictures of the marked membranes. Note also that I marked the points that are not used in the keyboard. I also wrote down the keycode:
--> continues in a second post. The software in the forum does not allow to include more pictures 
