Replacing Model M1 and M2 capacitors
Compared to their predecessors, these Selectric Touch Model M variants have quite a different logic board that makes use of surface-mounted (SMD) capacitors that tend to dry up when the keyboard is not used for an extended period, regardless whether they are Lexmark (US) or Greenock (UK) made. When this happens (or if the capacitors have leaked for a different reason), you will be left with an unresponsive keyboard with one or more of the lock-lights being stuck on. Thankfully, this can be fixed.
What you need
- Replacement 2.2uF 50V and 47uF 16V capacitors - I recommend non-SMT since they're easier to work with
- A screwdriver for your region's screw-type (see "Unscrewing the back" for possible screws)
- Fairly thin prying tools
- A lot of patience, especially with reassembly
Disassembly
Unscrewing the back
The first obvious step with disassembly is starting with the screws. Selectric Touch keyboards have only two screws on the back, however, the screw head differs between factory and time period.
In terms of the most common screw types used, Lexington/Lexmark used Phillips head and Greenock first used flat head and then moved to square head. There could be others yet undocumented too.
Removing the front cover
Unlike 'normal' Model Ms, M1s and M2s do not have a barrel plate as such, instead, the barrels are integrated as part of the front cover. This means that in order to get to the logic board inside, everything has to come a part. First start with removing the keycaps, remembering to take care where it comes to the longer keys since their stabiliser mechanisms are different to the metal bar or plastic rod types you are used to.
I would also advise that you take photos of the spring locations once the keycaps are off for reference later. Unfortunately, the easy part is now over. Like a normal Model M, the keyboard assembly is essentially a sandwich of multiple layers (except that the front cover is now a part of the equation), but instead of using rivets to hold everything together, we now get plastic stalks to deal with. Oh, and they are very easy to break.
To open up this sandwich, you will need to release these stalks simultaneously before you can lift the front cover off of the rest of the assembly. This is where the prying tools come in. I am using some of the metal tools that came with my general PC repair kit, but a popular cheap substitute that I have seen others use are desktop computer PCI slot brackets. Knowing how much pressure to use is key, so I would recommend doing your first attempt at prying in an area of the keyboard that has multiple stalks clustered together in case you break one since braking them will reduce the overall tightness of the assembly.
The force required is fairly minimal, so do not brute force your way in between the plastic stick and its grip. What you should find is that some of the stalks should hold their position outward once you've wedged in between them for a short while. Once you have worked your way around the board, you should be able to simply lift up the front cover.
Finding the capacitors
Once the cover is off, you should find that the keyboard's logic board is instantly visible in the top-right corner of the keyboard. The capacitors will be the two grey cylinders roughly in between the middle and right LED lights.
The smaller one is a 2.2uF 50V capacitor and the larger one is a 47uF 16V capacitor. You can confirm this yourself by reading the second and third values written on top of the capacitors.
Removing the old capacitors
Now comes the second challenging part - removing SMD capacitors. Being surface mount, you cannot simply desolder them from the other side of the PCB, nor can you pull them straight off with plyers since you will risk ripping out the solder pads.
This video by Mark Fixes Stuff shows an easy way to remove SMD capacitors without needing to use 'fancier' tools like heat guns or prying with a flat soldering iron.
