Mystery (to me anyway) Model 28D Typing Unit
 - solved!

• Please send info to nick@navy-radio.com
• Back to my 28ASR TTY restoration page

The mystery is a Model 28D typing unit - it has a different gear to drive the keyboard - is slightly larger and has more teeth that the usual typing unit - it will not fit into my 28KSR or my 28ASR. Why drive the keyboard with a different gear? What was it used for?

ANSWER - The 28D is a 7.00 unit typing unit - so the keyboard drive gear won't mesh with a 7.42 unit keyboard shaft gear.

This 28D typing unit also has some extra mechanism on the right-hand drum - looks like blades that trip a switch contact located near the main shaft. Is this some sort of tab mechanism? ANSWER - YES

28A typing unit - keyboard drive gear in center - 21 teeth	28A typing unit no extra switch contacts	28A typing unit no extra mechanism
28D typing unit - keyboard drive gear in center 26 teeth, 0.1" dia larger, tighter pitch	28D typing unit extra switch contacts in upper right	28D typing unit extra mechanism on right-hand drum