As other have told you F1 or Aux power to get the TIU power and the Red LED to light. No red LED lite and the TIU can not add DCS signal or be controlled.
F1 and F2 are hard wired to let power go through, but with out TIU energized the DCS does not work. VAR1 and 2 DO NOT pass power through and the TIU must be energized to control those channels and raise and lower voltage too.
The easy solution for you while experimenting is to just apply power to F1 via transformer and use jumper wires to feed input of F2. BUT remember a Z-750 is not much power and running 2 trains which probably max it out. G
