Duncan,

I have had at least three 28's with stuck keys In every case it has been because the keyboard clutch was not tripping.

There may be no need to take the keyboard apart. Try tripping the keyboard clutch with a screwdriver.

If the stuck keys pop up, you have found the problem.

If you hit another key and it sticks, trip the clutch manually again  and watch the clutch the next time you hit a key.

There are about a half dozen steps driven by springs before the clutch trips and finishes the job.

I have gotten two of my keyboards working that way. I know where the third is failing but cannot figure out why.

My bible is Bell System  Practices, Section P70.034, Issue 1, August 1953 (From Nick's site.) Page 10, Beginning on Paragraph 3.07

I had to study it, stare at the machine, study it some more, stare at the machine some more, etc. etc. etc. Until I finally began to understand each step. On my first ASR the code bar bail was frozen from critter pee.  A good oiling got it working. I forget what fixed the second one - but the clutch had not been tripping. Ditto for the third. I would have to study it again to refresh my memory on that point of failure.

Make sure your clutch is tripping. If not, trace the steps and find out why.

Tom   N3CRK