FWIW, I have put a copy (8MB PDF file) of the 1978 FAA manual "Maintenance of Data-Handling Terminal Equipment (Teletypewriter)" at http://www.navy-radio.com/tty/faa-tty-maint.pdf Covers M28, M35, M37 equipment and maybe some interesting info on FAA networks too. cheers, Nick K4NYW www.navy-radio.com