The speedo on my 84 k100 has been playing up for ages.
one day when it suddenly cut out I gave a gentle knock on the glass and it sprang back into life.
Remove the rear cover of the instruments, high up on the rearside of the speedo you find a three pin
connector (well known problem source, BMW have used several metods to solve this over the years).
Remove the center part with all contact pins and bulbs....then clean the three contact pins, and add a piece
rubber foam between the three pin connector and the rear instrument cover...to add pressure to the contacts.
everthing worked except the clock. The LCD was visible but not as clear as the original, and it quickly almost faded right out.
Same as above....bad connection. While you have things apart you can clean the two connectors to the clock
or you can swap the clock as one unit.
After this you should have two complete instruments, both with a functioning speedo and clock.