A bit of an update today...
In the first page of this thread I posted a picture of a spare 286 board that someone had taken to with a screwdriver to steal the PLCC 286 CPU and damaged the socket. Well today I replaced the socket with a new one.
Here is our patient:


Here's what we're working on:

After about 2 hours with my soldering iron and some copper braid, I stuck a blade under the CPU socket and tried to lever it out...

Not too bad for a first attempt... the rest of the pins just pulled out with pliers.

In with the new socket:


Some of the resin on the board surface burned off but none of the tracks were damaged.
So now, the moment of truth.... I stuck in an Intel 286 CPU and hit the power button...

Success!


The board springs back to live. Unfortunately its got one bad RAM chip (marked with a bit of red tape in the photo above) so I'm just running it with 512KB RAM for now. I'll need to dig up a replacement 256x1 DIP RAM chip.
Of course all this could have been avoided if someone wasn't so eager to steal the CPU and just used the proper PLCC extractor tool.... 😒
Another repair I had to perform recently was to the ST251 hard drive. It would work fine for a while but after an hour or so of use it would just go dead ('seek errors' when trying to load from disk and on rebbot it would fail on POST with a HDD errror). It would come back to life after leaving it for a while.
I found this page http://home.earthlink.net/~z100lifeline/data/HDrive.html which suggested replacing two 22uf surface mount caps near the power supply connector with 47uF caps.
Unfortunately I forget to take pics and the drive is mounted back in the PC (its a pain in the arse to remove without taking half the machine apart).
But this job was simple enough... a dab of solder on each end of the surface mount cap, then hit it with a bit of copper braid while sliding a blade under the cap and gently levering it up... repeat for the other side and it popped right off.
I got two 47uF electrolytic caps and soldered the legs of these to the surface mount pads, bending the caps over so they were nearly flat against the board.
So far its been good. I haven't had any of the issues I had before, drive is humming along perfectly. I can definitely recommend replacing the caps for anyone that's fiddling with an old ST251 drive.
If you are squeamish, don't prod the beach rubble.