Have some fun with the vacuum fluorescent display:
that's a sure way to make it really irrepairable... why would you want to do that ? those VFD's are unobtainium.
If i had a dead keithley and a working one i would carefully store the VFD of the broken one so that, when the vfd on the working one finally is too dim, i have a spare to revive my other one.
And if i don't have another keithely i'd still save the VFD as there may be someone else that can use it and i can get 20$ for it (or more)
simply destroying good working parts is idiotic.