Are you trying to operate it as a self contained system (the original intent of the system) or are you trying to run it as an output device from a PC?
If self contained than you need to have font and effect modules plugged into the motherboard. If as an output device, than you need to have an LMK board plugged into the Sprint motherboard and a cable from a PC sending it data and Gerber software with Sprint 15 and Sprint 30 drivers installed.
The original manuals will only help you if you are running it as a self contained lettering machine.
Do you have the plotters plugged into an A-B switch and the A-B switch plugged into the Sprint motherboard?