When I purchased the XL1201, it came with a Model 3600 keyboard. Unfortunately, several keys didn't work, including both hypers, and the left control and super. I disassembled it and couldn't see anything obvious wrong, so I assumed the key switches went bad. However, this keyboard uses hall effect switches, which have no electrical moving parts. (The keypress moves a magnet, which is sensed by an unmoving electrical component.) So, there aren't many things to break. I put it back together to deal with later.
Before reassembling it, I took some macro photographs of the PCB traces but didn't bother to look at them. A week later, I took a look, and saw this:
As you can see, the bottom solder joints for 79 and 80 are clearly suspect. 79 is the left hyper, and 80 is the left super.
I broke out my loupe and flashlight and looked at each of the 88 keys. I found five obviously bad solder joints, all on the bottom row and bottom joint.
I took out the Hakko and redid the five suspect joints, and tested the keyboard. The left hyper (key 79) was still intermittent and I re-examined the joint; my repair was bad. I wicked the solder and redid that joint and retested.
Now I had a fully working Symbolics 3600 keyboard!
Another minor thing to fix later: the two wood blocks with metal screw holes that were attached to the top side of the case have come unglued. I'll need to clean and re-glue these.
TODO: Add before and after pictures of each joint
TODO: Add repair workspace pic