That era of game is from before Gottlieb made manuals, so only a schematic would exist for it. Those aren't available online because the copyright is still retained. The schematic can be purchased from Pinball Resource though (pbresource.com).

Roto units are generally triggered by one of many switches throughout the playfield (e.g. a rollover switch, or standup switch, or even a dead bumper switch), which feed a Roto Unit relay, which then feeds a coil on the Roto Unit. This coil pulls in a plunger, spinning the Roto Unit.

So some things to check are whether the unit will spin if the plunger is pulled in manually (game off), to see if the issue is mechanical or not. If it spins freely manually, then need to work upstream from the coil there to see what's causing it to not activate. See if the Roto Unit relay is activating, but not the coil. If so, then it could be a switch on the stack there, or maybe a wire popped off (i.e. cold solder joint). Visual inspection often reveals a loose wire or misadjusted switch.

If the game has been sitting unplayed for a long time, switch contacts may have just oxidized and need to be cleaned.

If there's more than one way on the playfield to activate the Roto, try them all (or at least a few). If none work, then the issue is with a common point (like the Roto Unit relay). If at least one of them works, then the issue is local to that playfield feature.

Those are just some general guidelines though.

(This is assuming some experience working on EMs before though, apologies if that's not the case.)