On further study, I find that I misunderstood the nature of the print mechanism in the TI Silent 700 model 763. What I thought was the line advance solenoid is actually a head lift solenoid. It fooled me because it is actuated every time the line is advanced, but the actual line advance is performed by a stepper motor, like the newer variant of models 743 and 745, rather than by a solenoid as in the older variant of models 743/745.
The missing spring and knob prevent the head from being held in close enough proximity to the thermal paper. I'll have to figure out how to secure the parts in place in order to make it work reliably.
