Hi all,
While working on an
SGI Granite USB conversion I ended up with a few switches that were in a persistent closed state. After some puzzling, I hit on a fix this using paper shims in the contact assembly that seems to work pretty well. Using this method I successfully repaired the three problem switches on my keyboard, though I have no idea if it'll hold up.
I desoldered and removed the switches, since it was just three and it is easier to close them back up when they're loose, but you could do it with the switch on the board.
Here is the switch opened up with the contact assembly at the bottom of the picture. Notice how there are two thin plates with a small gap in between. We are going to add a couple small paper shims into that gap from the top.
The contact plate assembly removed. This is the side that gets actuated by the slider and presses the two plates together to complete the switch circuit. The spring clip serves both as the actuator, and a clip to hold the contact plates and the plastic contact assembly body tightly together.
Here is the reverse side. You can see the other side of the spring clip holding the top of the assembly together. With the spring clip removed, you can spread the plates apart a little bit at the top of the assembly. Don't go too far, you don't want to break the two plastic retainers (like rivets) that hold the assembly together at the bottom.
Here we are looking into the contact assembly from the top, with the clip removed. You can flip over the bottom half of the switch shell and use it as a stand if you want.
Now cut two small shims from a sheet of copy paper, and slip them inside the right and left sides of the assembly against the inside plate. The shims go between the inside plate and a thin plastic film that separates the inside and outside plates. I believe my failed-closed problem was caused by deterioration of this plastic film.
Hold the assembly (now with the shims) together and re-attach the spring clip. Test to make sure the contact assembly is no longer closed.
Test the actuator to be sure the contact assembly is working correctly, then reinstall.
NB: Before I started this project, I had never even seen the inside of a keyboard, so don't take this as expert advice. I'm just describing what seems to have worked in the case of my three problem switches.
Comments and criticisms are welcome - probably pretty much everyone here knows more about the subject than I do.
Jef
